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Publications Pests, Thresholds, and the Cotton Plant

Contents

Preface
Essential Facts about Pests and the Cotton Plant
The Cotton Plant
Scouting Protocol for Cotton Pests
Thresholds for Pests
Biology
Cutworms
Thrips
Plant Bugs
Tarnished Plant Bugs
Cotton Fleahoppers
Clouded Plant Bugs
Cotton Aphids
Spider Mites
Cabbage Loopers
Armyworms
Whiteflies
Boll Weevils
Bollworms
Tobacco Budworms
Pyrethroid Resistance Management in Cotton
Applying Pesticides
References

Preface

This handbook is designed to answer questions farmers often ask about thresholds, fruiting, insect life cycles, and control of pests.

While not comprehensive by any means, it at least begins to address a problem of many farmers, that of not understanding the cycles of a crop and the insects that attack it. When we understand these basic cycles, more refined methods of control and management can be developed to produce cotton economically.

Many of the age-old principles of control remain constant, but specific recommendations and rates should be obtained from the most current Insect Control Guide.

There is no substitute for correct timing, proper placement, and accurate calibration of pesticides. These factors all point to more efficient management on the part of the responsible user--the farmer.


Thresholds and Facts about Pests and the Cotton Plant

Pests, Thresholds, and the Cotton Plant

Pest identification is the number one consideration in control of cotton insects. Before making control decisions, you must correctly identify the pests or pest complex. This requires detailed study in pest characteristics and the type of damage associated with each pest species.

It is also essential that you develop and master a survey procedure (scouting) that permits reasonably accurate population estimates. This in turn can permit us to make control decisions only when populations reach a predetermined threshold. These thresholds may be based on pest population numbers, pest damage, or a combination of the two.

If population numbers or damage are below the threshold, treatment is not required; if thresholds are exceeded, treatment is required.

Cotton "scouting" involves taking systematic data on pests, beneficials, and plant characteristics from each field or decision unit. These data vary with stages of crop development.


The Cotton Plant

The cotton plant is unique among major agricultural crops. It is a woody perennial that has been adapted and bred to react as an annual. This fact presents some unique and at times difficult problems for those who deal with cotton as a crop.

Most wild cottons are confined to subtropical and tropical regions. They tend to occur in relatively arid habitats. This also complicates cotton production for the average Southern cotton farmer.

Establishing an adequate stand of vigorously growing seedlings is of first importance to producing a crop of cotton. It is at this phase that limits to potential yield are set. Everything that occurs subsequent to seedling establishment only maintains or decreases the potential yield of the initial stand of cotton seedlings. Temperature, moisture, and physical impedance regulate vigor and seedling emergence.

Seedling population, spacing uniformity, and vigor are the three prime determiners of yield potential. Rate of emergence is a good measure of plant vigor. A crop that emerges in 5 days stands a better chance of survival and has a greater potential for high yields than does one that takes even 3 days longer. If stand establishment takes more than 10 days, some studies show a potential of only about one-third of the yield.

Even if the planting operation places the seed in a good environment, soil crusting due to rainfall and low temperatures can cause poor emergence. These factors can be minimized by selecting planting dates based on good soil temperature and by planting on a well-drained seedbed. The earliest recommended date for planting is when the 10-day average of minimum soil temperatures is about 61 °F.

Emergence increases generally as temperatures increase up through 66 °F. Below 61 °F, less than 40 percent of the seed planted produce seedlings.

At the lowest node of the cotton stem, the two seedling leaves are borne on opposite sides of the stem. These are called the cotyledonary leaves and actually make up two tightly compressed nodes from which the leaves arise. Every node above the cotyledonary bears a single true leaf in a spiral arrangement of about 3/8 of a complete turn. Some of the spirals are from the left to right and others are from right to left.


Temperatures

The rate of vegetative growth before initiation of fruiting is temperature and water sensitive. Germination temperature can alter flowering branch initiation one node. The temperature after seedling emergence is of major importance in formation of the first fruiting branch. Thus, lower night temperatures immediately after emergence can cause the first fruiting branch to be lower. Chilling temperatures during germination can delay the time to first flower. The root system develops and expands during the 7-10 days it takes for the first true leaves to differentiate and expand. The first five true leaves are progressively larger.

The formation of the first squares on the plant may come as early as 10-14 days after emergence, when the plant has two to three expanded true leaves. Injury to these leaves or changes in day/night temperatures or light intensity during this stage of plant development can change the placement of the first fruiting node.

Most of the commonly used varieties of cotton today are bred to begin fruiting on the fifth or sixth mainstem node. The number of nodes from the cotyledonary nodes to the first fruiting node may vary considerably by variety and by cultural conditions, particularly close spacing. Vegetative branches are usually restricted to lower nodes but under some conditions may develop from upper nodes as well.


Spacing Plants

Crowded plants usually produce more total plant material per acre than do widely spaced plants. They bear few if any vegetative branches and fruit much higher on the main stem. These fruiting branches are usually shorter, producing only one or two bolls per branch. The close spacing also causes the bolls to be smaller. There may at times be a slight increase in yield due to the production of more bolls per acre. The bolls may mature earlier, especially when there is competition for nutrients and water.

Crowding tends to shorten the fruiting season, causing it to begin later and end sooner. Optimum yields for Mississippi conditions can be expected when plant populations fall between 35,000 to 60,000 plants per acre. In general, the better the soil, the lower the plant population should be for optimum yields.


Fruiting

After initiation of fruiting, new fruiting branches are produced by the terminal bud of the main stem approximately each 3 days during the plant's period of active growth (depending on temperature, variety, and other factors, this time may vary from less than 2 to more than 4 days). These branches develop from the first axillary bud at the base of the leaf that accompanies the flower. This causes the branch to have a slightly zigzagged appearance.

Fruiting occurs in an orderly manner. When a flower opens on the first node of a fruiting branch, usually the flower on the first node of the fruiting branch at the next higher node will bloom ± 3 days later. The buds on the fruiting branches develop about every 6 days (5 to 8 depending on climatic factors). Thus, the plant may bloom on the sixth node second position at about the same time it blooms on the eighth node first position. It follows this logical sequence up the mainstem and out the branches.

In general, plants produce many more fruiting forms than can be expected to mature. Most of these will be lost as small bolls within 5 to 10 days after bloom. Emphasis has been placed on prime fruiting positions in the last few years; thus, some consideration must be given to where a flower develops as well as when. Fruiting-rate counts can help to determine if the crop is on track and at times to give indications of problems. When rates are determined, climatic, cultural, and pest factors must be taken into account. These factors are best determined on a field-by-field evaluation.


Table 1. Fruiting Rates for a Mississippi Cotton Crop *

Week Node Height Sites/Plant Potential Fruiting Sites Minimum
sqs/acre bolls/acre
1 5 1 24,000 18,000 -
1 6 2 72,000 55,000 -
2 7 4 144,000 110,000 -
2 8 6 216,000 170,000 -
3 9 8 288,000 230,000 -
3 10 10 360,000 300,000 -
3 11 12 430,000 325,000 14,000
4 12 14 504,000 400,000 44,000
4 13 16 576,000 350,000 86,000
5 14 18 648,000 345,000 130,000
5 15 20 720,000 345,000 175,000
6 16 22 792,000 340,000 216,000
6 17 24 864,000 336,000 250,000
6 18 26 936,000 310,000 276,000
7 19 28 1,008,000 276,000 294,000
7 20 30 1,080,000 240,000 312,000
8 21 32 1,150,000 200,000 325,000
8 22 34 1,220,000 150,000 350,000 **

* Several assumptions were made in developing this table. Variations of these may be made on a field-by-field basis:
Assume: (1) 40,000 plants/acre; (2) effective fruit on first and second position; (3) 30-50% plants begin fruiting at fifth node; (4) ± 80% of sites will set squares; (5) ± 60% of sites will set bolls; and (6) most important fruiting occurs in first 4 weeks of fruiting.
** This represents a 2-bale-crop minimum.


Scouting Protocol for Cotton Pests

These general guidelines allow the scout to identify problems immediately in cotton fields before the problems become economically detrimental. The guidelines also provide timely information.
  1. At planting:
    • Place boll weevil traps (minimum of 1/50 acres).
    From emergence to third true leaf:
    • Check for thrips, cutworms, mites, and aphids. Make initial stand counts.
    • Check boll weevil traps weekly.
    Third true leaf until fifth node:
    • Check terminals for eggs, larvae, and weevils. Check for flagging of plants by plant bugs and weevils. Sweep borders of fields for plant bugs.
    • Record node height and position of pinhead squares.
    Fifth node to first bloom:
    • Remove and store boll weevil traps. Check terminals for eggs, larvae (minimum of 25 plants/field.) Percent square set (check minimum of 25 plants/field -- count squares on top five fruiting nodes and assess percent set). Begin sampling 100 green squares per field when 15 percent of squares reach one-third grown or larger. Check these squares for boll weevil, bollworm, or plant bug damage. Assess weekly mean node height and mean number of squares/acre. (Count number of squares on first two nodes of each fruiting branch on 40 inches of row in four locations; multiply by 1,000 to convert to per-acre counts.)
    • Count numbers of plant bugs and beneficials per 100 sweeps per field using sweepnet. Add clouded plant bugs to plant bug counts but also noted separately. (Drop cloth and/or visual counts can be substituted for sweeps.)
  2. Post first bloom:
    • Pull one-third grown or larger squares (100/field minimum) for worm and weevil damage counts. Check top 6 inches of plant for eggs and larvae. Check whole plants (10-20/field) for eggs, egg masses, and boll damage (note boll damage by "bugs" clouded plant bug, stink bug). Check blooms for weevils, worms, and clouded plant bugs.
    • Assess weekly number of squares and bolls per acre.

Thresholds for Pests

Cutworms

If cotton stand is reduced to fewer than 3-plants per-row foot, control measures are warranted. Damaged areas should be checked without regard to averages over whole fields and spot treatments applied. The size of an area is determined by the smallest unit one will treat.

Repeat treatment in 5 to 7 days if damage continues.


Thrips

Chemical control is warranted if one or more thrips per plant are found on seedling cotton. If systemic insecticides have been used, the presence of immature forms in counts indicates that treatments have broken down and spray also may be required. Repeat treatment in 7 to 10 days if thrips are still present.

Much higher thrips numbers can be tolerated on cotton at the 3- to 4-leaf stage or larger.


Fleahoppers and Plant Bugs

Presquaring cotton -- Initiate treatment if one plant bug flagged plant and one plant bug are found per 10 feet of row.

Squaring cotton -- Assume squaring begins at the 6th node with populations of 40,000-60,000 plants. If cotton is not setting 80 percent or more of its squares and holding them, treat for plant bugs when you find the following numbers:

During the first 2 weeks of squaring:
Drop cloth -- 1 bug/6 row feet
Visual -- 5 bugs/100 terminals
Sweepnet -- 7.5 bugs/100 sweeps
Third week of squaring to bloom:
Drop cloth -- 1 bug/3 row feet
Visual -- 10 bugs/100 terminals
Sweepnet -- 15 bugs/100 sweeps
After first bloom:
Drop cloth -- 2 bugs/3 row feet
Visual -- 15 bugs/100 terminals
Sweepnet -- 30 bugs/100 sweeps
These numbers convert to approximately 2,000 bugs/acre during the first 2 weeks of squaring, 4,000 bugs/acre from the third week until bloom, and to 8,000 bugs/acre after first bloom.

Determining Percent Square Set

Across a field on 25 plants, count the total number of squares in the top five nodes of each cotton plant. (Count no more than two squares per fruiting branch.) Assume that average Mississippi plantings of 40,000 plants/acre begin fruiting on the 6th node.

6th node cotton should have 25 squares/25 plants
7th node cotton should have 50 squares/25 plants
8th node cotton should have 100 squares/25 plants
9th node cotton should have 150 squares/25 plants
10th node cotton should have 200 squares/25 plants

Counts thereafter should maintain 200 squaring sites per 25 plants for the top five nodes. Therefore:

(Actual number of squares X 100) / Potential number (200 above 9th node) = % square set


Boll Weevils

Presquaring cotton -- When using pheromone traps, with trap densities at 1/20 acres, "pinhead" square applications should be made if four weevils total are captured per trap for the four weeks before squaring (an average of one per week). Consider cotton at "pinhead" when it reaches 5th node. Applications should not be made if cotton is extremely late, i.e., not squaring before June 25.

In the absence of traps, "pinhead" applications should be made if 25 or more weevils are found per acre. (This is approximately one weevil/500 feet of row.)

Squaring cotton -- After squares are 1/3 grown or larger, if you find 10 percent or more damaged squares, make applications every 4th day until you establish control. Three to five applications may be required to obtain control. Do not count flared squares.

Diapause control -- If weevils have been a problem during mid-to-late season or if the crop stays green long into the fall, stalk destruction is strongly encouraged as a cultural control for weevils. Adding boll weevil materials to defoliant is also strongly encouraged.


Bollworms

First-generation bollworms -- concentrate on management of the egg stage using ovicides. If egg hatch and worm populations are unacceptably high, use insecticides plus ovicides. In mid-to-late season, treat when you find four or more larvae per 100 terminals. The presence of eggs or squares' damage in combination with worms indicates multiple applications may be needed.

Control worms before they exceed 1/2 inch in length. In mid and late season, check entire plant for worms, worm eggs, and damage.

Cotton bolls are susceptible to damage by bollworms and should not be overlooked.


Aphids

Treat fields for aphids when low populations start to increase. If infestations are heavy in spots, consider spot treatments.

Spider Mites

Check the 5th fully unfolded leaf, starting at the terminal. Treat if low populations start to increase. If 50 percent of the leaves are infested, treatment is required. Fifty to 100 leaves should be counted. Spot treatments may be warranted early in the season to prevent mite spread. Look for "hot spots" along field borders and/or adjacent to buildings.

Clouded Plant Bugs

If you observe five or more clouded plant bugs (adults or nymphs) per 100 plants after first bloom, make whole plant counts to determine damage. Controls may be warranted if bolls or squares are being damaged. These pests occur in many cotton fields north of Mississippi Highway 8 from mid-June on. They are more prevalent in fields bordered by bodies of water where they build up on their primary host--button bush.

In mid-to-late season, make treatments if 5 percent or more of half-grown bolls are damaged by the bugs. Check bolls for damage by selecting half-grown bolls at random across a field and slicing them to check for necrotic spots within the bolls.

Stink bugs are also occasionally damaging to bolls. Damage is similar and "bug" counts should combine the two species.


Armyworms (Beet and/or Fall)

Treat when you find five egg masses and live larvae per 100 plants or when you find four or more worms in 100 blooms and bolls. Time applications to be made on young larvae.

Whiteflies

Treat fields for whiteflies when 50 percent or more of the plants are infested with adults. Multiple applications may be necessary.

Biology

Cutworms

Several species of cutworms may develop in weeds or cover crops and frequently attack cotton planted on such land. Five primary species attack cotton -- the black cutworm, pale-sided cutworm, variegated cutworm, granulate cutworm, and the army cutworm.

Cutworms damage cotton in the seedling stage by cutting plants off at the ground or they move up the plant to eat the leaves. This damage reduces stand, sometimes making replanting necessary. The worms also feed on roots and stems. Cool, wet weather is ideal for cutworm damage to occur. Cotton in low, wet areas with cool soils is also more likely to be attacked. Cover crops can increase the incidence of this pest in cotton fields.

Most cutworms overwinter as full-grown larvae beneath the soil in noncultivated or cover-crop areas. Some, however, pass the winter as pupae or as adult moths in the soil near their host plants.

Moths emerge in the early spring, mate, and deposit 300 to 500 eggs near their food plants. The eggs hatch in three to five days. Larval development varies, but the average development time is about 30 days. Worms hide during the day underneath clods or trash. When disturbed, worms generally curl into a ball and "play 'possum."

Larvae have four pairs of prolegs and may look similar to bollworms, except for their greasy smooth appearance.

Cutworm damage to cotton occurs erratically in Mississippi. Some low, wet areas sustain damage each year, but seldom is it enough to cause widespread replanting of cotton fields.

Scout for these pests by walking over suspect areas and looking for cut plants. One worm can cut a number of plants along a single row during a night. Some worms drag cut plants to daytime lairs and consume them during daylight hours.

Infurrow granules or hopperbox insecticides usually do not control cutworms. Generally, preventative treatments for cutworms are not economical because populations are erratic and material cost is high. When worms are destroying stands, two spot treatments 5-7 days apart give control.


Thrips

Four groups of thrips are pests of cotton in Mississippi. The pests are the tobacco thrips, the flower thrips, the onion thrips, and the variable thrips.

There are also three common predator species of thrips; these are banded thrips, spotted thrips, and the black hunter.

Thrips are minute, winged pests that rasp tender plant tissues and suck the juices from newly emerged cotton. Adults and larvae feed on cotyledonary plants. In heavy infestations, the cotyledon leaves may appear silvery on the underside. Heavy infestations may cause enough damage to kill plants and reduce stands.

Heavy damage results in a condition called " 'possum-eared" cotton. When thrips kill terminal buds, plants become distorted and branch excessively.

Thrips damage is magnified by plant stress, i.e., cold, wet weather, herbicide damage, and diseases, resulting in crop delay and reduced yields.

Thrips have complete life cycles including eggs, larvae, pupae, and adults. Adult females lay eggs in tender tissues of plants. Eggs are creamy white and very small and usually hatch in about four days. Larvae feed on plants for about six days then fall to the soil, where they pupate. New adults emerge in about four days. A generation is completed in about two weeks.

Thrips usually breed all season on many different host plants. The winter is passed in plant residues. Adults are winged and fly readily from plant to plant. The feeding cycle in cotton usually lasts from four to six weeks. Thrips may build up on grasses (i.e., wheat and legumes) before moving into cotton. As grasses and early season legumes mature, thrips migrate to cotton and other seedling crops.

In recent years there have been reports of mid- and late-season damage from western flower thrips in some states. These are present in Mississippi, but we have not been able to attribute losses to these pests.

Walk fields thoroughly as soon as cotton comes to a stand. Give special attention to border areas. Thrips can be dislodged from plants by beating plants over a piece of white paper or cloth. Cigar boxes also can be used for this purpose.

If cotton has received infurrow or other systemic treatments, the stage of growth of the pests is important. Immature thrips indicate that systemic treatments are not working and foliar applications are warranted. The threshold of one thrip per plant is a good general rule to follow when making a decision for treatment of seedling cotton. You should not wait to see " 'possum-eared" cotton before control measure are instituted. In cases where you find high numbers of adult thrips but there are no immatures, control is being obtained by systemics, but heavy migration is overwhelming the treatments. Foliar applications may or may not help.


Plant Bugs

Plant bugs or plant-bug relatives that do the greatest damage to Mississippi cotton are tarnished and clouded plant bugs and cotton fleahoppers. Rapid plant bugs and stink bugs also are seen in cotton.

Tarnished Plant Bug

Tarnished plant bugs are probably the best known and most damaging of the Hemipterous pests in cotton. Cotton fleahoppers are generally grouped with TPB because immature forms and damage are similar. All four species of plant bugs may attack cotton from the seedling stage until maturity.

Tarnished plant bugs and cotton fleahoppers are generally recognized as early season pests in Mississippi. Clouded plant bugs usually are most damaging in mid to late season.

Tarnished plant bugs overwinter as adults and may be active on green weeds throughout the winter. In early spring they feed on legumes and weeds. Preferred weed hosts include fleabane, vetch, winter mustard, lambsquarter, mare's tail, slim aster, and curly dock. Cultivated host crops are alfalfa, cotton, beans, carrots, sugarbeets, and fruits.

These pests are called lygus bugs from the scientific name Lygus lineolaris. Adult females lay eggs in tender stem tissues and midribs of leaves.

Eggs are deposited in plant tissues, and only the end of the egg appears on the surface. Eggs hatch in about eight days, and nymphal stages take about 11 days. This insect undergoes gradual metamorphosis. The immatures or nymphs are wingless. A generation requires 20 to 30 days, depending on climatic conditions. Nymphs are pale green when they first hatch, and an orange spot is visible on the middle of the abdomen. Shortly after feeding begins they become a darker green. Third, fourth, and fifth instar nymphs have four black spots on their backs. Adults are about 1/4-inch long and are brownish-yellow mottled in color.


Cotton Fleahoppers

The cotton fleahoppers hibernate in the egg stage on cotton, goatweed, sage weed, and other weeds and prefer these hosts over cotton. In Texas where this insect is an important pest, it migrates to cotton early and then deserts the crop for preferred hosts.

Eggs are deposited in plant tissue near the growing tips and hatch in about seven days. Nymphs pass through five instars and in 10 to 30 days emerge as adults that are about 1/4 inch long. The second antennal segment is spotted with four to five black spots. The body is pale yellowish-green with minute black specks over the upper surface.

The cotton fleahopper is in the same family as the tarnished plant bug, but it is not a Lygus. The favorite feeding site on cotton is the terminal bud cluster. As with all plant bugs, a toxin is injected into the plant as the insect feeds.

Feeding by plant bugs in presquaring cotton can cause "flagging," which results from repeated feeding on stems and wilting of leaves and plant parts above the injury. Feeding may cause lesions and knotted or swollen areas on main stems and petioles. Excessive feeding also causes excessive branching of the main stem. When these symptoms occur on one or more plants per 10 row feet and plants bugs are present, control is warranted.

During the first two weeks of squaring, give special attention to scouting for plant bugs. If cotton is squaring normally and loading fruit at capacity (i.e., 80 percent or more of the squares setting and remaining on the plant), plant bugs are not a problem. But if square loss is occurring, then scouts should estimate "bug" populations.

Three methods commonly used to estimate bug populations are drop cloth, visual, and sweepnet.

Thresholds for these methods vary as the season progresses and include the following:

First two weeks of squaring

  • Drop cloth -- 1 bug/6 feet of row
  • Visual -- 5 bugs/100 terminals
  • Sweepnet -- 7.5 bugs/100 sweeps
Third week of squaring until first bloom
  • Drop cloth -- 1 bug/3 feet of row
  • Visual -- 10 bugs/100 terminals
  • Sweepnet -- 15 bugs/100 sweeps
After first bloom
  • Drop cloth -- 2 bugs/3 feet of row
  • Visual -- 15 bugs/100 terminals
  • Sweepnet -- 30 bugs/100 sweeps
These thresholds apply to tarnished plant bugs and cotton fleahoppers. Scouting and new determinations of need for control measures should be made after each application. One application should control most plant bug populations. Square counts and bug numbers are the best indicators on which to base control decisions.

Clouded Plant Bugs

Clouded plant bugs overwinter as eggs. Eggs are inserted in plant tissue, and nymphs emerge and feed on their primary host (button bush) until after flowering. As flowers on these plants mature, bugs move to cotton, and feeding may seriously damage squares, blooms, and bolls.

Clouded plant bugs are about 1/2 inch in length. Color is mottled or rusty with a mixture of gray, browns, and yellows. Legs are yellow with darkened ends, and antennae are large in both adults and nymphs.

Scouting methods vary as the season progresses. Since plant bugs can seriously damage presquaring cotton, it is important to begin walking fields early to look for signs. At times bugs may also be migratory, especially if alternate host plants are nearby.

When you find clouded plant bugs in cotton, total fruit counts help to indicate amount of damage. Five "bugs" per 100 blooms indicate that high numbers are present. Check preblooming cotton with a sweepnet. Sampling during early morning and late afternoon often gives higher bug counts than does sampling at midday. The presence of nymphs in fields indicates that reproduction is occurring, and fruiting counts should be made. Control measures are warranted if plants are not fruiting normally.


Symptoms of Plant Bug Damage

  1. Wilted and blackened terminal leaves; "flagging" caused by tarnished or clouded plant bugs. Main stem and petiole lesions, knotted or swollen areas. All species. Excessive branching of the main stem. All species. Increase in plant height. All species. Terminal abortion--excessive vegetative branching. Tarnished, clouded, fleahopper. Small square (pinhead) shed. All species. Discolored anthers in square and blooms. Tarnished and clouded plant bugs.
  2. Green boll spots, misshapened bolls, lint discolored or absent, small bolls shed. Tarnished and clouded plant bugs.
These symptoms also may be caused by other factors such as diseases, mechanical damage, improper fertilization, or normal plant adjustments to stress, i.e., boll shed.

Cotton Aphids

Cotton Aphids are prolific--a generation may occur every five days. This rapid reproduction occurs because all in-field populations are females and give birth to living young without the need of a male. Many adults hibernate, but some live on green plants throughout the year. At certain times of the year, winged adults develop and fly to new hosts. Populations usually build up on alternate weed hosts and move from there to cotton. Crowding or scarcity of food stimulates the development of winged forms.

The aphids damage cotton by sucking juices from plants, and this damage reduces yield and quality and at times may kill plants. Heavy infestations on seedling cotton cause leaves to curl or crinkle -- stunting or killing plants.

When heavy infestations occur during the main fruiting period, older leaves may turn yellow and shed. Severe infestations may cause complete defoliation. Late-season infestations may cause premature boll opening and immature fiber development. Honeydew secretions from the aphids also drop on the fiber, making it sticky. A fungus often develops on the honeydew, staining the fiber and reducing quality.

Aphids usually are controlled by natural enemies, but at times insecticide applications for other pests kill natural enemies and permit high populations to develop.

Populations often build up in clumps or spots and may be controlled by spot treatments. Apply insecticides when aphids numbers are increasing. Control is essential when numbers reach 11 to 25 per terminal leaf.


Spider Mites

Various mite species attack cotton and often cause serious damage. The most important mite pest in Mississippi cotton is the two-spotted spider mite. Weather, beneficial insects, and predacious mites usually keep mite pests under control. Outbreaks of spider mites are most likely to occur after applications of insecticide destroy beneficials.

Spider mites go through five stages in their life cycles. These stages are egg, larvae, two nymphal stages, and adults. In most of the Cotton Belt, spider mites may remain active on various green plants and reproduce throughout the years. Development during cool periods is much slower than it is during the growing season, and there may be as many as 16 generations in one year. (A generation can occur every 7 to 12 days.)

Spider mites usually attack cotton during the latter part of the growing season. Infestations begin building in June but may not be high until later. Hot, dry weather favors population increases.

Most mites feed underneath the leaves. Both adults and immatures feed on plant juices, causing premature leaf drop and unthriftiness of plants.

Mites are found in the same area year after year. When checking cotton for mites, check turn rows, fence rows, and other areas where mites have occurred in the past. When chronic problems occur, spot treatments of these areas often are warranted. Two applications may be required at five- to seven-day intervals for satisfactory control.


Cabbage Loopers

Cabbage loopers are only occasional pests of cotton. There may be three to four generations each year, with population numbers increasing with each generation. If this buildup occurs too early, loopers can defoliate the crop, preventing boll maturity.

Adults are grayish-brown moths that can be easily identified by a small silvery spot near the middle of each wing. This spot resembles the figure "8." Looper larvae have three pairs of slender legs near the head and two pairs of thicker prolegs behind the middle. Loopers move in a characteristically looping motion. The body of the worm tapers to the head.

Loopers overwinter as greenish or brownish pupa in delicate cocoons of white tangled threads. Cocoons are usually attached to leaves of plants on which loopers feed. Adult moths emerge in the spring and lay 250 to 350 greenish-white eggs singly on upper and lower surfaces of leaves. Eggs hatch in two to three days, and worms develop in two to four weeks.

Looper caterpillars often are destroyed by a disease that causes their bodies to rot. Dead loopers often can be found hanging by their hind legs. When this disease is present, controls usually are not needed because the disease spreads rapidly.

Loopers are difficult to control with insecticides. Best control can be obtained when worms are small, but at this stage it is not known whether controls are needed.


Armyworms

Beet and fall armyworms are occasionally serious pests of cotton. Beet armyworms most often attack cotton in dry years after primary host plants dry up. Worms feed on bracts of small bolls, squares, and blooms.

Larvae are pale to olive-green. Mature larvae are about 1 inch in length. A dark stripe is visible down the middle of the back and pale stripes run along each side. There is a characteristic small black dot on each side, above the second pair of true legs.

The life cycle of this pest consists of egg, larva, pupa, and adult. In cooler climates these insects overwinter as pupae in the soil and in warmer areas as the adult moth.

Females lay 500 to 600 eggs in groups of 25-30 over a period of 4 to 10 days. Eggs hatch in three to five days and larvae feed for about three weeks. The entire life cycle is about 36 days. There may be as many as four generations per year.

Scout for these pests by checking blooms and looking at boll bracts for larvae. Look for eggs by examining the undersides of leaves. Identification and differentiation from other species are important because control measures vary greatly.

Fall armyworms usually attack cotton late in the season. Larvae feed in white blooms and may penetrate bolls from beneath the bracts. Entire bolls may be destroyed without visible signs of injury.

Fall armyworms are mid- to late-season pests in Mississippi. This insect does not overwinter in the state and must migrate into Mississippi from southerly climates.

Females can lay up to 1,000 eggs in masses of about 150 eggs per mass. The masses are placed on the undersides of larger leaves and are covered by body scales from female moths. Eggs hatch and young larvae disperse over host plants. Larvae may be distinguished from other species by the prominent wide inverted "Y" on the front of the head capsule, paired dots on each body segment, the lack of microspines, and a dark pigmented band behind the head.

The life cycle is about 30 days. Eggs hatch in two to five days and worms feed about 16 to 20 days before pupating. Pupation lasts 7 to 14 days and may occur in the soil or on host plants.

Check underneath surfaces of large leaves for egg masses. Inspect bolls and white blooms for the presence of fall armyworm larvae. Controls are warranted if five egg masses and small larvae are found on 100 plants or if four or more larvae are found in 100 blooms or bolls.


Whiteflies

Whiteflies occasionally are serious pests of cotton. Adults and larvae suck sap from the cotton plants, reducing vigor. As these pests feed, honeydew is secreted and accumulates on lower leaves and lint. Fungi grow on the honeydew, cutting down on photosynthesis in the leaves and lint quality of bolls.

Eggs are laid on the underside of leaves. Active pale-yellow six-legged crawlers hatch in 4 to 12 days. They move around for a short time on the lower side of leaves until they locate a feeding site. After about 3 days, crawlers begin the sessile or fixed stage.

Larva pupate on leaves. Adults mate within a few hours of emergence from the pupa.

The insect is active throughout the year on many hosts. The average life cycle occurs in 19 days in summer and 45 days in the winter.

Beneficial species help to control these pests, but controls are warranted when infestations of more than 50 percent of the terminals occur.


Boll Weevils

The boll weevil migrated into the United States from Mexico in the late 1800's and was attacking Mississippi cotton by 1911. Since that time, Mississippi farmers have had to deal with this pest. Cotton is still the only host plant for boll weevil in the Southeast, and even in other parts of the country, its host range is extremely limited.

Because of its tropical origin, the boll weevil does not do well in cold weather. As a result, only about 10 percent survive a normal winter, and a much lower number survive in extremely cold winters. For example, only about 1 percent survived in 1983 but some areas in Mississippi had problems with weevils in 1984. While numbers can be greatly reduced during cold winters, they build up quickly in spring and summer.


Annual Weevil Cycles

Many female weevils enter diapause already mated in the fall and emerge from overwintering capable of laying fertile eggs.

Boll weevils overwinter as adults in the litter of hardwoods near cotton fields. In mild winters weevils also may overwinter in bolls and other residue in cotton fields. In the spring, weevils emerge and feed on terminals of seedling plants and later feed on and reproduce in 1/3-grown or larger squares. Later in the season they attack and reproduce in bolls.

Eggs are deposited singly in squares but several may be deposited in bolls. The larvae feed for 7 to 14 days in either the squares (which fall to the ground in three to five days after egg hatch) or the bolls and pupate; then after three to five days, the adults emerge. Depending on temperature, this cycle may vary from 14 to 28 days. Average for most Mississippi cotton is about 21 days. Females of the new generation begin laying eggs after three to four days. As many as five generations may develop each season.

Overwintering boll weevils usually begin moving out of hibernation in March, with individuals emerging as late as July or August in some years. Peak emergence generally occurs in late May or early June. Thus, weevil pheromone traps placed at planting generally will give a good indication of weevil numbers in an area. Peak numbers may emerge following rains, especially after dry periods.


When To Control Boll Weevils

Weevils are more easily controlled at the fringes of the season, i.e., spring and fall. Spring applications for overwintered weevils are commonly called "pinhead sprays" because initiation of squaring triggers the applications. (Many people have problems in identifying pinhead squares. Therefore, when 5th node is reached, application can be initiated. The earlier in the season pinhead squaring occurs the more effective the applications will generally be. Pinhead application should never come after June 20 in any area of the state. No more than two pinheads should be made. Pinhead-square sprays usually prevent buildup of the first-generation weevils into economic levels until August.

The success of in-season control efforts depends on timing. Since freshly emerged callow weevils require three to four days to mature before they begin laying eggs, 3- to 4-day spray intervals must be maintained for adequate control of weevils. To break the reproductive cycle, these intervals and multiple applications must be maintained until no more weevils emerge from fallen fruit in the field.

At pinhead, materials directed over the tops of smaller plants are more effective; thus, lower rates are usually recommended. Do not make more than two pinhead applications.

In-season -- Apply materials at no greater than 4-day intervals to break the cycle.

Late season -- After the last in-season application, treat on a 10- to 14-day schedule until stalks are destroyed.

Pyrethroids are effective against this pest; therefore, applications made for bollworms and budworms also control weevils.


Bollworms

Of all the insects that attack cotton, the cotton bollworm and tobacco budworm are probably the most damaging. These two groups of insects are often referred to as the "Heliothis Complex" because of their scientific names, Heliothis zea (Bollworm) and Heliothis virescens (Budworm). In the larval stage, behavior and damage are similar for both species. Both species attack many plants.

Bollworms feed on grass crops (i.e., corn, grain sorghum, and legumes) as well as cotton, tobacco, and other solanaceous plants.

Budworms feed almost exclusively on broadleaf plants, including tobacco, cotton, ground cherry, geranium, and other solanaceous plants.

Bollworms may develop as many as three to four generations per year in cotton.

Bollworms overwinter as pupae 2 to 6 inches below the soil surface and emerge in early spring, usually March. Newly emerged moths mate and fly to green plants, mostly legumes, where they deposit eggs. Bollworms may complete as many as two generations on other hosts before attacking cotton. Bollworms may complete two generations on corn or grain sorghum, the first in the whorls of the plants and the second in the ears of corn or the seed head of grain sorghum. As the culture of grain sorghum coincides with cotton, it may in fact attract many of these pests away from cotton.

Each female moth lays from 250 to as many as 1,500 eggs over a span of three to 12 days. Pearly white eggs may be laid on any part of the cotton plant but tend to be placed in the upper one-third on upper surfaces of smaller leaves or in terminals. At higher temperatures in summer months, moths will place eggs deeper in plant canopies, on square or boll bracts, and stems. Eggs hatch in three to five days, and larvae feed from 12 to 15 days before pupating in the soil.


Bollworm Hosts during the Year

  • Wild host (one to two generations) -- March through mid-May, September through October
  • Corn (one to two generations) -- mid-April through mid-June
  • Grain sorghum (one to two generations) -- mid-May through July
  • Cotton (three to four + generations) -- late June through October

Tobacco Budworm

The tobacco budworm life cycle coincides closely with that of the cotton bollworm. Budworms can be found on different host species but usually do not build into tremendous numbers in Mississippi until they move into cotton.

Budworm generations overlap those of bollworms, and both may be found in the field at the same time.

Budworm moths are about 1 1/2 inches across the spread wings. Front wings are light-green and crossed by four oblique light bands, the inner three of which are edged with black. They vary in color from shades of olive green to grayish brown. The front wings of a light grayish brown are marked with darker brownish irregular lines and with a dark area near the tip of each wing.

In early and midseason, scout for bollworms/budworms by looking for eggs and small larvae at the upper one-third of the plant. Larvae usually double in size each day for four days after hatch. Freshly hatched larvae of 1/16-inch length will be more than 1/2 inch along by the fifth day of life. Full-grown larvae are greater than 1 inch in length. As larvae grow, they migrate downward to squares, blooms, and bolls. In late season when moths are more subject to lay eggs all over the plants, whole-plant examinations for eggs and larvae are very important. One larva can destroy a plant terminal and six to eight squares or bolls before reaching maturity.

Budworms traditionally are harder to control than are bollworms. Timing of insecticidal sprays to kill young larvae is important with both species. Once larvae move to squares and bolls they are protected from insecticides. Adding ovicides often increases the effectiveness of control.


Pyrethroid Resistance Management in Cotton

Tobacco Budworms

Many research and Extension people have recognized that development of resistance by insects to widely used pesticides is a real problem. One of the recent occurrences of this problem is the development of resistance to synthetic pyrethroids by the tobacco budworm.

Rationale

Bollworms and tobacco budworms are the most economically important pests of cotton beltwide. Pyrethroids are relatively inexpensive and have been extremely effective against both species. The pyrethroids also have some efficacy against boll weevils. Since the advent of the pyrethroids, cotton yields generally have increased, while the length of the cotton production season has decreased. The widespread use of pyrethroids for Heliothis control and introduction of shorter-season varieties have been responsible for most of these changes. Few alternative insecticides are available for Heliothis control.

The Situation

Historically, management of cotton insect pests such as tobacco budworm and bollworm has been hampered by the rapid development of resistance to insecticides used to control them. Tobacco budworms and bollworms have developed resistance to insecticides in all three major classes (organochlorines, organophosphates, and carbamates) used to control them before the advent of the pyrethroids in the late 1970's. Usually, tobacco budworm develops resistance to insecticides more readily and more rapidly than bollworm.

Pyrethroids are now the principal insecticides used to control bollworms and tobacco budworms in cotton. Generally these insecticides remain effective on both species. However, tobacco budworms from some localities in Arizona, Texas, Louisiana, Arkansas, and Mississippi have become increasingly more difficult to control. Resistance-monitoring studies using four evaluation procedures (adults in insecticide-coated vials, larvae in insecticide-coated vials, and larvae on insecticide-treated plants) have demonstrated that the occurrence of resistant genotypes of tobacco budworm are higher in localities where control problems have occurred than in areas with no control problems.

The primary mechanism of resistance to pyrethroids in tobacco budworm appears to be nerve (target site) insensitivity (or knockdown resistance). However, the manner of inheritance of knockdown resistance is a potential weak link that may be exploited. Knockdown resistance to pyrethroids is probably a recessive trait; thus when a resistant tobacco budworm moth mates with a susceptible moth, the resulting larvae will be susceptible. Hence, it is imperative to attempt to maintain pyrethroid-susceptible tobacco budworms to dilute out resistance as it occurs.


Management Plan

  1. Avoid late planting. Planting during a 20-day period from April 20-May 10 should be optimal. The use of an in-furrow fungicide is recommended to help avoid stand loss. You should consider using an in-furrow insecticide in fields with high yield potential and where you expect thrips problems. The goal is to get a uniform stand of healthy cotton seedlings. Insect control (plant emergence to first bloom). Control of economically damaging levels of insects such as thrips, aphids, plant bugs, boll weevils, and Heliothis must be obtained in order for the cotton plants to mature normally. Thresholds should be flexible to account for yield potential, cost of insecticide, cost of application, complexes of insects, and the price of cotton. Avoid the use of pyrethroids during this period. The goal is to avoid selection of the first generation of tobacco budworm in cotton with pyrethroids but still keep the cotton plants in a fruiting mode. Insect control (first bloom to end of season). Pyrethroids can be used for bollworm and tobacco budworm control as necessary, but do not increase pyrethroid dosage rates. When tobacco budworms are present, application intervals need to be shortened and a full recommended dosage of pyrethroid plus chlordimeform or other insecticide should be applied. The goal is to have most of the bolls set by the end of July. When this occurs, the cotton plant and fruit will be so mature that they will not be very susceptible to attack and damage by the August generation of tobacco budworms. Avoid late irrigation that would delay maturity into September.
  2. Scout the crop. Close monitoring of the cotton crop for insects, fruit set, and fruit development is mandatory throughout the growing season. Timing insecticide applications against eggs and early instar larvae is critical.

Applying Pesticides

Once you decide to control a cotton pest, you must consider application method and equipment. Important factors in controlling any pest include timeliness, choice of insecticide, rate, and placement.

Insect control guides are written with the idea of giving farmers and other users a guide to the best material for controlling particular pests. Each chemical also has a label that gives a substantial list of pests that can be controlled. It is important that these be followed.

Chemicals can be selected for a number of reasons. Some are longer residual than others, some are less damaging to beneficials, while others are nonselective and kill almost everything. The best choice often depends on economics and on the task you are trying to accomplish.

Rate is probably affected most by calibration. There is absolutely no way to tell what a chemical will do when an uncalibrated sprayer is used to apply pesticides, because there is no way to determine what the rate is.

Calibration is probably the most economical practice on the farm, yet it is probably the most neglected. An application of material below or above rate that gives unacceptable results causes hardship on farmer, consultant, and industry alike. Complaints of "bad material" or "no-good chemical" often can be traced to poor calibration.

Apply insecticides with a hollow cone-type nozzle. Insects are most often down in the canopy of the crop, and penetration into that canopy is essential for adequate control. In recent years the use of soil or water and oil mixtures, has placed emphasis on penetration of crop canopies. Oils or oil and water mixtures have been shown to aid in placement of chemicals and should be considered for use, especially late in the season when cotton is tall.

Most cotton in Mississippi is sprayed by conventional agricultural airplanes or by tractor or high clearance-type sprayers. Few farmers have any control over the applications by air. They may be able to determine the chemical, rate, and volume to be used but will have little control over the actual application. Many farmers, especially those with average field size of fewer than 20 acres, have found that aerial applications are somewhat less than desirable in heavy insect pressure. When aircraft can get down to the crop and place pesticides into the canopy, they have proved to be more efficient, economical, and desirable than ground equipment.

Ground application using either a tractor or high clearance sprayer can be quite effective in pest control. Coverage is usually better and farmers have control over all facts of the operation. Most applications by ground utilize volumes of from 4 to 20 gallons, depending on material used and target pest. Lower volumes are sometimes used with directed type sprays. Most applications should be made at no less than 30 psi and no more than 70 psi. Ground speed is variable. Many farmers are adding insecticide rigs to their cultivator bars and have found these to be effective in banded-type applications in the early season. The drawback to this practice is that the crop is usually too big for those applications after the second or third cultivation.


References

Applying Pesticides Correctly: A Guide for Private Applicators, USDA/EPA

Biology and Control of Thrips, MSU-ES Information Sheet 1226

Biology and Illustrated Key of 20 Species of Economically Important Noctuid Pests, A. D. Oliver and J. B. Chapin

Cotton Insect Control, D. F. Young

Control Plant Bugs, MSU-ES Information Sheet 1127

Cotton Scouting Manual, MSU-ES Publication 988

Destructive and Useful Insects, Metcalf, Flint, and Metcalf

How a Cotton Plant Grows, Kamal El-zik

Insect Pest Management in Arkansas Cotton, R. T. Allen and E. P. Rouse

Planting Pattern, Plant Population, Irrigation, and Insect Interactions in Cotton, Leigh, Grimes, Dickens and Jackson

Cotton Physiology, The Cotton Foundation Reference Book Series No. 1, edited by J. Mauney and J. Stewart.

The Cotton Plant, Tharpe

Arkansas--Don Johnson (X) and Marvin Wall (X), University of Arkansas.
Louisiana--Jack Baldwin (X), Gene Burris (R), Jerry Graves (R), and James Tynes (X), Louisiana State University.
Mississippi--Doug Gaydon (X), Bob Head (X), Tom Helms (R), Bill Kitten (R), Randy Luttrell (R), Jack Reed (R), Roy Reid (X), Rick Roush (R), and Mike Williams (X), Mississippi State University.
USDA--Gordon Snodgrass, Southern Field Crop Insect Management Laboratory USDA-ARS.

(X) -- Extension Entomologist
(R) -- Research Entomologist


By Dr. Robert B. Head, retired Extension Entomologist and Dr. Michael R. Williams, Extension Entomologist

Mississippi State University does not discriminate on the basis of race, color, religion, national origin, sex, age, disability, or veteran status.

Publication 1614
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director


Copyright by Mississippi State University. All rights reserved.

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