Insect Scouting and Management in Bt-transgenic Cotton

With the use of Bt-cotton, the number of sprays that target tobacco budworm/bollworm will be reduced, and this reduction will have an impact on populations of other cotton insects. In some cases, this impact will be positive, but in other cases the impact will be negative, allowing certain other pests to increase in importance.

On the positive side, reduced foliar sprays will allow populations of beneficial insects to increase and be retained in fields. These beneficial insects will help suppress populations of aphids, whiteflies, mites, armyworms, and loopers. In addition, reduced sprays targeting tobacco budworm/bollworm will mean less selection for resistance in populations of non-target pests.

On the negative side, the reduction or elimination of sprays targeting tobacco budworm/bollworm also reduces the control that these sprays may be providing against pests such as boll weevil, tarnished plant bugs, stink bugs, and fall armyworms.

Proper scouting of Bt-cotton requires additional time and labor. Changes in scouting schedules, acres allotted per scout, and scouting fees may be required to gain the maximum benefit of using this method of control.

Agronomic Considerations: The varieties in which the Bt-gene is initially available are later maturing and have smaller seed than average. (A limited seed quantity of several earlier maturing Bt varieties will be available in 1997). Follow recommended agronomic practices for managing these varieties. Pay particular a ttention to timely planting, and avoid excessive plant density, nitrogen levels, and late irrigation.

The following guidelines address only those areas in which scouting and management of Bt-cotton may differ from conventional cotton. These guidelines are based on a limited amount of research with transgenic Bt-cotton and are subject to modification as ad ditional research findings and transgenic Bt varieties become available. Refer to Extension Publication 343 Cotton Insect Control Guide and other Extension publications and information sheets for additional information on identification, biology, scouting techniques, and control of individual pests .

Guidelines for Scouting and Management of Specific Pests

Thrips, Aphids, Mites, Whiteflies, Cutworms

Tobacco Budworm/Bollworm

• Like conventional cotton, Bt-cotton should be scouted for tobacco budworm/bollworms at 3- to 4-day intervals. Supplemental treatment with foliar insecticides may be required in some situations.
• Beneficial insects are important in management of Bt-cotton. The need for supplemental foliar treatments will be more common where beneficial insect populations are low, but may be reduced where beneficial insect populations are high.
• Although Bt-cotton is active against both tobacco budworm and bollworm, bollworm is less susceptible to the Bt toxin than tobacco budworm. In addition, bollworm larvae may be found more often lower in the plant canopy where Bt-toxin expression is lowe r (relative concentration of toxin is particularly low in pollen). Consequently, the need for supplemental treatment is more likely when high populations of bollworms are present. However, Bt-cotton does have "good" activity against bollworms and will usu ally provide adequate control of low to moderate populations.
• Because tobacco budworm and bollworm differ in their susceptibility to Bt toxin, it is important to know which species of moths are present. Use moth flushing counts, pheromone traps, or other methods to monitor species composition of moths present (b ollworm or tobacco budworm).
• Scout Bt-cotton similarly to conventional cotton, by examining randomly selected plants for the presence of eggs and larvae. In addition, carefully check blooms and bolls for presence of eggs and larvae. Record counts as percent of plants infested wit h eggs or larvae, and note size range and location of any larvae that are found.
• After first bloom, a "modified whole plant" sampling procedure may be a more time efficient method of scouting for bollworms in Bt-cotton. Randomly select plants with a first position bloom present and examine the bloom and 3 bolls immediately below t he bloom for larvae. Be sure to check under any stuck bloom tags present on bolls, but do not preferentially sample bolls with stuck bloom tags. Also, note the percent of plants on which eggs are found in this region and the percent of bolls that are dama ged. Supplement this method with terminal counts.
• Positively identify any larvae found. Fall armyworm, beet armyworm, and other species may occur in addition to tobacco budworm/bollworm. Thresholds and treatments differ greatly, depending on species present.
• Newly hatched larvae must eat some plant material before mortality can occur. Therefore, do not use numbers of eggs and/or very small larvae as a treatment criteria. High numbers of eggs or small larvae indicate a need to tighten scouting intevals an d intensify sampling in the lower portion of the plant.
• Base need for treatment on number of larvae approximately ¼ inch in size and/or percent of damaged fruit. (Do not count superficial damage caused by newly hatched larvae.)
• Supplemental treatment may be needed when the percent of plants infested with larvae surviving to approximately ¼ inch or greater exceeds current threshold:
        Before first bloom...............8% infested plants.
        First bloom to cutout............4% infested plants.
        After cutout.....................8% infested plants.

        Additional factors, such as numbers of beneficial insects or need to treat for other pests, must also be considered in management decisions.

• Follow the resistance management plan.
• Do not apply foliar Bt products to designated refuge areas.

Boll Weevils

Prebloom

• Use scouting and management techniques similar to those for conventional cotton. Apply pinhead square treatments to reduce populations of overwintered boll weevils if trap captures indicate need for treatment.

Postbloom

• Intensify scouting efforts and begin a series of 3 to 5 treatments if percent punctured squares exceeds 10 percent. Expect to apply more postbloom treatments specifically against boll weevils.

Diapause

• Because of the relatively lower use of foliar insecticides, boll weevil populations may build to higher levels in Bt-cotton than in conventional cotton. Consequently, maintaining effective late-season/diapause control of boll weevils will be especially im portant in managing Bt-cotton.

Tarnished Plant Bugs

• Bt-cotton has no activity against tarnished plant bugs. However, Bt-cotton is no more susceptible to tarnished plant bugs than nontransgenic parent lines. Reduction of tobacco budworm/bollworm sprays will reduce coincidental control of tarnished plant bu gs.

Prebloom

• Scout and treat similarly to conventional cotton. Use published thresholds for bug numbers and fruit retention. Avoid automatic or prophylactic type treatments.
• High levels of resistance have been documented in some tarnished plant bug populations to pyrethroids and other classes of chemistry. Avoid prebloom use of synthetic pyrethroids against tarnished plant bugs.

Postbloom

• Intensify scouting, and use published thresholds. Expect to apply slightly more mid- to late-season sprays specifically targeting tarnished plant bugs. Some pyrethroids are effective against non-resistant tarnished plant bugs.

Stink Bugs

Prebloom

• Stink bugs are rarely a threat at this time.

Postbloom

• Intensify mid- to late-season scouting. Treat as necessary. Stink bug infestations are often clumped and are often higher near field edges. Spot treatments may be effective in some situations.

Clouded Plant Bug

Postbloom

• Intensify scouting and treat as necessary.

Beet Armyworms

• Bt-transgenic cotton has some limited activity against beet armyworms but can be severely damaged by high populations.
• Reduction of tobacco budworm/bollworm sprays will allow increased populations of beneficial insects to aid in suppression.
• The advantage of increased populations of beneficial insects will be lost where frequent foliar applications are necessary to control boll weevils, tarnished plant bugs, aphids, etc.
• Scout as for conventional cotton.
• Higher thresholds may be used where beneficial insect populations are maintained.

Fall Armyworms

• Bt-cotton has little direct activity against fall armyworm. This pest may become more important in Bt-cotton.
• Reduction of mid- to late-season tobacco budworm/bollworm sprays will reduce coincidental control of fall armyworms.
• Intensify mid- to late-season scouting. The modified whole plant search procedure suggested for bollworms will also be useful in detecting fall armyworms.

Table 1. Relative activity of Bt-transgenic cotton (Bollgard) against specific cotton insects

Resistance Management-Bt-Cotton

The gene for resistance to Bt-cotton has already been identified in tobacco budworm, and laboratory cultures exist that can survive on Bt-cotton. It is anticipated that if Bt-cotton were used on 100 percent of Mississippi cotton acreage without any resistance management effort, resistance could develop in tobacco budworm in as little as 2 to 3 years.

One of the essential components of managing resistance in Bt-cotton is the use of refuges, which are simply plantings of cotton that do not carry the Bt gene. The objective is to provide a source of susceptible tobacco budworm moths to mate with any moths that might survive on Bt-cotton. For this concept to work, the number of moths emerging from the refuge area must be much greater than the number emerging from the Bt-cotton. Also, the refuge areas must be located near the plantings of Bt cotton so that moths from the refuge will have a high probability of mating with any resistant moths that develop in the Bt-cotton.

Refuge Options

Option 1. 20 percent refuge, treated - For each 100 acres of Bt-cotton planted, growers would plant 25 acres of non-Bt-cotton.

• This 20 percent refuge area can and should be treated with conventional foliar insecticides to control tobacco budworm and bollworm as well as other pests when numbers exceed economic thresholds.
• Note that foliar Bt products may not be used on designated refuge areas. However, this does not preclude their use on the remaining acreage of non-Bt cotton a grower may have.

• This 4 percent refuge area should not be treated with insecticides that control tobacco budworm and bollworm. (See list of products that should not be used on the 4 percent refuge areas.)
• Take care to plant the refuge in an area where it is easily identified and not likely to be inadvertently treated with products that control budworm or bollworm.
• Note that the 4 percent refuge acreage can/should be treated to control all noncaterpillar pests, such as boll weevils, tarnished plant bugs, etc.
• It is important that the refuge crop be managed for maximum crop production, similarly to the Bt-cotton. A neglected, poor fruiting crop will not function as an effective refuge.
• According to the 1997 license agreement, certain insecticides cannot be applied to the 4 percent refuge crop. This list of insecticides is provided for information purposes. (See Footnote 1).

Do not apply the following insecticides to the 4 percent Non-Bt Refuge:

• acephate (Orthene)
• amitraz (Ovasyn)
• endosulfan (Phaser, Thiodan)
• methomyl (Lannate)
• thiodicarb (Larvin)
• profenofos (Curacron)
• sulprofos (Bolstar)
• synthetic pyrethroids (Capture, Baythroid, Karate, Ammo, Cymbush, Asana, Ambush, Pounce, Scout, Fury, Danitol, Decis)
• Foliar Bts (Biocot, Condor, Design, Dipel, Javelin, MVP, and other foliar Bts)
• helicoverpa zea nuclear polyhedrosis virus
• pyrolle
• spinosad
• pepper spray
• garlic spray

Size and Spacing of Refuges

In addition, if refuge areas are too small, they might not effectively maintain a population of susceptible moths. This is a particular concern with the 4 percent refuge option where growers choose to plant strips of non-Bt cotton within fields of Bt-cotton. In such cases, it is recommended that such strips be several planter passes in width. Obviously, the number and size of such strips will have to be large enough to meet the 4 percent refuge requirement.

Also, leave a skip, or blank row, on either side of a refuge planting to prevent movement of larvae from non-Bt plants to Bt plants and to aid scouts in identifying the refuge areas. Growers using the strip method of planting the refuge should be aware of the list of insecticides that cannot be used on the refuge, but may be used on the Bt-cotton, because this could create problems when applying treatments.

Footnote 1: The above recommendations for resistance management in Bt-cotton are intended as general recommendations only. They are not intended to represent or replace the full details or requirements of any license agreement into which producers may enter with providers of transgenic genes, seed, or technology. Producers should maintain full knowledge of the details of any license agreements into which they enter regarding use of Bt-cotton.

By Blake Layton, Ph.D., Extension Cotton Entomology Specialist

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Publication 2108
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

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