Cotton Insect Situation

No. 15
August 3, 2001

Blake Layton
Extension Cotton Entomology Specialist

Area Specialists:
Dr. Gordon Andrews
Dr. Scott Stewart
 Dr. Mike Williams

Cotton Insect Hotline: 1-800-445-4931

Crop Situation: The Weather and Crop Report for the week ending July 29 indicated that 96% of the crop was setting bolls, which is the same as the 5-year average. Seventy seven percent of the crop was rated as good to excellent. Statewide average temperatures were one degree above normal and statewide rainfall averaged 0.58 inches above normal. However, this rainfall figure is a bit skewed, because there were some areas that got several inches above normal.

Insect Situation: Much of the earlier maturing cotton will likely escape heavy pressure from the August budworm/bollworm flight. However, moths will concentrate their egg laying in the later maturing fields and these may experience heavy pressure. Beet armyworm and fall armyworm numbers have been relatively light so far, but larvae of both species are present and it is important to stay alert for these. August is also the time when stink bugs populations are likely to be heaviest. Loopers are also beginning to appear, and will become more prevalent as the season progresses. When scouting Bt cotton, it is important to keep in mind that different varieties of Bt have different levels of toxin expression, and control of caterpillar pests, other than tobacco budworm, can vary among varieties. This year we have already observed that some varieties of Bt cotton sustained heavier infestations of bollworms, and this has been observed with pests like loopers and beet armyworms in previous years. Pests like bollworms, loopers, and armyworms are inherently less sensitive to the toxin in Bt cotton and are often able to survive. Don’t assume that a field is safe from these late season caterpillar pests – just because it’s planted to a Bt variety.

Treatment Options for Tobacco Budworms: Tobacco budworm populations have been relatively light so far this year, but some fields of non-Bt have experienced relatively heavy infestations. Non-Bt cotton, especially later maturing fields, is likely to see heavier budworm pressure this month. Pyrethroids are no longer recommended for control of tobacco budworms. Treatments recommended for budworm control are listed below.

            Suggested Treatments for Control of Tobacco Budworms in non-Bt Cotton

Treatment

lbs ai/acre

Number of Acres Treated per Gallon

Tracer 4SC

0.067 - 0.089

60 - 45

Denim 0.16EC *

0.01

16

Steward 1.25SC *

0.11

11.4

Larvin 3.2SC

0.8

4

Curacron 8E

1.0

8

Curacron 8E + Larvin 3.2SC

0.67 + 0.32

12 & 10

* Denim is available under Section 18 Emergency Exemption

 

Bollworms and Pyrethroids: For several years we have participated in a regional project sponsored by Cotton Incorporated to monitor for pyrethroid resistance in bollworm moths, using pyrethroid treated vials. Significant levels of pyrethroid resistance have been documented in bollworm populations in more eastern states, such as South Carolina and Georgia. And, in some states pyrethroid resistance levels have been high enough to result in field control failures. However, in past years, moths from our Mississippi location near Greenwood rarely survived in the pyrethroid treated vials. This year however, we have had some bollworm moths to survive in the pyrethroid treated vials. During June, only 71 moths were tested, but six, or 8.5%, survived. In July 7.2% of the 333 bollworm moths tested in vials treated with 10ug of cypermethrin survived. While these levels of survival are not high enough to suggest that we have a problem with resistance in Mississippi bollworms, they do indicate a shift in bollworm tolerance to pyrethroids.

Cotton Square Borer: This is more of a curiosity than a pest, but this year there has been somewhat of an "outbreak" of this insect. Several consultants have reported finding more than one of these. As the name implies, this insect is most commonly found feeding in cotton squares. These are the caterpillars of the gray hairstreak butterfly, which is a small, bluish to gray-colored butterfly with orange markings on the wings. The larvae, which are about ½ inch long when fully grown, have a somewhat flattened body covered with short velvety hairs. The most distinguishing characteristic is that the head is usually drawn under the front of the body, but can be extended to feed deep into a square. Historically, this insect has probably been more familiar to home gardeners than to cotton scouts. It will also feed on lima beans, snap beans, okra, and hibiscus, and is also known as the lima bean pod borer. However, sightings will likely become more routine in the reduced spray environment in which cotton is now grown.

 

Pheromone Trap Counts:

 

Avg. No. Moths/Trap/Week

County

Budworms

Bollworms

Beet Armyworms

Boll Weevils

Attala

0

95

0

0

Bolivar

9

38

46

0

Calhoun

3

19

1

0

Chickasaw

5

6

3

0

Choctaw

0

56

2

0

Clay

27

26

0

0

Coahoma

25

40

13

0

Hinds

0

7

0

0

Holmes

0

25

2

0

Humphreys

4

49

65

0

Issaquena

0

64

50

0

Lafayette

4

9

2

0

Lee

36

32

0

1

Leflore

19

91

4

0

Lowndes

10

0

0

0

Madison

0

22

1

0

Monroe

36

25

0

0

Noxubee

1

0

5

0

Quitman

0

22

14

0

Sharkey

31

97

26

1

Sunflower

19

122

51

0

Tallahatchie

20

21

4

0

Tunica

13

22

8

0

Union

6

35

2

0

Warren

2

5

0

0

Washington

4

31

1

0

Webster

2

34

4

0

Yazoo

1

97

95

0

The information in this newsletter is provided for educational purposes only. Mention of specific products or trade names does not imply their endorsement above other products that are labeled and recommended for a similar use.

 

Insecticide of the Week:

Trade Name and Formulation: Larvin 3.2
 Common Name: Thiodicarb
 Producer: Aventis CropScience
 Chemical Class: Carbamate
 Target Pest(s): loopers, bollworms, tobacco budworms, armyworms
 Use rate range: 0.25 to 0.9 lbs ai/acre
 Toxicity (rat/oral LD50 of technical product): 166 mg/kg

 

Larvin first appeared in the Cotton Insect Control Guide in 1986. It was originally marketed by Rhone Poulenc, but is now an Aventis CropScience product. Larvin is recommended primarily for control of a wide range of caterpillar pests. However, the label also indicates that it will provide suppression of boll weevils and plant bugs. Thiodicarb, the active ingredient in Larvin, belongs to the carbamate class of insecticides, as do Lannate, Vydate, Temik, and Furadan. The chemical structure of Larvin looks much like a double molecule of Lannate, but Larvin does not cause the reddening of cotton that is sometimes associated with Lannate. Since its introduction, Larvin has gained a reputation as a product that will control a broad spectrum of caterpillar pests, and it was often the material of choice against mixed infestations of caterpillars during late season. However, overall use of Larvin was somewhat limited by cost and because it is relatively less rainfast than many other products. This tendency to get "washed off" is one of the key factors limiting the use of Larvin. Although it has some contact activity, its main mode of action against caterpillar pests is as a stomach poison. Compared to most of the other, older, products Larvin is relatively "easy" on beneficial insects. Because of this, it was often recommended for control of early season caterpillar infestations. Perhaps one of the more important uses of Larvin was, and continues to be, as an ovicide for bollworm and tobacco budworm eggs, at a rate of 0.25 lbs ai/acre or one gallon to 12.8 acres. Another important use of Larvin has been as a "salvage treatment" for large/escaped budworm/bollworms. When used at a rate of around 0.8 lbs ai/acre (1 gallon to four acres), Larvin will often provide surprisingly good control (50 to 60%) of larger budworm/bollworm larvae. The data presented in the graph below show that the efficacy of Larvin against soybean loopers is similar to that of many of the newer caterpillar products (all rates are expressed as lbs. Ai/acre).

 

1999 Small Plot Soybean Looper Trial

Northeast Counties - Mike Williams: I've not had the opportunity to look at any Mississippi cotton this past week, but wanted to share with you some of the things I have learned, anyway. I attended the PREP 2001 IPM in Agriculture: Principles, Practice, and Policies workshop in East Lansing, Michigan. This meeting reviewed pest management in all crops and pointed to some of the changes we have made and are going to have to make. It is estimated that about 71% of all major crops practice some phase of IPM. Some phase of IPM is practiced in about 86% of the cotton acreage in the US. With that compliance pesticide use is up 4% overall. Organo-phosphate, carbamate and other old line pesticide use is down by 14% since 1999. It is estimated that the increase in pesticide use is due to the increased use of herbicides, especially Roundup.

Reports from most of the participants indicate that we are experiencing a fairly easy insect year across the country. One new pest that we saw was the soybean aphid, a new species which has been reported in the upper Midwest . These pests cause severe yellowing of plants and in heavy numbers will stunt and kill plants.

It's important that we continue to remain vigilant and follow through by continuing to walk fields until the crop is safe. Stink bugs can and will move into the crop up until it opens. The drop cloth is still the best way to find bugs. The bolls you pull and check might be the most important ones you see all year. Select quarter sized (25 cents) bolls and pop them open looking for warts and stained lint. If 20% of the bolls are damaged and if you find bugs in the cotton, SPRAY.

We've also had weevils caught in the line pheromone traps again this past week, so that may mean they are beginning to move around a little. Continue to look for weevil punctures and for weevils in blooms. If you know of anyone who has ornamental cotton, please ask them to destroy it or at least check it for weevils.

Delta Counties - Gordon Andrews: Plant bugs and something else seems to summarize the observations from the delta cotton fields. Difficult to control plant bugs seem to be the cotton insect subject of the week. Dr. Gordon Snodgrass, a USDA scientist, has detected pyrethoid resistance in plant bug populations from several locations in Washington Co., MS. Tests conducted by USDA scientists and reports form consultants indicate single applications of standard rates of organophosphates insecticides are not cleaning up plant bug infestations.

Many delta fields have cutout and are no longer susceptible to plant bug and bollworm damage. Stinkbugs have been observed at sub-threshold levels in cotton fields but seem to be increasing. The predominate foliage feeder seen this week is the salt marsh caterpillar. With recent rains and bolls opening boll rot is becoming a threat. Some defoliation may be welcome in some fields but we still need a canopy to fill the top bolls with lint.

Southern Counties - Don Parker: Many fields have reached 5 NAWF. There are several fields that have 15-20% egg lay with tarnished plant bug present. There have been some areas reporting moderate densities of stink bugs. Overall the cotton crop looks good and will soon be matured.

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