Alabama Cotton: Bollworm Management Options in 2 Gene Crops

Bollworm attacking green cotton boll. Photo: North Carolina State University

Problems with damaging levels of escape bollworms were observed in 2017 on two gene cotton in numerous fields throughout the state. At harvest time, worm damaged bolls were noticed by those operating harvesters in many other fields. These escape bollworms were associated with all 2 gene varieties.

This event had been noticed in a few fields statewide for several years, but the problem increased tremendously in 2017. This situation may increase in 2018 and in future years on 2 gene cotton due to bollworm resistance to both genes.

Three gene varieties have been available to a limited degree for the past couple of years. More varieties will be available with 3 genes in 2018, but supplies of many will be limited. Until the 3 gene varieties are widely planted, we will need to focus on closer scouting and better management of bollworms in the 2 gene varieties.

A major effort has been underway since last season to aid fieldmen on scouting, thresholds, insecticide choices, and timing of applications for bollworms. Following is a discussion on how we might manage bollworms and other cotton insects in 2018.

Do not let plant bugs become embedded.

When this happens, multiple applications will be required to prevent economic damage. An embedded situation is when a subthreshold level has been allowed to go untreated for more than a generation. This results in a plant bug population of all ages and stages from adults, to all ages of immatures and eggs.

In Alabama, most adult plant bugs move from wild hosts to cotton by July 10, early bloom. This is the perfect time to control plant bugs with one timely spray and then stay out of the fields with hard chemistry until the bollworm escape window begins or until stink bugs become economic.

Some of the more commonly used choices for an early bloom plant bug spray are Bidrin, Transform, Centric, or bifenthrin to control the adults. Since egg deposition has been going on for 2-3 weeks prior to that time of the season, we will have plant bug immatures hatching for the following 2-3 weeks.

This is the time we want to stay out of the fields with a hard chemical. So what do we do? Add the IGR (insect growth regulator) Diamond 6-9 oz/ac to the adulticide. This will zero out most of the immatures that hatch for the following 2-3 weeks. At which time, we shift our focus to escape bollworms and stink bugs or in a worse case scenario, late migrating plant bugs.

Now back to our bollworm situation.

Monitor cotton closely to detect the major peak of corn earworms coming from corn. This historically has occurred about July 10-15 in south AL; July 20 in central AL; and about Aug 1-5 in north AL. This egg lay may last from 7-10 days in any given location.

When this flight is detected by increased egg numbers or five to ten 1-2 day old larvae in white blooms, have your control plan ready and implement it within 24-48 hours.

Timing is everything here. You should already have your chemical choice in hand. We basically have 2 choices, a pyrethroid at the highest labelled rate, or one of the diamide selections: Prevathon 14-18 oz or Beseige (Karate + Prevathon) at 7-9 oz/ac. The pyrethroid will cost $2.00-$4.00/ac, the diamide class $14-$18/ac.

If the bollworm larvae are 5 or more days old, the diamide may be no more effective than a pyrethroid. Diamides are most effective when the residue is on the plant when the egg hatches. In order to make this happen then, we must trigger the spray on or shortly after the historical date for the bollworm moth flight determined by egg or small larvae counts.

Fieldmen will need to check sentinel fields every 3-4 days during these critical periods. In my research plots at Prattville, AL, I will be evaluating both types of chemistry with varying timing sprays.

Until I convince myself that the pyrethroids will not work on bollworms when the application is well timed, I will be recommending a pyrethroid. This is for two reasons, one the diamide chemistry is expensive. The most we will want to spend in Alabama will be one application. The second reason I will first choose a pyrethroid is the fact that a diamide, a few days late, may be no more effective than a pyrethroid.

Are all pyrethroids equal for bollworm control?

I am not sure, but they are not all equal on other species that infest cotton. Most all agree that the bifenthrin pyrethroid is better on bugs. Maybe we should never have assumed that they were all equal on bollworms. Some are better on fall armyworms than others.

Originally, Pydrin was the best on worms while Pounce and Ambush were better on boll weevils.

One more point on these escape bollworms. You really have one shot at the peak of this moth flight. That is the reason that first spray must be so timely. By the time you find escape worms behind this spray, it is too late for a “clean up.”

Following this one well timed spray, scout only for more eggs or tiny hatching larvae. A second pyrethroid application may be needed during each moth flight. The older worms will just have to cycle out.

My summary suggestion is to have an escape bollworm plan of attack for 2 gene cotton in 2018, implement it very quickly when the problem is detected, then move on.

Do not call for help, panic, or waste a lot of time surveying or counting large escape worms. If you miss your target, it is too late. You only have a narrow window of time for a well timed spray. Keep in close touch with other fieldmen in your area.

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