Bollworm trap catches are highly variable across much of the cotton-growing regions of Louisiana. Trap catches at the Northeast Research Station, Macon Ridge Research Station and Red River Research Station are beginning to slightly increase from week to week. Louisiana typically experiences a large bollworm flight the week of July 4, and it appears this year will be no exception.
However, Louisiana’s corn acreage is staggered in maturity, which may create a longer — but less intense —bollworm flight.
Insecticide sprays based on egg lay may be prudent for some technologies and not others. Bollgard 3, TwinLink Plus and Widestrike 3 all contain the VIP3A gene. These insect-protection packages do not offer complete protection against bollworms; however, they do offer better protection than their second-generation counterparts (Bollgard 2, TwinLink and Widestrike). Sprays based on egg lay for the third-generation cottons are not recommended.
For producers who planted second-generation Bt cottons, the decision to spray based on egg lay is more difficult. Bollgard 2 and TwinLink both experienced field control failures with bollworms last year. Yet last year’s bollworm pressure was very intense from the first week of July until August.
Individual fields and pressure may vary, but as a general rule, we need to give the technology a chance to work before making automatic applications, especially under light pressure. Bollgard 2 and TwinLink both contain Cry2 proteins. Insect bioassays conducted last year indicate a very high level of resistance to Cry1 but varying levels of resistance to Cry2. These pockets of resistance may or may not be experienced again this year, and Cry2 resistance does not appear to be widespread across our populations.
Ultimately, the decision to make an application based on egg lay should be made on the amount of eggs and the presence of live worms in cotton. Be aware that as the season progresses and corn continues to mature, moths moving out of corn into cotton may have been exposed to multiple Bt toxins, so control in cotton may decrease. The LSU AgCenter bollworm threshold for cotton is 6 percent fruit injury with the presence of live worms.
Widestrike cotton (499, 312, etc.) — but not Widestrike 3 — should be managed similarly to conventional cotton. Widestrike contains Cry1Ac and Cry1F. Our bollworm populations are highly resistant to Cry1, and Cry1F is marginally effective against bollworms. All commercial Bt cottons are very effective against tobacco budworms.
Bioassay results from 2018 indicate that populations collected in late April from clover were highly resistant to Cry1 but highly susceptible to Cry2. These populations weren’t selected in corn, and results may change as the cropping season continues.
Another thing to consider is the natural mortality of bollworm eggs. Bollworm eggs, when exposed to hot, dry weather, may desiccate before they become viable. Although this phenomenon isn’t guaranteed, it may offer some welcome relief to cotton undergoing bollworm moth flights.
If an insecticide application is warranted, the use of pyrethroids is strongly discouraged. Louisiana bollworm populations have the highest level of pyrethroid resistance in the United States, and pyrethroid applications may not provide adequate control.
The LSU AgCenter recommends the diamide chemistry (Prevathon, Besiege) for control of bollworms in cotton. Beware that Besiege contains a pyrethroid. Using it may inadvertently flare secondary pests. Keep in mind that bollworms are cryptic feeders, and worms that have established in squares and bolls may not be controlled by diamides.
Finally, all insecticide applications, including ones targeting bollworms, benefit from adequate gallonage and correct nozzles. The proliferation of auxin-tolerant crops has forced many producers to adopt coarse-droplet nozzles. These nozzles work well for herbicide applications but limit efficacy of insecticides by reducing coverage.
The vast majority of our insecticides are coverage dependent, so investing in a set of hollow cones will help increase insect control in all crops. Insecticide applications should be made at a minimum of 10 gallons per acre by ground and 3 to 5 by air.
