Bollworm Management in Bt Cotton: Things to Consider at Planting Time

As the 2018 cotton season approaches, it is a good time to look back at 2017. From an insect management standpoint, the number one thing that stands out about last year is the amount of foliar sprays that were made to two gene Bt cottons (Bollgard II, TwinLink, and Widestrike) for bollworm. A lot of fields got 2 applications and a few fields got 3 applications in 2017. This is not sustainable for cotton growers given the technology fees and other input costs throughout the season.

As we talked about a lot through the winter at grower meetings, Dr. David Kerns at Texas A&M reported high levels of bollworm resistance to the Bt proteins in the two Bt gene varieties. That is the major reason we saw higher than expected survival in all Bt cottons and why we needed so many sprays. So the question becomes, “How do we avoid this in 2018?”

The simple answer is that we can’t avoid the problem in 2018, but there are some things we can do to manage the problem to ensure that cotton production remains profitable in our area. Probably the most important thing is to have a plan in place to manage bollworm when they show up. Growers need to talk to their consultants, and consultants need to talk to their growers now about what to expect and what to do when the time comes.

Avoidance – Nobody wants to make 2-3 applications for bollworm on Bt cotton, so the obvious thing to do is try to avoid the problem. Similar to what we talk about with tarnished plant bug management and what we talked about for tobacco budworm management before Bt cotton, one of the most important things to do is manage for earliness. Several factors that contribute to this include planting date, proper fertilization, and early season pest management.

All of these factors contribute to getting the crop off to a good healthy start from the beginning and not necessarily just planting as early as possible. Plant the crop early, but not too early. Waiting a week to 10 days to plant when soil temperatures are warmer and getting the crop up and through the thrips, seedling disease, and weed stage as quick as possible is probably more important than planting too early. The bottom line is that we may be able to avoid one spray without sacrificing yield simply by planting at the right time.

2-gene vs. 3-gene Technologies – Currently, all of the companies now have 3 gene cottons that produce the Vip protein in addition to the proteins in the 2 gene varieties. These include Bollgard 3, TwinLink Plus, and Widestrike 3. The varieties with the Vip protein will provide significantly better control of bollworm than their 2-gene counterparts. Because of that, it will be critical for growers and consultants to know which “type” of cotton is planted in each field rather than just the seed company (ie Phytogen vs. Deltapine vs. Stoneville). It is highly likely that there will be times when the 2-gene cotton needs to be sprayed, but the 3-gene cotton does not, so knowing where the 3-gene cottons are planted is very important.

Also, because Bollgard 3, TwinLink Plus, and Widestrike 3 will provide much better control of bollworms than Bollgard II, TwinLink, and Widestrike, growers should plant the 2-gene cottons first. Bollworm pressure will likely be more intense later in the year. If planting gets delayed because of weather and some of the cotton has to be planted during the latter part of May, it will be much better to have 3-gene cotton in that planting window rather than 2-gene cotton.

Thresholds and Spraying – It is highly likely that the 2-gene cottons will have to be sprayed at least once for bollworm in 2018. The 3-gene cottons are not immune to bollworm and there may be rare cases where they have to be sprayed as well. Again, it is going to be critical to know where the 2-gene varieties and the 3-gene varieties are planted because our thresholds are different.

On the 2-gene cottons, we suggest spraying based on the presence of eggs. Although it is not published in the Insect Control Guide, the thresholds we talked about all winter are

20% eggs anywhere on the plants for the 2-gene cottons (Bollgard II, TwinLink, and Widestrike)

4% Worms or 6% Damage for the 3-gene cottons (Bollgard 3, TwinLink Plus, and Widestrike 3)

We will post more about insecticide choices, rates, and specific timings as it gets closer to time to spray.

Finally, start thinking about bollworm management now. Carefully plan out when and where the 2-gene cottons and 3-gene cottons will be planted. Having a good plan now and getting everybody on the same page about where each cotton type will be planted should help to avoid making unnecessary applications on 3-gene cotton because somebody wasn’t sure where it was and reduce overall costs.

Presentations at the 2017 Row Crop Short Course were recorded by the Plant Management Network in the Focus on Cotton webcasts A couple of them dealing with bollworm resistance to Bt and their management are listed below.

David Kerns, Texas A&M – Bollworm Resistance

Jeff Gore, MSU – Bollworm Management