Since mid-July, bollworms have hit Arkansas and Mississippi soybeans and some producers have found control difficult.
On Wednesday morning, Delta Farm Press spoke with Scott Akin, University of Arkansas Extension entomologist, about the situation in Arkansas. Akin outlined the typical scenario and recommendations. Among his comments:
On reports of high bollworm numbers in soybean fields…
“We have been receiving a number of calls from growers and consultants regarding bollworm control in soybean with pyrethroid insecticides. The calls started coming in around mid-July for us, as bollworms are typically more problematic in R2-R4 beans. These calls have run the gamut from ‘leaving a few more in the field than normal’ to ‘not killing any at all.’
“Before I even use the ‘R-word’ – resistance -- it is important to note that there are several factors that come into play when considering evaluation of insecticide efficacy.”
“First, it takes a few days for insecticides, even pyrethroids, to do their job in a field scenario. Some of the reports have resulted from going in shortly after the REI to evaluate, and often that is too soon to make an accurate assessment.
“Second (and we make this point with all other insect pests), insecticides should be evaluated on a percent control basis instead of whether it ‘zeroes them out’ or not. If you have 20 bollworms in 25 sweeps pre-spray, then end up with 4/25 sweeps at four to five days after treatment, you still ended up with 80 percent control, which is often considered decent performance.”
On treatment options…
“As hard as it seems to do, as busy as everybody is, it will help make subsequent treatment decisions if infestations levels are somehow quantified prior to the insecticide application.
“It is also important to note that not all pyrethroids are the same -- although bifenthrin-products (Fanfare, Discipline, Tundra, etc.) are both extremely popular and effective for sucking pests, it is not the best choice for bollworms. Karate Z or other products containing lambda-cyhalothrin are more active, thus a better fit for this pest.
“While on the subject of insecticide brands, I would be remiss not to caution our growers and consultants to be careful of insecticide formulations vs. rates -- some generics are formulated with half the amount of the branded products, so be careful when choosing rates. Entomologists across the mid-south also recommend adding a half-pound active ingredient per acre of an acephate product to help with worm control. However, I have received mixed reviews from this strategy this season, as well.”
On treatment coverage…
“If I had to make a prediction, I would guess that at least part of the issues we are facing involves the application itself. Canopies are closing up quickly, and we all know how hard it can be to get insecticides to where the worms are.
“Although applications are better to be made during the cooler hours of the day (before 9 a.m.), not all applications can be made at these times. Our pilots are working like crazy to get various applications out and have their hands full as it is.
“During the hot summer temperatures, we have recommended previously that using an adjuvant such as a crop oil (at least 1 percent volume-to-volume) may help get the insecticide deeper into the canopy, particularly by air.
“When going out by air, be assured that going out with as high of a spray volume that you and your applicator can stand -- typically 5 gallons per acre -- is much more effective than traditionally lower volumes (in the two or three gallons per acre range).”
On the crops getting attention…
“Most of the calls are in soybean.
“I haven’t yet heard widespread reports of bollworms causing significant fruit damage in Bollgard II cotton, as reports indicated last year in Mississippi and Louisiana. Of course, we’re finding bollworms and budworms in non-Bt cotton -- most has received at least one dedicated insecticide treatment for worms.
“So far, the dual-toxin Bt cottons (WideStrike and especially Bollgard II) seem to be holding up pretty well. That, however, could be helped by pyrethroids that are being added as tank-mix partners with plant bug insecticides.
“Some calls that we have received that are raising an eyebrow here and there regard pyrethroids not providing typical control of bollworm in grain sorghum, a crop where canopy penetration is rarely an issue as larvae are more exposed on the heads. Pyrethroids are still providing some control in these cases, but reportedly they have not performed as well as in the past.”
More on possible resistance…
“Now for the R-word: It is important to keep in mind that tobacco budworms can also be found in soybean, and our traps numbers of this pest have been relatively high this year.
“Those who have been around long enough to remember tobacco budworms before Bt cotton was introduced know that pyrethroid resistance is a bonafide issue already. I am sure we have budworms out there, but fields in Southeast Arkansas with reported control issues have consisted of less than 25 percent budworm at the highest, thus far.
“As for potential pyrethroid resistance in bollworm, we have sent larvae from three locations with questionable control to Ryan Jackson with USDA-ARS in Stoneville, Miss., to screen for pyrethroid resistance. (We) should receive word of the results in a couple of weeks.
“For now, improving application quality and adding acephate to pyrethroids, or using labeled Lepidopteran-specific products such as Beltor Steward are recommended for control of bollworm.”