With many consultants reporting flushing high numbers of moths over the last 7-10 days, bollworm larvae are beginning to turn up at treatable levels in some fields now. Over the last couple days I am hearing reports of treatments going out in some areas but at this time treatments are not yet widespread.
It will be very important to monitor fields closely in the coming weeks for bollworms. Bollworms are preferentially attracted to wide row flowering beans but over the last few years the “old rules” don’t seem to apply as much. Most of the calls lately have been in narrow row beans and we had numerous reports last year about infestations starting in R5 beans with no flowers, so assume nothing, scout everything.
Mississippi’s threshold for bollworms in soybean is 9 larvae 1/2 inch long per 25 sweeps or 3 per foot of row. I have witnessed numerous fields over the last couple years that had substantial pod injury where folks were never able to sweep a threshold of larvae out. In nearly every case these were very large narrow row soybeans. Remember the threshold only works if you are able to get an accurate sample. In some situations it is not possible to get an accurate sample in chest high rank soybeans with a sweep net and you cant do drops in narrow row beans so you have to supplement the sweeps with visual samples. We do not actually have a visual threshold, so what do you do? We know that the drop cloth threshold is 3 per foot of row, so I take the handle of the sweep net and bend the plants over and visually examine them. Because you are only able to actually examine a small part of the plant like this, I use less than half the drop cloth threshold and trigger on about 1 per foot of row visually. I admit up front, we have no data to suggest this is a perfect solution but what’s the alternative? There is none currently, so this is our best guess based on our experience with sampling rank chest high soybeans.
What to treat with? You may remember a time where bollworms were very easy to control with a low rate of a pyrethroid insecticide in soybeans. In fact, many of the pyrethroid labels still include extremely low rates for control of bollworm in soybean. Over the last 3 years we have seen the level of control with straight pyrethroids fall off dramatically. Because of the high number of control failures and less than acceptable control, we no longer recommend pyrethroids for control of bollworm in soybean. This does not mean you will get zero control with a pyrethroid but 7 out of 10 pyrethroid sprays target a bollworm in the Delta region of the state in recent years resulted in poor control or outright failures.
Preferred treatments are the new insecticides in the diamide class of chemistry. These include Belt by Bayer, Prevathon by Dupont, and Besiege by Syngenta. These materials are very good on caterpillar pests and have long residuals. It will not be uncommon to get 12-14 days residual on bollworm (maybe little longer) and 24-28 days on soybean loopers. With the exception of Besiege (which is a premix that includes the pyrethroid Karate) these materials will have to mixed with another product to control pests other than worms.