Bean Leaf Beetle Numbers Picking Up in Mississippi Soybeans

Bean leaf beetle numbers have been increasing steadily over the last couple weeks. We have actually made bean leaf beetle targeted sprays on a number of acres in recent days. Yield loss is caused by defoliation and less frequently by pod feeding. Bean leaf beetles were once very easily controlled by very low rates of pyrethroid insecticides in Mississippi; however we began seeing control problems with even the highest rates of pyrethroids in 2006. Shortly after Dr. Fred Musser, Entomologist at Mississippi State University documented and published a paper confirming that unfortunately we have pyrethroid resistant bean leaf beetles in Mississippi. This obviously changes our management decisions.

Thresholds: Our bean leaf beetle threshold is 20% whole plant defoliation, or when 50% of the plants have feeding on one or more pods per plant. There was a time when we recommended treating on 50 BLB’s per 25 sweep but now we only recommend treating on defoliation. The reason is in late July and August when early beans begin turning we get very high numbers of BLB’s invading greener fields. Using the number threshold alone, we were reaching threshold and making multiple treatments on numbers but defoliation rarely even approached 10%. By waiting until we approach 20% defoliation we are allowing enough time in many cases for the immigration into the field to slow requiring only one application or we reach R6.5 firsts, which is safe from defoliators. This makes a lot of folks nervous because we are generally on once per week checks and the fear is we come back next week and all the foliage will be gone if we ride the higher numbers. I followed a 3X (150 per 25 sweeps) several years ago that I found moving into some Group V beans. I caught them at about 2-3% defoliation and monitored them for 2 weeks. It took 12 days for a 3X threshold to go from 3% defoliation to 20% defoliation so you have some time.

Defoliation: Bean leaf beetles defoliate the top of the plant first. Defoliation thresholds are based on the whole plant. So for example, if you have 50% defoliation in the top 50% of the plant it would equal 25% percent on a whole plant and this is how you should think about it. We tested these thresholds and they held up.

Pod Feeding: We do see pod feeding that can be an issue on occasion but the actual damage is much worse when beans are going through R5 stages because the seeds beneath the feeding usually rot but I have never seen much of an issue from pod feeding past R6. Also, I have never been able to figure out any pattern of when bean leaf beetles will move, if they move, to pod feeding. It seems random and does not happen in every field but we do need to monitor it. Keep in mind bean leaf beetles will not feed into the seed. They feed on the outside of the hull and don’t feed into the seed. If you seed pod feeding that makes it into the seed, it is likely a bollworm.

Control Options: A few years ago we conducted numerous tests on pyrethroid resistant bean leaf beetles to determine best control options. Best control options were Endigo at 4-4.5 oz, Orthene at .75-1.0 lb A.I., Bifenthrin at 1:20, (Although Bifenthrin is a pyrethroid it separated itself from the rest and provided decent control) and Orthene + pyrethroid mixtures. In the Orthene mixtures we have been successful dropping the rate to .5 lb but only in the mixtures. Also, we do not use these much, but Sevin XLR and Larvin were very good also. In fact, without rain, they provide the most residual but are gone with the first rainfall event. A newer product that initially has looked very good is Belay at 4-6 oz. Remember we do not have to have 100% to be a success.

Expectations: the options mentioned above will work fine but will not provide enough residual to hold bean leaf beetles out of the field for 7 days. So again if you spray to early do not be surprised when you return if numbers are back up again. Ride them as long as you can and let defoliation levels be your guide.