Bean Leaf Beetle Problems in Mississippi Soybean

Bean leaf beetle numbers have been much higher than they have been in the last couple years. We have actually made bean leaf beetle targeted sprays on a number of acres this year. 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; 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 thresholds in MS are 50 beetles per 25 sweeps or 20% whole plant defoliation, or when 50% of the plants have feeding on one or more pods per plant.

Defoliation: Bean leaf beetles defoliate the tops of the plants. Defoliation thresholds are based on the whole plant. So for example, if you have 50% defoliation in the top 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.

Now for some explanation of the threshold, first, I wish we would completely eliminate the 50 per 25 sweep threshold. The reason is bean leaf beetles move around a lot this time of year and we can reach this threshold very easy then make an application and next week have to repeat the application, and again the next based on numbers alone. We would be much better off forgetting about the numbers and waiting until defoliation approaches 20% even if numbers are much higher than 50 per 25 sweeps. Then we can usually get away with one application rather than several because they are finished moving into the field. This makes a lot of folks nervous because we are 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.

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.

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 don’t be surprised when you return if numbers are back up again. Ride them as long as you can.

Bean Leaf Beetle Pod Feeding (Click to Enlarge)

Red Color Phase of Bean Leaf Beetle