The first call I received about corn rootworm in peanut a couple weeks ago seemed like one of those interesting oddities that show up in a field or two every year but do not amount to much more than that. Since then I have gotten a couple more calls, and it seems infestations are more widespread than I would have guessed. We are seeing two species of rootworm adults in fields: southern corn rootworm and banded cucumber beetle, and I have recently seen confirmation of rootworm larvae and damage to developing pods. Rootworms are not a common pest of peanut in Georgia. They prefer heavy, moist soils with high organic matter, two things that tend to be rare in our peanut production areas. Nevertheless, fields with high clay content and center pivot irrigation can be infested. To date I have gotten reports of pod damage occurring in Sumter, Pulaski, and Lee Counties and have talked to several folks who are seeing very high numbers of adult beetles.
Like for most soil insect pests, control options for rootworms are limited. The only product available for use in peanut that has proven efficacy against rootworm is chlorpyrifos, and post emergence applications are limited to the granular formulation. This product is most effective when used as a preventative before damage occurs. I spoke with my colleague at NC State two weeks ago at the APRES meeting in FL about rootworm in peanut. He said that insecticide applications made after the first week of August in NC do little to prevent pod damage, and he recommends that no treatment be made after 5 August. This “cut off” date could be even earlier in GA, though no data are available.
I expect rootworm infestations and damage to be sporadic, but the insect is certainly more abundant than “normal” in peanut right now. We do not have any information collected from Georgia to help growers make treatment decisions for rootworm. Most of the reports available from North Carolina and Virginia suggest that treating fields after larvae are already feeding on pods will not be very effective. In spite of this, it will be difficult to do nothing if we know pod damage is occurring. Growers who are doing the math on a chlorpyrifos application should also budget for at least one additional insecticide application to control the foliage feeding caterpillars that almost always follow chlorpyrifos treatments.