Issue No. 15, Article 2/July 1, 2005
Rootworm Adults and Silk Clipping
We have begun to receive a relatively large number of telephone calls and e-mail messages regarding extremely large numbers of rootworm adults clipping silks in more advanced cornfields. Reports of as many as 25 rootworm adults per ear have not been uncommon. We get questions about the effects of rootworm adults feeding on corn silks every year when rootworm densities are large. However, the wrinkle--and challenge--this year is that rootworm adults are clipping silks on plants in fields in which stand development has been uneven, as a direct result of our weather this spring. Consequently, rootworm adults are "ganging up" on the plants that are tasseling and silking early, and much of the rest of the field has not begun to flower.
So the question of the day is "What is the economic threshold for rootworm adults clipping silks in these types of situations?" And the answer of the day is "We don't know." The answer may seem facetious, but it's honest. I believe everyone realizes that when crop plants are suffering from environmental stresses, the impact of insect injury is worse than if the injury were being inflicted on healthy crop plants. But how much worse? The truth is that most economic threshold research is conducted over a few years, and then the data are averaged. It is uncommon for such research to include experiments conducted when environmental conditions are extreme. So we're left with reliance on experience and judgment when such situations arise. Kelly Cook has written an article about Japanese beetles in the Bulletin this week that deals with the same issue.
The economic threshold for rootworm adults clipping silks during "average years" is five or more beetles per plant before pollination is complete and silk clipping is observed. So, what if there are 15 beetles per plant and only 10% of the field has silked? There is no easy answer to this question, so your experience and judgment will have to help you make a reasonable decision. If densities of rootworm adults are high and silk clipping is extensive on the early-silking plants, an insecticide with reasonable residual activity probably is warranted. Assuming you can get 10 to 14 days residual from the selected insecticide, you may get through the pollination process before the active ingredient no longer is providing control and densities of rootworm adults begin to increase again. Maybe a few stragglers (i.e., late silkers) will still get clipped, but most of the pollination process will have been protected.
There are a number of products labeled for control of rootworm adults to protect pollination in corn (Table 3), but we are not certain that the pyrethroids will provide acceptable levels of control when temperatures are high. Pyrethroid insecticides have a track record of losing some efficacy when temperatures exceed 90°F. The nonpyrethroid choices are Dimethoate 4EC, Lorsban 4E (or Nufos 4E), Penncap-M, and Sevin XLR Plus. Although not necessarily cheap, Sevin XLR Plus has a long residual and provides excellent control of rootworm adults. A couple of cautions: If corn leaf aphids are present in the same field with corn rootworm adults, Sevin XLR Plus is not the best choice. Sevin does not control aphids, but it will kill lady beetles, which otherwise likely would hold corn leaf aphids in check. Spraying Penncap-M to flowering corn could affect honey bees that might be foraging. If Penncap-M is the product of choice, we encourage you to make the application early or late in the day. So make your choice of product appropriate to the situation.--Kevin Steffey