No. 14 Article 1/June 25, 2004

Japanese Beetle Update

After a flurry of e-mails and phone calls last week, things have been considerably quieter the past several days. Of course, Japanese beetles and corn rootworms are still headlining this week, but we have a few other updates to offer as well.

They've arrived 2 full weeks ahead of last year's first sighting, and Japanese beetles are slowly making themselves known. Late last week, some leaf feeding on corn plants was observed, although the feeding was noted as "very limited." Most Japanese beetles spotted in fields were actually feeding on weeds, such as velvetleaf and Pennsylvania smartweed.

Japanese Beetles on Pennsylvania Smartweed.

E-mails this week have been quite different. With cornfields edging closer to tassel in parts of the state, concern over economic levels of Japanese beetles is starting to take hold. Most reports are still noting only low levels of beetles, with little feeding in corn, but I've also received some questions on economic thresholds and treatment options. Before deciding to go ahead with an insecticide treatment, be sure to get a good idea of the Japanese beetle pressure in the field. Populations of Japanese beetles are usually clumped within a field. As adult beetles emerge, they move toward flowering plants in the area to feed. Female beetles also emit a pheromone, attracting other adult Japanese beetles. It is very possible to observe lots of beetles in the field edges or borders and then find none as you make your way into the field. Growers are urged to scout throughout the cornfield to determine what level of Japanese beetles is present. Densities may be over- or underestimated if sampling is done in only a few locations.

While populations in most areas are still small, populations may reach economic levels in the future. When densities of Japanese beetles reach or exceed three beetles per ear before pollination is complete and silk clipping is obvious, an insecticide application may be warranted. With that said, also make a decision that is economically justifiable. An entire field may not need to be sprayed if silk clipping is occurring only in certain areas. Insecticides labeled for control of Japanese beetles in corn are listed in Table 1. Science and experience have taught us that pyrethroid insecticides, in general, lose some efficacy when temperatures are high. Also, keep in mind that, while Japanese beetles continue to emerge and move around, initial efficacy of almost any product will seem questionable because of beetles immigrating into sprayed fields. Keep this information in mind when you assess the efficacy of any product applied to control Japanese beetles in corn.

Please refer back to last week's issue (issue no. 13, June 18, 2004) for more information on the Japanese beetle, and feel free to share any information concerning Japanese beetle problems. --Kelly Cook

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