Issue No. 14, Article 3/July 6, 2012
Pest Observations and Management Perspectives
The hot and dry weather persists throughout much of Illinois, with the 7-to-10-day forecast showing daytime highs above 90°F and little rain (except widely scattered showers) expected for most of the state. Justifiably, producers' focus has turned from most pest-related concerns to the weather, yet the hot and dry conditions will likely exacerbate infestations of twospotted spider mites throughout many soybean fields in the coming weeks.
If you are seeing discolored leaves (yellowing or bronzing) along field margins, I encourage you to tap the leaves over a sheet of white paper to dislocate mites, if they are present, from the lower leaf surface. If you observe tiny specks moving on the paper, chances are very good you have mites in that field.
Continued hot and dry conditions will very likely cause the infestation to become more widespread throughout the field. If you find discolored leaves along field margins, detect mites, and determine that the midrange forecast (7 to 10 days) will bring more hot weather and no rain, you should consider a rescue treatment.
In 1988, the last year that twospotted spider mites caused widespread damage to the soybean crop (an estimated 6 million acres in Illinois received a treatment), the organophosphate insecticides chlorpyrifos and dimethoate were the primary insecticides of choice. Of the two, dimethoate provides some systemic activity. To be effective and provide longer residual activity, dimethoate must be absorbed by the leaf and translocated. Plants that are under severe drought stress are less able to absorb and translocate dimethoate as effectively. Chlorpyrifos (Lorsban 4E or generic products) is not systemic, and the residual activity may be reduced to as few as 3 to 5 days due to photodecomposition under intense heat and sunlight.
With high humidity and heavy morning dews, a fungal disease may help suppress mite densities. Cooler temperatures at night will help spread the disease. For now, keep scouting for mites and be prepared to make a rescue treatment as needed. In 1988, initial sprays that were directed only at field margins proved ineffective, and mite infestations spread quickly to field interiors. If mites are found throughout a field, border rows are bronzed, and the forecast for the next week to 10 days is hot and dry, consider treating the entire field.
Western corn rootworm and Japanese beetle adults are now commonly found throughout most of the state. The focus over the next 10 days should be scouting for evidence of excessive silk clipping, which could interfere with the pollination process. Plants under severe moisture stress may not be able to generate sufficient silks to stay ahead of clipping by the beetles. In general, densities of 3 Japanese beetles per ear and 5 western corn rootworm adults per plant are sufficient to cause potential pollination problems. Don't neglect to scout for these insects.
Is there any good news to report? So far, soybean aphids are a no-show for much of the Corn Belt; with the exception of the upper Midwest, the very hot conditions have suppressed densities. I don't anticipate this scenario changing any time soon this growing season.
We begin our annual corn rootworm digs on July 6 to evaluate the effectiveness of various Bt products and soil insecticides. I look forward to sharing the results of these trials later this summer and during fall and winter meetings. It appears that we have very good pressure in our checks.--Mike Gray