University of Illinois

No. 19/August 1, 1997

Drought Stress and Insect (and Mite) Problems

In several previous issues of this Bulletin, we have discussed corn leaf aphids in corn, potato leafhoppers in alfalfa, and twospotted spider mites in soybeans. The essence of many of the articles written about these pests is that hot, dry conditions exacerbate the problems. None of these pests has relented. In areas where crops are suffering from a lack of moisture, these pests have gained a foothold and continued to cause problems for growers and retailers for at least the past 2 to 3 weeks (longer for potato leafhoppers). If the weather continues to be unkind, the pests will not go away.

Problems with corn leaf aphids continue to plague cornfields in drought-stressed areas of western Illinois. Although the research literature suggests that corn leaf aphids are most detrimental before pollination, our experience with these pests all the way back to 1980 has taught us that heavy infestations of corn leaf aphids in drought-stressed corn in August could result in reduced yields, primarily due to poor grain fill. In 1980, densities of corn leaf aphids in east-central Illinois were so intense that leaves above ear level died. Consequently, affected plants were either barren or produced pathetic ears. Those of you battling corn leaf aphids should try to assess whether their densities are increasing or decreasing. Increasing densities during droughty weather won't bode well for corn yields.

Twospotted spider mites are showing up in numerous locations throughout Illinois. As we have experienced in the past, mite problems linger as long as the weather remains hot and dry. In drought-stressed counties in northern Illinois, spraying for spider mites has been going on since second week in July. In other areas (northeastern, western, and southwestern counties), spraying activity has accelerated within the past couple of weeks. Again, we are not experiencing the same intensity of spider mite problems as in 1988, but sustained vigilance is recommended. Remember that densities of spider mites began increasing early (June) in 1988. Our experience thus far in 1997 resembles the problems we had with spider mites in 1983, when their densities increased late in July and into August. Regarding less-than-satisfactory control of spider mites with dimethoate, apparently other folks have experienced this, too. Be aware of this information if you must make a decision about what to use for control of spider mites in soybeans. Also, make certain you diagnose the problem correctly. Despite our urging for proper diagnosis, people still make costly mistakes. I am aware of a couple of incidents in which growers have had their soybeans sprayed for control of spider mites, but the problem in the field was something elseŠsoybean cyst nematodes in one instance. These mistakes are not beneficial for anyone involved.

Finally, as people in alfalfa-growing areas already know, densities of potato leafhoppers this year are very large. Battles with potato leafhoppers in alfalfa began in June; and the hot, dry weather has aggravated the problem. Don't drop your guard. Research has shown that injury caused by potato leafhoppers through August and into September threaten alfalfa's ability to survive the winter.

Kevin Steffey, Extension Entomology, (217) 333-6652