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The Watch for Soybean Aphids Continues

July 13, 2001
Although we have received only one report of the occurrence of soybean aphids in Illinois (McHenry and Boone counties) from someone not associated with our survey efforts, the numbers of this pest continue to increase in some fields. For example, in a field in Kendall County that our survey team visits weekly, the surveyors found about 10,000 aphids per 50 plants (about 200 aphids per plant) during the week of July 9, a significant increase from the numbers they recorded from the same field during the week of July 2. On the other hand, numbers of aphids in some fields have decreased, possibly as a result of the heavy rains that fell in some areas of the state recently. As you may recall, heavy rains in northern Illinois last year began the demise of soybean aphid populations in mid-August. Then the cleanup crew of predators (multicolored Asian lady beetles, green lacewings, syrphid fly larvae) did the rest.

On the "new sightings" front, we have received word from entomologists in other states that the soybean aphid (pending confirmation of species) has been found in Indiana, Ohio, and New York, and entomologists in Pennsylvania are searching. In most instances, the numbers of aphids found were quite low, although almost every state has a report of at least one field with more than 100 aphids per plant. In Illinois, we can add Sangamon County to a slowly growing list of counties in which we have found soybean aphids. David Voegtlin, aphid specialist with the Illinois Natural History Survey, found a moderate number of aphids during a casual search of a soybean field just at the edge of Springfield. David Onstad, Department of Natural Resources and Environmental Sciences, has sent a team to resurvey some areas of north-central and northwestern Illinois. The discovery of the aphid in Sangamon County probably will persuade us to boost our survey efforts in more south-central counties.

Because of what happened last year (people being caught unaware by a "soybean aphid explosion"), trigger fingers are itchy and some people want to treat early to avoid the buildups that occurred last year. Most entomologists strongly agree that we would do more harm than good by treating early. One study conducted in China revealed that early infestations by small numbers of aphids could result in some yield loss; however, that study, cited by some scientists, was based on an even, manual infestation of small plants. In reality, uniform infestations in most production fields are not common. Usually we find a plant here and there that harbors a relatively large number of aphids, but the other plants are infested by very few aphids.

Waiting creates some anxiety, but we really believe that waiting is the best policy right now. We could really disrupt the natural scheme of things by spraying an insecticide too early. In some areas of Illinois, hot and dry conditions are encouraging buildups of twospotted spider mites. An early insecticide spray right now might kill soybean aphids, but it also will kill important predators and parasitoids. Without these natural enemies, populations of soybean aphids and twospotted spider mites could resurge strongly without any "good guys" to stop them. And keep in mind that most pyrethroids, if that is your chosen class of insecticide, are notoriously not very effective against spider mites. So we urge a watchful eye and patience. Let's give natural control (whether by natural enemies or environmental circumstances) a chance.--Kevin Steffey, Mike Gray, and Sue Ratcliffe

Author: Kevin Steffey Mike Gray Susan Ratcliffe

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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