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Soybean Aphid Update: Preliminary Results of Insecticide Efficacy Trials

August 18, 2000
Questions concerning the wisdom of applying insecticides to rescue soybean fields infested with soybean aphids persist. Dr. Christina DiFonzo, Extension Entomologist, Michigan State University, recently reported the results of an unreplicated insecticide strip trial in Ottawa County (adjacent to Lake Michigan), Michigan. Four products were evaluated in the trial: Baythroid (1 ounce per acre), dimethoate (1 pint per acre), Lorsban 4E (1 pint per acre), and Penncap-M (1 pint per acre). Please note that Baythroid is not labeled for use in soybeans. Baythroid was included in the trial for experimental purposes only. Treatments were applied on August 12 (TerrogatorÒ at 27 gallons carrier per acre), and plants were evaluated on August 15. The percentages of leaves still infested with soybean aphids that had been treated with Baythroid, dimethoate, Lorsban 4E, and Penncap-M were 36%, 91%, 37%, and 81%, respectively. In the untreated strip, 95% of the leaves remained infested with aphids. The numbers of live aphids per leaf three days after treatment with Baythroid, dimethoate, Lorsban 4E, and Penncap-M were 1.9, 5.5, 1.3, and 3.0, respectively. On untreated plants, nearly 10 aphids per leaf could be found. Although the numbers of aphids declined after treatments were applied, significant numbers of surviving aphids remained. Because soybean aphids, like many other species of aphids, can rebuild their populations quickly, the final value of the insecticide application remains uncertain at best.

Soybean aphids on soybean leaf in Kendall County. (Photo courtesy of Gary Bretthauer, Kendall County Extension Unit.)

Dr. DiFonzo noted that in some heavily infested fields, populations of soybean aphids were crashing. She observed many natural enemies hard at work. These biological control agents included predators such as lady beetles, parasitoid wasps, and a pathogenic fungus. In some fields, the pathogenic fungus was especially devastating to the soybean aphid population. This is great news!

Before applying an insecticide to control soybean aphids, please consider the following points:

(1) Fields with plants that are not showing any signs of stress due to feeding injury (yellowing or cupping of leaves) will most likely not benefit from a rescue treatment.

(2) While looking at injured plants and aphids, also check for the presence of natural enemies.

(3) Even though Dr. DiFonzo reported on an insecticide trial in which insecticides were applied with a ground applicator (27 gallons of carrier per acre), control was marginal at best.

(4) At the current commodity prices of soybeans, considerable uncertainty remains regarding the return on this investment.

(5) Don’t panic if you find soybean aphids in your soybean fields and automatically assume that a treatment is warranted.

As mentioned in an earlier update, the life cycle of soybean aphids is somewhat complex. Later this growing season, winged forms of this aphid species will leave soybean plants and fly to their overwintering host. The Chinese literature (1962) indicated that this overwintering host, upon which eggs are laid, is buckthorn (Rhamnus davuricus). A more current search of the literature suggests that at least three species of the genus Rhamnus can be found in Illinois: Rhamnus lanceolata (lance-leaved buckthorn), Rhamnus cathartica (common buckthorn), and Rhamnus davurica (buckthorn or dahurian buckthorn). All of these buckthorn species are woody perennials. Lance-leaved buckthorn plants can be found in at least 45 counties throughout northern and central Illinois. It also occurs in more isolated counties in southwestern and southeastern areas of the state. Common buckthorn plants are prevalent in northern counties of Illinois; isolated observations have occurred in other counties in central Illinois (24 counties collectively). Dahurian buckthorn plants have been reported only in DuPage County; however, this may simply mean that more exhaustive surveys for this species of buckthorn are warranted.

We have much to learn about the potential attractiveness of these three species of buckthorn to the soybean aphids that will exit soybean fields later this season. This story will continue to unfold, and we will provide updates throughout the remainder of the growing season. Expectations for next year will be forthcoming as entomologists from the University of Wisconsin, Michigan State University, and the University of Illinois learn more about this new pest of soybeans in the upper Midwest.--Mike Gray and Kevin Steffey

Author: Kevin Steffey Mike Gray

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

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