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Time to Step Up the Vigil for Soybean Aphids and Spider Mites

July 20, 2001
If you haven't done so already, you should increase your search efforts for soybean aphids in soybean fields. And while you're at it, look for twospotted spider mites, too. Although numbers of soybean aphids are relatively low in most fields, numbers have built up to very noticeable levels in some areas. In addition, we (entomologists in Illinois and elsewhere) continue to find soybean aphids in new locations. Entomologists in Kentucky and Pennsylvania have found soybean aphids in isolated fields. In Illinois, our survey team found soybean aphids in a soybean field in Madison County near East St. Louis. Before this observation, Sangamon County represented the southernmost verified occurrence of soybean aphids in the state.

In northern Illinois, crop scouts and consultants are reporting that they, too, are finding soybean aphids. Ryan Stoffregren, a certified crop adviser in Kingston (DeKalb County), has found soybean aphids in almost every field he has scouted in northern DeKalb and Kane counties, as well as in Boone and McHenry counties. He reported a range of infestations from 2 to 50 aphids per plant. Our survey team reported the following maximum number of aphids per 30 plants in soybean fields in counties in northern Illinois: 10 (DeKalb), 62 (Lee), 40 (Marshall), 717 (Ogle), 176 (Putnam), 83 (Whiteside), and 36 (Woodford). Maximum numbers of soybean aphids per 30 plants in Macon, Madison, and Sangamon counties were 57, 11, and 147, respectively.

Initially soybean aphids were found mostly on the new trifoliates. However, the members of our survey teams recently have noticed a slight change in the aphids' location on the plants. As the colonies build in numbers, the aphids are moving onto stems and older leaves.

There is little doubt now that soybean aphids have become established in the United States. The range of their occurrence indicates that they are either widely distributed or are being widely dispersed within a year, or both. So, now we have to address the tougher question: If we find soybean aphids in soybean fields right now, what should we do? The answer to this question is not straightforward. As you know, we know very little about the potential impact of soybean aphids on soybean yields in North America. A preliminary and inconclusive study conducted by Wisconsin entomologists in 2000 suggested that 80 to 100 aphids per leaflet resulted in a yield loss of about 8 bushels per acre. But thus far, that's all we have.

As I have indicated in previous articles in the Bulletin about soybean aphids, we will conduct insecticide efficacy trials and study the effects of soybean aphids on soybean yield this year, but we haven't initiated much of these efforts yet. We are waiting for numbers of soybean aphids to increase to a level that will allow us to conduct meaningful research. However, in one small field in Kendall County, David Onstad, Department of Natural Resources and Environmental Sciences, and his survey team arranged for a block-type demonstration of an insecticide (Warrior) and an untreated control. The insecticide was sprayed on July 17, so it's still too early to tell what, if any, effect the treatment will have. We will offer our observations at an appropriate time after evaluation.

As numbers of soybean aphids continue to increase in some counties, producers and their dealers are becoming a bit anxious about what needs to be done. We still hold the line that it's too early to apply insecticides. Although the survey teams have not found an abundance of predators accompanying the soybean aphid colonies, there is still hope. In addition, heavy rainfall often reduces the numbers of aphids in soybeans, at least temporarily. Unfortunately, a lack of rainfall may exacerbate the problem. And the buildup of twospotted spider mites in the same fields in which soybean aphids are being found adds another layer of complexity to decision making.

The most reasonable approach at this time is to watch the numbers of soybean aphids within any given field over time. This approach is analogous to something we recommended for corn leaf aphids many years ago. In 1980, we had very little useful information regarding the effects of corn leaf aphids on corn yields if the infestations increased after pollination was complete. However, we were aware that corn leaf aphids were killing the tops of corn plants that were suffering from a lack of moisture. So we suggested that people randomly select a few infested plants, count the aphids (or at least estimate their numbers) on each plant, mark the plants, and then return in a few days to count aphids again. If the numbers of aphids had increased and drought conditions prevailed, we suggested that an insecticide treatment was warranted. If the numbers of aphids had decreased, we urged continued scouting. At this time, this may be the best approach for addressing the concern about soybean aphids in soybean fields.

If soybean aphids and twospotted spider mites are present in the same field and the plants are suffering from a lack of moisture, a treatment for spider mites may be warranted. A spot treatment (for example, along the field margin) may be appropriate. However, before you decide on a spot treatment, make certain that spider mites are not present in the rest of the field. Spider mites may be feeding in areas where symptoms have not appeared yet.

The choice of insecticide may be the final consideration. Dimethoate and *Lorsban 4E are labeled for control of twospotted spider mites in soybeans. Lorsban 4E is labeled for control of soybean aphids (incorrectly called "Chinese aphids" on the label) in soybeans. The labeled rates of application are 1 to 2 pt per acre. Lorsban should not be applied to soybeans within 28 days of harvest. Lorsban and dimethoate provided good control of soybean aphids in our insecticide efficacy trial conducted in Carroll County in August 2000. We recently learned that Syngenta issued and approved a supplemental label for *Warrior for control of soybean aphids (also referred to incorrectly as the "Chinese aphid"). The labeled rates of application are 1.92 to 3.2 oz per acre. Warrior should not be applied to soybeans within 45 days of harvest. Lorsban 4E and Warrior are restricted for use by certified applicators.

Keep us apprised of the situation with soybean aphids in your neck of the woods. The more reports we have, the more likely we will be able to provide an accurate picture of developments with this invasive species in Illinois and throughout the Midwest.--Kevin Steffey

Author: Kevin Steffey

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

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