No. 18 Article 1/July 27, 2007

Soybean Aphids at Economic Levels in Some Illinois Soybean Fields

Our regular surveys of soybean aphids have paid off in 2007, allowing us to anticipate some population explosions in some fields in north-central and northwestern Illinois. As we have indicated many times already this season, ideal weather conditions (temperatures in the 70s to mid-80s Fahrenheit) would promote rapid development of populations of soybean aphids, especially in the relative absence of natural enemies. So it is not surprising that some northern Illinois fields are now harboring near-economic levels of soybean aphids. Although the economic threshold for soybean aphids is 250 aphids per plant (field average) with 80% of the plants infested, the economic injury level (generally when the cost of control equals the cost of the yield loss) is slightly greater than 600 aphids per plant. The economic threshold is conservative to allow for at least 1 week to arrange for an insecticide application.

Table 1 reveals the average densities of soybean aphids in 26 soybean fields in northern Illinois sampled on July 23 and 24. The overall message from the data is that all fields are not created equal in terms of population densities. In Stephenson County alone, the average densities ranged from a currently nonthreatening 14.35 aphids per plant (Stephenson 2) to a near-economic threshold level of 180.4 aphids per plant. The average densities in the other 16 fields are all well below the economic threshold. But the densities in fields in Woodford County and the "transect" counties (Bureau, Lee, Marshall, Ogle, Putnam, Whiteside) have increased noticeably over the past two weeks. For example, the average density in Woodford 6 has increased from 0 (July 10) to 46.25 (July 20--sampling delayed by storms) to 68.85 (July 24) aphids per plant. This same pattern was representative of Stephenson 6--8.55 aphids per plant on July 10, 48.30 on July 18, and 180.4 on July 24.

Dave Feltes, University of Illinois Extension IPM educator in the Quad Cities, has been monitoring populations of soybean aphids in a couple of soybean fields in JoDaviess and Stephenson counties, and he too has reported a marked increase in numbers over the past two weeks. In both fields, 100% of the plants (R2-R3) were infested, with more than 800 aphids on one plant in the JoDaviess County field and more than 100 on individual trifoliolates in the Stephenson County field. (On July 25, numbers of aphids on all individual plants in the JoDaviess County field were more than 250 per plant.) Dave reported that the aphids can be found virtually everywhere on infested plants (leaves, stems, pods) and that winged aphids are relatively common. He also has observed honeydew, the sticky substance excreted by aphids.

Data not shown in Table 1 indicate that the numbers of soybean aphids on individual plants were very large in some fields:

The data also indicate that in some fields with low average densities (e.g., Woodford 8, 24 aphids per plant), colonies of soybean aphids are noticeably large on some plants (nearly 200 aphids on two of the plants sampled) and nonexistent on others (eight of the plants sampled). This situation indicates that soybean aphids have colonized the field more recently and that spread of the aphids within the field is likely.

We also have been asked what's going on with soybean aphids further south in Illinois. Although we are not regularly surveying fields farther south than Champaign County (where numbers still are quite low), we have received a couple of reports of soybean aphid activity in Sangamon County. Matt Montgomery, University of Illinois Extension crop systems educator in Springfield, reported that the density in one field had reached the economic threshold on July 20. However, the producer, who is diligently scouting for aphids, had indicated that the population density did not increase over a 3-day period, possibly due to the plentiful natural enemies (types not reported) in the field. In addition, many of the aphids being observed in this field were "white dwarfs" (more on this issue later in the article). Nathan Bengston, an agronomist with Pioneer Hi-Bred International in Litchfield, spot-checked a handful of fields in Sangamon County on July 24 and observed a range of 1 to 15 aphids on individual plants in all fields. He too reported finding white dwarfs.

Soybean aphids on a trifoliolate from a Sangamon County soybean field (photo courtesy of Matt Montgomery, University of Illinois Extension).

It is apparent that soybean fields throughout Illinois should be scouted soon and frequently (at least every 3 to 4 days), especially in northern counties. The average densities of aphids have increased dramatically over the past 2 weeks, and yield losses could result from heavy infestations. It is also apparent that not all fields will have economic infestations of soybean aphids, so "blanket" insecticide treatments are not appropriate. Decisions about the need for an insecticide should be made field by field. Insecticides (and rates of application) suggested for control of soybean aphids in Illinois are published in chapter 1, page 13, of the 2007 Illinois Agricultural Pest Management Handbook (Adobe PDF). Please follow all label directions and precautions if applying insecticides. Keep in mind that spraying blossoming soybeans can be extremely hazardous to bees. Please coordinate with local beekeepers before applying insecticides to soybean fields.

Regarding white dwarfs, these remain something of a mystery to entomologists. Following is a quote from Purdue University's Pest & Crop Newsletter, August 12, 2005: "These are soybean aphid. They are not all 'baby' aphids. They are not diseased aphids. In literature they are referred as 'white dwarfs.' Aphids of many different species do this in response to change. With soybean aphid, this change in morphology may be due to hot temperatures, higher humidity, shorter day-length, nutritional quality, predator populations, etc. In short, we do not know what is causing this. We do know that they are living, feeding, and reproducing aphids. They should be included in the total plant population count when determining treatment."

Clearly, it is recommended that these aphids be included in counts of all aphids in any given field, although it is possible (maybe even likely) that these white dwarfs do not cause as much injury as the typical "Mountain Dew-colored" soybean aphids.

As the season progresses and soybeans mature, the economic threat posed by soybean aphids will lessen. However, with many soybeans currently in vulnerable stages of development (R2, R3, R4), soybean aphid infestations could cause yield losses. At the risk of being redundant: scout now and scout frequently.--Kevin Steffey

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