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Just the facts (Part 4): A review of the biology and economics behind insecticide recommendations

University of Minnesota: Bruce Potter, Robert Koch & Phil Glogoza
Iowa State University: Erin Hodgson
Purdue University: Christian Krupke
Penn State University: John Tooker
Michigan State University: Chris DiFonzo
Ohio State University: Andrew Michel & Kelley Tilmon
North Dakota State University: Travis Prochaska & Janet Knodel
University of Nebraska: Robert Wright & Thomas E. Hunt
University of Wisconsin: Bryan Jensen
University of Illinois: Kelley Estes & Joseph Spencer

Costs of treating soybean aphids too early

While some newer insecticides target a narrower range of insects, most insecticide applications are not specific. They will kill beneficial insects (lady beetles, parasitic wasps, etc.) as well as pests, later allowing soybean aphid populations to rebound in fields without those beneficial insects to slow them down. By using the ET, natural enemies will have a chance to suppress the aphid population and possibly prevent it from reaching economically damaging levels. After application, insecticide residues will kill insects for a short time, but insecticide activity invariably declines over time (generally, this is considered a good thing). With most insecticides registered for soybean aphid control (such as pyrethroids), soybean foliage emerging after treatment is not protected. Insecticides that are absorbed and translocated within soybean plants typically move upward only a leaf or two and eventually leave unprotected foliage, especially when applied early in the season.

Applying treatments early can result in a false sense of security and a reduced reliance on scouting. If a re-infestation is not detected before reaching the EIL, yield may be reduced. If detected, the cost of additional insecticide applications are incurred. Early treatment can reduce or eliminate the cost efficiencies of a single, well timed threshold-based treatment. Finally, unnecessary insecticide applications do nothing positive for a short-term return on investment. Importantly, long-term returns can be reduced if insecticide resistance becomes fixed in the soybean aphid population. This has happened many, many times in the history of pest management. We know that managing pesticide resistant pests is seldom "cheap and easy" (for example, consider the problems with herbicide resistant weed control).

How do you know if you have a soybean aphid problem?

All soybean fields are not equally likely to have a soybean aphid problem. Geographic, landscape, biological and agronomic factors all influence soybean aphid populations. The research results generated on commercial and University farms across the North Central Region can help identify when and where to target early- and late-season aphid scouting efforts. Early aphid infestations are often found in smaller fields near buckthorn (1) and are often more abundant near field edges. Soybean aphids prefer moderately dry soil moisture conditions (20). Soybeans grown in soils testing low in potassium contain higher levels of amino acids favorable for soybean aphid development (25) and aphid feeding can intensify potassium deficiency symptoms on these soils. Aphids are often most abundant in late maturing fields.

During vegetative growth stages, soybean aphids are often on the upper, newly expanding leaves. During reproductive growth stages, soybean aphids tend to move to leaves, stems and pods lower in the canopy (15). Soybean aphids should be scouted by examining individual plants throughout the field (7). Because soybean aphids remain attached to the plant while feeding and can occur throughout the canopy, use of a sweep net is not recommended for assessing populations of this pest.

It takes time for aphid populations to grow. It's important to note that sometimes soybean aphid populations never grow at all - a single aphid does not invariably lead to hundreds! Soybean aphids can initially colonize a field and be rapidly wiped out by a combination of inhospitable environmental conditions and predatory insects. Once established, populations can grow at a rate where numbers double every 1-½ days to as many as 6 days depending on environment and natural controls (14, 23). Most often, research documented that in-field populations doubled every 3 days (23).

Soybean fields should be scouted on a regular basis. Soybean aphid populations can increase rapidly, particularly with winged aphids migrating into fields. Early-season scouting can focus on fields with high risk for colonization or a history of early colonization by aphids. As aphid populations develop, more fields should be scouted. There may not be a need to visit every field every week, but fields with a history of high populations may need to be scouted weekly or more. The following resources may help to develop an efficient scouting strategy (Scouting guide for North Central Region).

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