Soybean aphid continues to be the dominant pest issue being faced by farmers in the region. Field scouting to determine the need for treatment on a field by field basis is strongly recommended.

Most questions about soybean aphid pertain to temperatures affecting the population growth, and how insecticide treatments might be impacted by high temperatures. The following comments were prepared by Dr. Ken Ostlie, Extension Entomologist with the U of Minnesota and should compliment earlier remarks published in previous issues of the newsletter.

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Since the arrival of soybean aphid in Minnesota, the occurrence and geography of outbreaks (2001, 2002, 2003, and 2005) have been associated with drought conditions. Drought potentially benefits aphids in several ways. First, slight drought stress may improve the nutritional quality of soybeans for aphids. Second, fewer rain events mean less chance to wash off during thunderstorms. Third, drought conditions (low humidity, less rainfall) reduce the activity of aphid-attacking fungi periods of high humidity. So it is not surprising that soybean aphid problems are emerging in Minnesota.

Aphid Situation:
The soybean aphid situation developing this year is exactly opposite what we saw in 2001, when populations exploded first in SE Minnesota with treatable levels progressing north and west during the summer. While scattered, small sheltered fields especially along rivers have been treated over the last 10 days, infestations have reached treatable levels on a widespread basis in NW and WC Minnesota. Meanwhile, populations farther south and east are increasing at a slower pace with scattered fields reaching threshold over the last week. Unless overall aphid populations collapse, fields needing treatment this early may be re-colonized and require treatment in three-four weeks. Don‘t assume that a single application will solve the situation for the entire summer!

The production of winged aphids from heavily infested plants fuels local redistribution and long-distance dispersal. In heavily infested NW and WC fields, alates and nymphs developing wings were abundant. Large scale movement will accelerate as vegetative growth declines during pod set and fill. Expect the aphid situation to change markedly over the next two weeks as aphids continue to disperse.

Development of the soybean canopy also moderates the conditions that aphids experience. Canopy cover is approaching 100% in earlier planted fields with adequate moisture. As the canopy closes, the highest temperatures and the greatest temperature fluctuation occurs near the top of the canopy. It's not surprising that the highest aphid concentrations are on new vegetative growth, which is well below upper canopy leaves, and on mid-canopy leaves.

Does Hot Weather Affect Insecticide Performance?
Higher temperatures and low humidity can affect insecticide performance in several ways.

1. The metabolic activity of insects increases as temperature increases. Enhanced metabolism may either enhance the toxic effects of the insecticide or increase insect ability to detoxify or tolerate insecticides. The toxicity of organophosphate insecticides, such as chlorpyrifos (Lorsban), dimethoate or methyl parathion (Penncap-M), increases as temperature increases. In contrast, the toxicity of pyrethroid insecticides, such as Asana, Baythroid, Decis or Delta Gold, Mustang Max, permethrin (Ambush, Arctic), and Warrior, decreases as temperature increases.
2. Low humidity and high temperatures may cause small insecticide droplets to evaporate before reaching and penetrating the soil canopy. Evaporation may reduce insecticide control. Evaporative effects can be reduced by avoiding application during the heat of the day, increasing water volume, increasing droplet size by changing nozzles, or by adding crop oils (consult labels or companies for specific recommendations).
3. While heat increases insect movement, aphids may be moving away from the upper canopy, which receives the greatest concentration of insecticide.

Should Spidermites be Considered When Treating Soybean Aphid?
With drought intensifying, two-spotted spidermites may become a factor in treatment decisions. The first fields reported treated for mites during the drought of 1988 were associated with recently cut alfalfa and roadside ditches.

Spidermites are favored by prolonged drought. Hot, dry weather enhances spidermite reproduction, increases the quality of soybean as a host, and decreases fungal diseases on the spidermites. Infestations build up more quickly along field edges, especially near alfalfa fields where movement into soybean may be triggered by cutting the alfalfa. Spidermites work from lower leaves upwards so examine lower leaves of plants near the field edge for these minute mites (smaller than soybean aphids) or their feeding marks (minute white spots on leaves called stiples). If spidermites are found along the field edge, check plants further into the field. The presence of spidermites should be considered in selection of insecticide and when scouting for aphids.

Insecticide efficacy differs between soybean aphids and spidermites. Organophosphate insecticides work better against spidermites than pyrethroid insecticides. During the 1988 outbreak, chlorpyrifos (Lorsban) and dimethoate provided excellent control while pyrethroids varied widely in their effectiveness. Some pyrethroid insecticides can "flare" spidermites, i.e., trigger rapid increases in spidermite populations. Flaring is caused by removal of predators that would keep spidermites in check and by stimulatory effects of some pyrethroids on spidermite reproduction. In short, widespread use of pyrethroid insecticide during prolonged drought can trigger or aggravate two-spotted spidermite problems. If a pyrethroid insecticide is used, check the fields in two weeks for spidermite problems, especially if drought persists.

Do aphid infestations differ among planting dates?
Initial colonization reflects proximity to buckthorn and planting date. Populations build up more rapidly in earlier planted fields. As the summer progresses, aphids re-distribute locally and preferences shift toward younger, relatively uninfested fields. Then as long-distance dispersal becomes more prevalent and soybean canopies close, soybean aphids become more equal-opportunity colonizers.

Should thresholds be adjusted for drought conditions?
The current economic threshold (250 aphids/plant average on 80% or more of the plants economic threshold (ET) and economic injury level (EIL) of 4000 cumulative aphid-days were developed from data gathered under drought conditions. Aphids aggravate drought stress on soybean so there's less room for error. Monitor soybeans closely and treat as soon as possible after populations exceed threshold. Removing the damaging aphid populations leaves the soybean crop in a better position to take advantage of later rains.

Dr. Ken Ostlie, Extension Entomologist
University of Minnesota