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. A quick look at the calendar reminds me that there's still plenty of time for aphids to reach economic levels. As fields become completely infested, the rate of population increase will become more obvious. A change in aphid numbers from 1 to 2 aphids per plant may seem inconsequential; the change from 10 to 20 noticeable; the change from 100 to 200 dramatic; even though populations have only doubled in each case. 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. Last Friday, observers at several locations in southern Minnesota reported winged females (alates) were common in fields with low-level infestations, suggesting widespread movement is underway. 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.

A snap shot of soybean aphid status across Minnesota this week see the end of this article or check out the Soybean Aphid Web Page at www.soybeans.umn. Just click on the aphid. View what others are seeing and report you own situation by clicking on "2006 Field Observations" tab on the right.

Will the Forecasted Hot Temperatures Control Soybean Aphids?

With temperatures forecasted to reach the high 90s and even over a 100°F this weekend, it's logical to wonder about the effects of hot temperature on soybean aphids. Temperature affects the longevity and reproduction of soybean aphids, but high temperatures will not control aphids Research at constant temperatures by McCornack, Venette and Ragsdale (see Fig. 1) indicates soybean aphids are temperate insects that do better at cooler temperatures. Longevity declines with temperature. The cooler the temperature, the longer aphids survive. Reproduction is also better at cooler temperatures. At 90°F constant temperature, soybean aphids longevity drops dramatically and reproductive success is almost nil.

Research at fluctuating temperatures does not find these dire effects of high temperatures. Why? Daily temperatures fluctuate with nightly lows 20 to 25 degrees less than the high temperature for the day. Transitory high temperatures have little effect as long as nightly lows drop below into the 70s. Brief periods of hot temperatures don't affect the rate at which females produce nymphs, only the survival of the nymphs. Nymphs born in the heat of the day don’t survive well when temps exceed the 90s. The net effect: aphid populations may not increase as quickly under hot temperatures. Hot temperatures will not stop or control aphid populations. Keep in mind that the 2001 outbreak developed under fairly hot temperatures (highs frequently above 90oF) while the 2003 outbreak developed during a cool drought.

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 humidities 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. Steve Commerford, crop consultant, reported one field that required treatment for spidermites already. Soybeans planted into an alfalfa stand and a pyrethroid insecticide application helped create the problem in this field. I hope that this field is not a warning of things to come. The first fields reported 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. Two situations bear special attention when scouting. First, soybeans planted after peas are especially attractive and populations can build very rapidly to economic levels. At least one and possibly two sprays may be needed this year. Second, regrowth after hail damage, provides an abundance of succulent, young foliage that’s extremely attractive to aphids.

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.

Local Reports of Aphid Status in Minnesota

For a quick overview of soybean aphid infestations, see results of the latest MDA soybean aphid survey at: www.mda.state.mn.us/pestsurvey/pestreports/default.htm. These maps are produced from ca. 250 fields sampled around the state during the last week. Keep in mind that aphid populations can vary significantly from one field to the next. These maps, and the comments below, are intended to provide you with a general idea of what’s happening and should never be used as the basis for treatment decisions. There's no substitute for scouting your fields!

Crookston – Aphid populations took a significant jump over the last week with fields now exceeding threshold and others remaining steady. Ian MacRae, Extension Entomologist, reports a field at Northwest Research and Outreach Center with aphid densities in the range of 700-1000 per plant and honeydew coating the pants of scouts. Widespread spraying is occurring.

Moorhead - Soybean aphid populations are increasing with 25 – 30% of fields at or above economic thresholds, confirms Phil Glogoza, Regional Extension Educator. Earlier patterns of higher aphid numbers near woods or in sheltered areas have changed with aphids even infesting larger, exposed fields in the Red River Valley that aren’t even near buckthorn. Aphid levels are less severe on the beaches and farther east.

Fergus Falls – Over the weekend, fields began to reach economic threshold and widespread treatment is beginning from Breckenridge to Underwood. Populations not yet at "Wow" or "Ouch" levels so Doug Holen, Regional Extension Educator, is encouraging growers to scout fields now for timely treatment.

Morris – Aphid populations extremely variable with a few fields over 150 aphids per plant and 3-4 days from treatment. However, the majority of fields have 40-70 aphids per plant, which puts these fields ca. 7 – 10 days from reaching threshold, according to Paul Gronenberg, Centrol crop consultant.

Andover – Drought concerns predominate in farmers' minds on the Sands with aphids at generally low levels, says Krishona Martinson, Regional Extension Educator. Despite initially higher infestation levels a few weeks ago, populations have declined. Brian McCornack, who's conducting aphid research at Becker, finds few aphids in dryland soybeans this summer but aphid populations in irrigated fields at Becker exceeded 1000 per plant last week. These populations dropped to about 100 per plant this week, probably reflecting activity of natural enemies and departure of nymphs that developed wings.

Hutchison – Aphid levels have been increasing with aphids at low levels, reports Dave Nicolai, Regional Extension Educator. Populations in the range of 10-20 aphids per plant are common near Brownton with slightly higher levels 20-30 near Atwater and lower levels (5-15 per plant) near Hector. Higher levels, up to 70 per plant, are occurring on lighter soils and some treatable fields have been reported near Grove City and Monticello says Daryl Ike, crop consultant.

Rosemount – Populations have been increasing steadily, but not dramatically, with 100% of plants infested with 15 to 40 aphids per plant, says Ken Ostlie. Planting dates differ only slightly in aphid density. Arriving alates common (1-2 per plant) last Friday on plants dominated by small nymphs so aphid levels could increase markedly this week.

Lamberton - Aphids numbers climbing even in areas that received heavy rainfall earlier this summer. Worst field at SWROC now at 75-80% of plants infested with 5-35 aphids per plant. Out of the heavier rainfall zone, aphid infestation levels are higher in pockets near Worthington, Pipestone and near the rivers, e.g., New Ulm. Aphids are as uniform as Bruce Potter, IPM specialist, has ever seen.

New Richland – Low levels of aphids throughout the area, reports Mark Bernard, crop consultant. Fields commonly have 50 to 70% of plants infested with aphids and field averages of 2 to 10 aphids per plant. However, alates were common Friday suggesting numbers should increase faster than expected this week.

LeSueur – Soybean aphid levels low in field interiors with aphids concentrated mainly in field perimeters, says Dave Pfarr, Extension Educator. Scattered protected fields near the Minnesota River are higher with scattered treatments occurring.

Rochester – Aphids levels are increasing slowly with field averages in the range of 6 to 50 aphids per plant. Fritz Breitenbach, IPM Specialist, mentions that scattered fields are reaching threshold levels and advises growers to check the status of aphids in your fields.

Albert Lea – Ryan Miller, Regional Extension Educator, reports extremely low aphid levels with aphids not common in fields. Mike Mertens, a local Pioneer dealer near Austin, concurs with the low infestation levels.

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