Department of Entomology
Entomological Notes

Aphis glycines

The soybean aphid is a new pest of soybeans in the United States. The pest was first identified in Wisconsin in 2000 and 2001 in Pennsylvania. However, populations high enough to warrant treatment were not observed in Pennsylvania until 2003. Although very few fields in Pennsylvania required treatment in 2003, over 3 million acres of soybeans in Minnesota were treated. Because of Minnesota's experience with the pest, the soybean aphid is considered a potential major threat to Pennsylvania soybean fields.

DESCRIPTION

Figure 1. Click on image for a close up of wingless soybean aphid.

The soybean aphid (Aphididae: Aphidinae) is a small (1/16 in.) greenish-yellow aphid with reddish-brown eyes (Figure 1). It has a plump oval-like body with two black cornicles at the top rear of its abdomen. The winged forms have two pairs of transparent delicate wings. The nymphs are wingless and look identical to adult females but only smaller in size.

LIFE CYCLE

The soybean aphid has a complex life cycle with two different reproduction phases. During late summer and fall, aphids migrate from soybean fields to buckthorn shrubs and reproduce sexually. Mated females lay overwintering eggs on buckthorn buds. After hatching in the spring, the aphid completes 2 to 3 generations on buckthorn and then it migrates to soybean plants.

However, the soybean aphid has not been documented to overwinter in Pennsylvania. Our populations may result from Midwest migrants that are carried along with major storm fronts moving east out of the Midwest. In any case, it is the summer asexual reproductive phase that is responsible for soybean injury.

During the summer phase all individuals are asexually, live bearing females that can be winged or wingless. The first females to produce numerous nymphs in a soybean field are called stem mothers . Stem mothers are easily identified; they usually have a number of smaller nymphs (their offspring) surrounding them. Offspring during this period are predominantly female. Each female produces, on average, 45 offspring, over a 15-day period (at 77 ºF). At 77 ºF, the newly born nymphs develop into reproducing females within 5 to 6 days. Newly born nymphs begin feeding immediately.

In 2002 and 2003, soybean aphids were first observed on soybeans in Central Pennsylvania during early- to mid-July. Populations then increased about 10-fold each week and reached their highest density about the fourth week after first detection. At which time, the population rapidly collapsed and did not recover.

DAMAGE SYMPTOMS

During the vegetative stages (V1 to V7), the soybean aphid is commonly found on the top portion of the plant and feeds on the underside of newly emerged or emerging leaves. As the aphid population increases and plants enter the early reproductive stages (R1 - R3), aphids are found feeding throughout the whole plant. In laboratory studies aphids prefer cooler temperatures (77 ºF). However in the field, the plant canopy protects aphids from inadequate environmental factors, such as rain and hot weather, increasing their survival.

Soybean aphids feed on the sap (phloem) of soybean plants causing water stress. Rainfall patterns, therefore, influence the number of aphids needed to cause the plant to become water stressed. High soybean aphid infestations that occur during early reproductive soybean growth stages (R1-R3) can stunt the plants causing reduced pod and seed counts. Later in the season, highly infested plants exhibit yellow distorted leaves and can have a charcoal-colored residue on older leaves and stems. This charcoal-colored residue is the sooty mold growing on the honeydew excreted from the aphids. Yield reductions of 20% or more can result from high soybean aphid infestations, particularly in dry years or sites.

MANAGEMENT

Soybean aphid management requires field scouting to assess aphid and natural enemy population levels and the use of economic thresholds (ET). Few studies have been conducted to assess the impact of aphid population under variable crop populations, row spacing, and timing of infestation, but limited research has provided an estimate for an ET. The economic threshold currently accepted is 250 aphids per plant. This threshold is designed to provide a one-week window to take action before the economic injury level is reached (ca. 2,500 aphids per plant). Research is currently underway to see if this ET is appropriate for Pennsylvania. Until this research is completed, however, the above value is a reasonable approximation.

In 2003, research conducted under ideal moisture conditions at the Russell E. Larsen Research Center indicated no impact on yield with aphid populations up to 4,000 per plant. In 2003, the Midwest experienced moisture deficiencies during the period that soybean aphids were causing damage. Therefore, it is likely that the impact of the aphid will depend on moisture conditions.

Based on current knowledge a decision to treat a field should be made, if the economic threshold is reached and less than 1 ladybird beetle per plant is observed. However, if ladybird beetle numbers exceed one per plant and aphid populations are at 250 aphids per plant or below, the aphid populations will likely collapse and treatment will not be necessary. It should be noted, that one Asian ladybird beetle per plant can eat 160 aphids per day (see Figure 3).

Adult

Immature

Figure 3. Adult and immature stages of the Multicolored Asian Ladybird beetle.

For soybean aphid scouting and Economic Threshold information visit website: http://www.soybeans.umn.edu/crop/insects/aphid/aphid_sampling.htm

For currently recommended insecticides, see the Agronomy Guide .

Warning

Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.

Authors: Wilma Aponte, Masters Student and

Dennis Calvin, Professor of Entomology

July 2004

DISCLAIMER

This publication is available in alternative media on request.

Where trade names are used, no discrimination is intended and no endorsement by The Pennsylvania State University or Pennsylvania Department of Agriculture is implied.

Entomological Notes are intended to serve as a quick reference guide and should not be used as a substitute for product label information. Although every attempt is made to produce Entomological Notes that are complete, timely, and accurate, the pesticide user bears the responsibility of consulting the pesticide label and adhering to those directions.

Issued in furtherance of Cooperative Extension Works, Acts of Congress May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture and the Pennsylvania Legislature. D. Jackson, Director of Cooperative Extension, The Pennsylvania State University.

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