June 30, 2006

Soybean aphids found in northeast Nebraska

Also see: Soybean aphid management Benefits of predator insects

Description

The soybean aphid is light green to pale yellow, less than 1/16 inch long, and has two black-tipped cornicles (cornicles look like tailpipes) on the rear of the abdomen. It has piercing-sucking mouthparts and typically feeds on new tissue near the top of soybean plants on the undersides of leaves. Later in the season the aphids can be found on all parts of the plant. It is the only aphid in North America that forms colonies on soybean.

Life cycle

The seasonal life cycle of the soybean aphid is complex with up to 18 generations a year. It requires two different species of host plant to complete its life cycle, common buckthorn and soybean. Buckthorn is a woody shrub or tree and is the overwintering host plant of the aphid. Soybean aphids lay eggs on buckthorn in the fall. These eggs overwinter and hatch in the spring, giving rise to wingless females. These females reproduce without mating, producing more females. After two or three generations on buckthorn, winged females are produced that migrate to soybean. Multiple generations of wingless female aphids are produced on soybeans until late summer/fall, when winged females and males are produced that migrate back to buckthorn, where they mate. The females then lay eggs on buckthorn and the eggs overwinter, completing the seasonal cycle.

Hundreds of soybean aphids can be found on a single leaf.

Soybean aphid populations can grow to extremely high levels under favorable environmental conditions. Reproduction and development is fastest when temperatures are in the 70s through the mid 80s (°F). The aphids do not appear to do well when temperatures are in the 90s, and are reported to begin to die when temperatures reach 95°F. When populations reach high levels during the summer, winged females are produced that migrate to other soybean fields. Like a number of other insect species (e.g. potato leafhoppers), these migrants can be caught up in weather patterns, moved great distances, and end up infesting fields far from their origin. These summer migrants were most likely the major source of infestations in Nebraska during the last couple of years.

Injury to soybean

Soybean aphids injure soybeans by removing plant sap with their needle-like mouthparts. Symptoms of a soybean aphid infestation include yellowed, distorted leaves and stunted plants. A charcoal-colored residue also may be present on the plants. This is sooty mold that grows on the honeydew that aphids excrete. Honeydew in itself makes leaves appear shiny. Soybean plants are most vulnerable to aphid injury during the early reproductive stages.

Begin scouting . . . soybean fields once or twice a week in late June to early July. The current recommended threshold for late vegetative through R5 stage soybeans is 250 aphids per plant (field average) with 80% of the plants infested and populations increasing.

Soybean aphid occurrence in Nebraska

In much of the soybean aphids range, significant aphid infestation has often occurred in the early vegetative stages. These infestations then undergo rapid population growth to reach high populations during the flowering stages (R1, R2). During the last few years in Nebraska, however, very few aphids have been found during the vegetative stages. We find a few in late June to early July, but it is usually mid-July before we begin to regularly find aphids, while soybeans are entering or in R3 (beginning pod stage). During 2003 and 2004, a few Nebraska aphid populations reached economically damaging populations in late July, but most reached economically damaging populations in mid to late August, while soybeans were in the mid-reproductive stages (R4-R5). During 2004 there were many fields where the aphid populations peaked in late R5 (beginning seed) to early R6 (full seed).

Identifying a treatment threshold

Figure 2. Yield loss by peak population, 2004, UNL Haskell Ag Lab, Concord, NE

We did not have enough soybean aphids in 2005 to do a thorough threshold study, but in 2004 we did. A yield trial conducted at the Haskell Agricultural Laboratory at Concord in 2004 examined yield loss from late July infestations of soybeans. It was part of a North Central Soybean Research Program Project that has been repeated in several states. In this study 16 soybean plots (irrigated) were infested with very low numbers of soybean aphids on July 23. The aphid population in these plots were allowed to increase; four additional plots were kept aphid free. The aphid populations peaked on approximately August 30, with average peak populations ranging from 952 aphids per plant to 3,634 aphids per plant. The results indicate that late season infestations can result in at least 20% yield loss (see graph). The major portion of the population curves and all population peaks occurred in soybean stage R5 to beginning R6. The data also indicate that the economic injury levels (EILs) would be approximately 1000 aphids per plant, depending on various factors including management costs and crop value. Because a farmer’s goal should be to keep the aphid populations from reaching the EIL, the economic threshold will be lower than the EIL and should allow the farmer time to set management tactics in motion.

Tom Hunt, Extension Entomologist
NEREC Haskell Ag Lab, Concord
Keith Jarvi
PM Assistant, NEREC, Norfolk

Copyright 2006 by the University of Nebraska Board of Regents. All rights reserved.

Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.

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