Soybean aphid

Soybean aphids on a single leaf, 2002. In Minnesota, as many as 13,000 soybean aphids have been found on a single plant.

June 29, 2007

We haven't found any soybean aphids yet, but I'm sure that they are out there. States further east reported aphids on seedling soybeans earlier this month and in Nebraska, we usually begin seeing them in late June. If you aren't already scouting, now is the time to begin.

Soybean Aphid Description

The 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 species of host plants to complete its life cycle — common buckthorn and soybean. Buckthorn is a woody shrub or tree that 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.

Soybean aphid populations can grow to extremely high levels under favorable environmental conditions. Reproduction and development is fastest when temperatures are in the 70s to mid 80s. The aphids do not do as well when temperatures are in the 90s, and are reported to begin dying when temperatures reach 95°F. When populations reach high levels during the summer, winged females are produced and 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 two years.

 Soybean aphids injure soybeans by removing plant sap with their needle-like mouthparts. Plant symptoms may include yellowed, distorted leaves and stunted growth. 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

Soybean Aphid Occurrence in Nebraska

In much of the soybean aphids range, significant aphid infestation often has 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. This is when soybeans are entering or in R3 (beginning pod stage). A few Nebraska aphid populations reach economically damaging populations in late July, but most reach economically damaging populations in August, while soybeans are 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).

(From left to right) Lady beetle eggs, compared to the size of a penny, lady beetle larvae feeding on soybean aphids, and adult Asian lady beetle. (Photos courtesy of Dr. Chris DiFonzo, Michigan State University. )

Soybean Aphid Natural Enemies

Understanding how natural enemies impact aphid populations is an important component in developing a comprehensive management plan, so the Nebraska Soybean Board funded a project at the Haskell Agricultural Laboratory near Concord to examine the effects of predators and parasitoids on soybean aphid populations. Tierney Brosius, a UNL Department of Entomology graduate student, was placed in charge of this project and has discovered some interesting relationships that add to our understanding of the soybean aphid in Nebraska.

Minute pirate bug nymph

=For one thing, although the most visible soybean aphid predator is the Asian lady beetle, the minute pirate bug (Orius insidiosus) is the most common predator. Found throughout Nebraska, this predator feeds on a variety of small insects and spider mites. The study indicates that naturally occurring predators, primarily the minute pirate bug, can significantly slow soybean aphid population growth, and that initial populations of predators may reduce the rate of successful colonization of the soybean aphid. In addition, the presence of minute pirate bugs causes the aphid population to be distributed throughout the canopy, with a higher proportion lower in the canopy than in soybeans without significant levels of minute pirate bugs.

This has important implications for soybean aphid management. Preserve your natural enemy populations. Premature or unneeded insecticide use can rid your soybeans of these important control agents and leave your field open for aphid colonization and rapid population growth. For example, do not add an insecticide to a glysophate application simply to clean up the field. This can lead to later problems with not only the soybean aphid, but with other pests such as the twospotted spider mite. Always remember to treat only when necessary. Aphids must be present with populations growing and reaching economic thresholds. Also, when scouting for aphids make whole plant aphid counts. A significant number of aphids may be in the middle of the canopy.

Of course, this does not mean we can leave the work to the natural enemies and not scout. When environmental conditions favor soybean aphids, they can overcome the effects of the natural enemies and aphid populations can skyrocket. We will continue to do research to determine the environmental and biological conditions that favor the soybean aphid and its natural enemies.

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