Although at Low Levels, Soybean Aphids are in Nebraska

By Wayne Ohnesorg and  Thomas Hunt et.al

Soybean aphids have been observed in eastern Nebraska soybean fields, although at extremely low levels. Aphids have not been a widespread problem for soybean fields in the north-central region of the United States for a number of years now. If you have not yet begun to scout for aphids, start now. The good news is that we also observed soybean aphid natural enemies in these fields, so they may hold the populations in check, or at least slow their population growth.

During the last few years, soybean aphids have been at low levels in many Nebraska soybean fields, but they did not reach economically damaging levels in most fields. Soybean aphid natural enemies (predators, parasitoids and pathogens) are essential to maintaining low, non-economic populations of many soybean pests. Insecticide treatments applied prematurely may cause population outbreaks not only for soybean aphid, but also for two-spotted spider mite. Both pests can flare up from eliminating natural enemies. Early treatment followed by cool, mild temperatures may cause soybean aphid populations to surge. Premature treatment followed by hot, dry conditions may cause spider mite populations to surge (more information is available on spider mites in soybean). Preserving natural enemies should always be a goal under an integrated pest management approach, and treating only when pest populations reach economic or treatment thresholds to let them do their work.

Soybean Aphid Description

The soybean aphid is soft-bodied, light green to pale yellow, less than 1/16^th 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 on the undersides of leaves near the top of recently colonized soybean plants. Later in the season, the aphids can be found on all parts of the plant, feeding primarily on the undersides of leaves, but also on the stems and pods.

Soybean Aphid 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. Common buckthorn is a woody shrub or small 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 winged and wingless female aphids are produced on soybeans until late summer and early fall, when winged females and males are produced that migrate back to buckthorn, where they mate. The females then lay eggs on buckthorn, which overwinter, thus completing the seasonal cycle.

Nebraska lacks significant and widespread buckthorn populations, and so early season soybean colonization by aphids migrating from buckthorn appears to be limited.

Soybean aphid populations can grow to extremely high levels under favorable environmental conditions. Populations increase fastest when temperatures are between 70°F through the mid-80s. Aphid numbers can change rapidly. Populations can double in two to three days under optimal conditions in the field. Aphids do not do well when temperatures are in the 90s, where mortality is reported as temperatures reach 95° F. When temperatures drop below 48°F, aphid development stops.

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, move great distances and end up infesting fields far from their origin. These summer migrants are likely the major source of initial infestations in Nebraska.

Soybean Aphid Natural Enemies

There are many insect predators of soybean aphid. The most visible soybean aphid predator is the multicolored Asian lady beetle, but the tiny (1/10-inch long) insidious flower bug (or Orius) is the most commonly occurring and important predator. This is a common predator that feeds on a variety of small insects and spider mites. Naturally occurring predators — primarily the insidious flower bug — can significantly slow soybean aphid population growth, particularly during our hot July weather. Resident populations of predators also help reduce the rate of successful colonization of soybeans by the soybean aphid. Other common predators include green lacewings, brown lacewings, damsel bugs or Nabids, and spined soldier bugs, among others.

Other groups of natural enemies include parasitoids and pathogens. The presence of aphid “mummies” (light brown, swollen aphids) indicates the presence of parasitoids. These mummies harbor immature parasitoids, which will become adults, emerge from the mummy and parasitize more aphids. The presence of “fuzzy” aphid carcasses indicates fungal pathogens are present, which occasionally can lead to dramatic reductions of aphid populations.

Soybean Aphid Injury to Soybean

Soybean aphids injure soybeans by removing plant sap with their needle-like mouthparts. Symptoms of soybeans infested by soybean aphid may 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 by itself makes leaves appear shiny. Soybean plants are most vulnerable to aphid injury during the early reproductive stages. Heavy aphid infestations during these stages can cause reduced pod and seed counts.

Soybean Aphid Occurrence in Nebraska

Soybean aphids have been reported in most soybean producing regions of Nebraska, although the highest likelihood for economically damaging populations is in northeastern Nebraska. 

In much of the soybean aphid’s range — northeast and further east of Nebraska — significant aphid infestation can begin during the vegetative stages of soybean. These infestations can undergo rapid population growth to reach high populations during the flowering stages (R1, R2). During most years in Nebraska, however, very few aphids have been found during the vegetative stages. This may be in part because in Nebraska we have less of the soybean aphid’s overwintering host — common buckthorn — than in states further east and north. We usually 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).

Historically, Nebraska aphid populations have reached economically damaging levels in late July, but more commonly the economically damaging populations occur in August. During this time, soybeans are in the mid-reproductive stages (R4-R5). There have been some instances when the aphid populations peak in late R5 (beginning seed) to early R6 (full seed). Once soybean reaches R6, it takes even greater populations of aphids to be economically damaging. Of course, there are always exceptions to any rule, so one should always be watchful for soybean aphid colonization and population increase.

Treatment

For treatment recommendations, including products and timing, see Soybean Aphid Scouting and Management.