Soybean Aphids Found in Northeast Nebraska; Scouting Warranted July 28, 2016
This week soybean aphids were found in several northeast Nebraska fields at low numbers. Although it has been too hot for soybean aphids to thrive, populations could quickly increase with cooler temperatures. Scouting is recommended at this time.
On Thursday, July 28 low numbers of soybean aphids, about 5-10 aphids per plant, were found in a Wayne County soybean field. The infestations appeared to be recent as all the aphids were found on the top 1-2 inch leaflets. The field was in early R3, which makes sense as aphids prefer later maturing soybeans (e.g., late planted soybean). Other fields with aphids averaged about 2-3 aphids per plant. Natural enemies of the aphid, such as lady beetles, green lacewings, insidious flower bugs, and other insect predators were found along with the aphids. These natural enemies may help hold the populations in check or at least slow their growth.
While the field should be monitored, it does not require any management action as the aphid numbers are well below the 250 aphids-per-plant threshold and the field has plenty of soybean aphid predators. It does, however, signal the need to begin scouting soybean fields for soybean aphids.
Many of these fields could be considered to be “seeded” with soybean aphid. When temperatures decline, we could see soybean aphid populations increase significantly. In past years we have monitored soybean fields that were almost devoid of aphids in mid-July, but by mid-August were well over 2000 aphids per plant. Given this, it’s time to review soybean aphid biology and management. If you have not started to scout for aphids, start now.
The economic threshold for late vegetative through R5 stage soybeans is 250 aphids per plant with 80% of the plants infested and populations increasing.
Soybean Aphid Description
The soybean aphid is soft-bodied, light green to pale yellow, less than 1/16th 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 aphids can be found on all parts of the plant, feeding primarily on the undersides of leaves, but also on stems and pods.
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 appear to be 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 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 plant to complete its life cycle: common buckthorn and soybean. Common buckthorn is a woody shrub or small tree and is the overwintering host. In the fall soybean aphids lay eggs on buckthorn. 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 and early fall, when winged females and males are produced and 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 so early season soybean colonization by aphids appears to be limited.
Soybean aphid populations can grow to extremely high levels under favorable environmental conditions. Reproduction and development is fastest when temperatures are between 70° and the mid 80°s when populations can double in two to three days. The aphids do not do well when temperatures are in the 90°s, and are reported to begin to die when temperatures reach 95°.
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 are likely the major source of infestations in Nebraska.
Soybean Aphid Natural Enemies
Soybean aphids have many insect predators.The most visible predator is the multicolored Asian lady beetle, but the tiny (1/10-inch long) insidious flower bug (or Orius) is the most common and important predator. It 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 hot July weather. 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 Occurrence in Nebraska
Soybean aphids have been reported in most soybean-producing regions of Nebraska, although the highest and most economically damaging populations typically occur in northeastern Nebraska.
In much of the soybean aphid’s range, northeast and east of Nebraska, significant aphid infestation has often begun during vegetative stages of soybean. These infestations then 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). Nebraska aphid populations can 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). In some years there are many fields where the aphid populations peak in late R5 (beginning seed) to early R6 (full seed). Of course, there are always exceptions to any rule, so one should always be watchful for soybean aphid colonization and population increase.
For treatment recommendations, including products and timing, see Soybean Aphid Management.