Soybean aphid season under way
University of Nebraska researchers are beginning to get reports of soybean aphids in southwest Minnesota and South Dakota. The numbers are low, but relatively widespread.
There have only been a couple reports of soybean aphids in Nebraska, again at very low numbers, but this is when we generally start to find them.
Start scouting now, as populations can start late and build fast. In 2011 we monitored a soybean field in Dixon County that was almost devoid of aphids on July 22, but by August 18 was over 2000 aphids per plant in untreated field areas.
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 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 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 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. Reproduction and development is fastest when temperatures are between 70° and the mid 80°s. Aphid numbers can change rapidly (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°. When temperatures drop below 48°, 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, moved great distances, and end up infesting fields far from their origin. These summer migrants likely have been the major source of infestations in Nebraska the past several years.
Soybean Aphid Natural Enemies
The soybean aphid has many insect predators. The most visible 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. 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. Resident populations of predators also help reduce the rate of successful colonization of soybeans by the soybean aphid. Other common predators include green lacewing, brown lacewing, 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 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 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 northeast Nebraska.
In much of the soybean aphid’s range, significant aphid infestation often has occurred in the early 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 states further east and north do. We usually find a few aphids in late June to early July, but it is usually mid-July, when soybeans are entering or in R3 (beginning pod stage), before we begin to regularly find aphids.
While Nebraska aphid populations can reach economically damaging populations in late July, it usually occurs in August, when 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.
Soybean Aphid Management
The current recommended economic threshold for late vegetative through R5 stage soybeans is 250 aphids per plant with 80% of the plants infested and populations increasing. Depending on economic conditions, this generally gives you about five to seven days to schedule treatment before populations reach economically damaging levels. If populations do not increase during these five to seven days, you may be able to eliminate or delay treatment. Determining if the aphid population is actively increasing requires several visits to the field. Factors favorable for aphid increase are relatively cool temperatures, plant stress (particularly drought), and lack of natural enemies.
Soybean aphids in Nebraska usually reach the economic threshold and require treatment in late July through August, with a few fields requiring treatment earlier in July. Treatment during this period usually is enough to keep aphid populations from resurging because there is not enough time for populations to build up before they would naturally leave the fields in late August and early September.
The earlier a field is treated, the greater the chance that any surviving aphids can later reproduce or new aphids can repopulate the field. Also, insecticide treatment will kill many natural enemies, reducing control from predators or other natural means. Even insecticides with a relatively long residual cannot last when insecticide treatment is done in early or mid-July, particularly during a year when aphid populations are thriving. If you have to treat early, make sure to closely monitor the field until early September for a resurgence of aphids.
Another practice that can result in aphid population resurgence is unwarranted insecticide treatment, either because fields were treated well before the threshold was met or fields were treated along with an herbicide (in some cases a fungicide), regardless of aphid presence. These treatments kill natural enemies and are usually done relatively early so there is plenty of time for aphids to resurge, or re-colonize a field.
Aphid populations below or even at the economic threshold do not cause yield loss, so treating before populations reach 250 aphids/plant only increases the probability of aphid resurgence. In addition, we have observed that many fields support a non-increasing, low population of aphids (e.g., less than 100 aphids/plant) through August. Treating these fields would be a waste of time and money.
Tank-mixing insecticides with glyphosate or other herbicides can be problematic because application methods for herbicides (e.g., lower pressures, large droplet-producing nozzles) are not optimal for good insecticide efficacy. Tank-mixing with fungicides can be effective because application methods for fungicides and insecticides require high water pressure for adequate penetration and coverage; however, only conduct this practice if soybean aphid thresholds are met.