Soybean aphid colonization has been slow to start this year, however, we are beginning to get reports of soybean aphids in southwest Minnesota and a few other states. While we have had only one report of soybean aphids in Nebraska, and it was at a very low number, this is when we generally start finding 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 Aug. 18 was over 2000 aphids per plant in areas of the field that were left untreated.
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.
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.
Soybean aphid populations can grow to extremely high levels under favorable environmental conditions. Reproduction and development is fastest when temperatures are between 70 F and the mid-80 F when populations can double in two to three days. The aphids do not do well when temperatures are in the 90s F, and are reported to begin to die when temperatures reach 95 F.
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.
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.
Symptoms Of Soybean Aphids
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.
There are two methods to scout and determine if an insecticide treatment is warranted: a conventional scouting method using the 250 aphids per plant economic threshold, and a speed scouting method.
Conventional 250 Aphids Per Plant Method:
Begin scouting soybean fields once or twice a week in late June/early July. Check 20 to 30 randomly selected plants in various areas of each field. Aphids are most likely to concentrate at the very top of the plant, although they will move onto stems and within the canopy as populations grow and/or the plant reaches mid to late reproductive stages. If a tree line or woodlot is adjacent to the soybean field, make sure and include a few sampling locations near these areas. Soybean aphids often are found first in fields near wooded areas.
Counting aphids is not as difficult as it may at first seem. First, walk to a random spot in the field. Pull a plant, turn it upside down, and give it a quick scan to see where the aphids are located. Get a feel for what 10 or 20 aphids look like and count by 10s or 20s.
The current threshold for late vegetative through R5 stage soybean is 250 aphids per plant with 80% of the plants infested and populations increasing (see above for details). Thresholds for early R6 have yet to be determined, but are likely in the 400 to 500 aphids per plant range. Insecticide treatment during or after mid-late R6 has not been documented to increase yield.
Speed Scouting Method
This method uses a spreadsheet adapted from a sampling plan developed by the University of Minnesota. (See Enumerative and Binomial Sequential Sampling Plans for Soybean Aphid (Homoptera:Aphididae) in Soybean, E.W. Hodgson, E.C. Burkness, W.D. Hutchison, and D.W. Ragsdale, 2004, Journal of Economic Entomology 97(6): 2127-2136). It is different from conventional scouting in that it relies on the number of "infested" plants.
Plants are considered "infested" when there are 40 or more aphids on a plant. The scout does not have to count or estimate the number of aphids on a plant to determine whether it has reached the threshold of 250 aphids per plant. One simply determines if a plant is infested and enters this in the spreadsheet. The spreadsheet then recommends further scouting or treatment options based on the number of "infested" plants in a given area.
Managing Soybean Aphids
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 should give you 5 to 7 days to schedule treatment before populations reach economically damaging levels. If populations do not increase during this period, you may be able to eliminate or delay treatment. Factors favorable for aphid increase are relatively cool temperatures, plant stress (particularly drought), and lack of natural enemies.
Treatment Timing. The earlier a field is treated, the greater the chance that any surviving aphids can reproduce or that new aphids can repopulate the field. Insecticide treatments will kill many natural enemies, so any aphids that do re-infest a field are not constrained by predators and other natural controls. Even insecticides with a relatively long residual cannot last when an insecticide is applied in early or mid-July, particularly when aphid populations are thriving. If you have to treat early, make sure to closely monitor the field until early September.
Unwarranted insecticide treatments, such as in fields treated well before the threshold was met or treated along with a 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 per 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 per 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.
General Management Guidelines
Look for the presence of aphid natural enemies, such as lady beetles, green lacewings, insidious flower bugs, aphid mummies, fuzzy aphids, and other insect predators. Predators and parasitoids may keep low or moderate aphid populations in check. Often soybean aphids can be found by examining plants where lady beetles are observed.
Take note of winged aphids or "broad-shouldered" nymphs. Nymphs with broad or squared-off shoulders will become winged adults. A magnifying glass is helpful to see the "broad-shouldered" nymphs, but the winged adults are easy to see with the naked eye. If the majority of aphids are winged or developing wings, the aphids may soon leave the field and treatment can be avoided.
If the plants are stunted or covered with honeydew or sooty mold and aphids are present at threshold levels, an insecticide treatment may still be of value but the optimum time for treatment has passed.
Good insecticide coverage and penetration is required for optimal control of soybean aphid because aphids feed on the undersides of the leaves and within the canopy. For ground application use high water volume (≥15 gallons per acre) and pressure (≥30 psi). Aerial application works well when high water volume is used (≥3 gallons per acre).
Several insecticides are labeled for soybean aphid; their rates, preharvest intervals, etc. are listed at http://entomology.unl.edu/instabls/soyaphid.shtml. Pyrethroids have a relatively long residual, and work best at temperatures below 90 F. Organophosphates have a fuming action, and may work well in heavy canopies or high temperatures. Dimethoate is least effective.
Spraying flowering soybean poses a threat to honey bees. Communicate treatment plans to nearby beekeepers and follow label precautions to minimize honey bee kills. When there is concern about honey bees, pyrethroids are the better insecticide choice and spraying late in the day is preferred.
Host Plant Resistance
Plant resistance is another soybean aphid management strategy. Certain soybean cultivars have genetic qualities that prevent them from being heavily damaged by the soybean aphid when compared to other soybean cultivars. These varieties use the Rag genes. Soybean aphids feeding on varieties with this resistance reproduce at a drastically slower rate and are less healthy. However, the soybean aphid can circumvent this resistance. Resistance has been documented to some of the earliest deployed genes (e.g., Rag1) in states to the east. In light of this, growers shouldn't rely on a single method of pest management and should continue to scout fields thought to be resistant.