The soybean aphid (Figure 1) is a relatively new insect pest that has quickly become a primary pest of soybean in the United States. Soybean aphids were first discovered in Wisconsin in 2000 and have since spread to most soybean growing areas of the United States. My first encounter with soybean aphids was in Kossuth County, Iowa, during July 2001.
Soybean aphids are tiny, soft-bodied insects that are about the size of a pinhead. They are yellow-green in color, have black cornicles, and have winged and wingless forms. The wingless form is most common and usually found on the underside of younger soybean leaves in the upper plant canopy. If soybean aphid populations become large, they may also be found on soybean stems and pods. The winged form is produced when migration becomes necessary.
The life cycle of the soybean aphid is complex and involves sexual and asexual reproduction along with the requirement of two specific hosts, buckthorn, which is a common woody shrub, and soybean. In the fall, winged male and female soybean aphids migrate to buckthorn plants where they mate and the females lay very winter-hardy eggs. This soybean aphid generation is the only generation that is reproduced sexually during the year; all other generations will be asexually reproduced with the females giving live birth to offspring.
In the spring, the eggs hatch and 100% female wingless soybean aphids emerge. These aphids undergo three to four wingless generations on buckthorn, depending on the environmental conditions, before a winged generation is reproduced that migrates to soybean.1 Once on soybean, the aphids produce 15 to 17 more generations consisting mostly of wingless females along with some winged females that will migrate to other places in the soybean field or to other soybean fields.1 In the fall, a winged generation of males is produced that flies back to the buckthorn where they are joined by a winged generation of females, starting the cycle over again.
Soybean aphids cause damage to soybean plants by using their piercing-sucking mouthparts to feed on plant sap. As they feed, the soybean aphid excretes a sugar-rich substance called honeydew, which can accumulate on plant tissue and stimulate the growth of a sooty mold that can interfere with soybean photosynthesis. Their feeding activity can be particularly harmful if the soybean crop is under moisture stress. Viral diseases, such as soybean mosaic virus, can also be transmitted during feeding.1
Because of the numerous generations they have during the growing season, their populations can grow rapidly and cause economic damage to the soybean crop. However, populations of soybean aphids may not reach the economic threshold every growing season or at the same time during each growing season, so scouting is the best way to monitor them.
The most critical time to scout for soybean aphids is during the reproductive stages R1 (beginning flower) through R5 (beginning seed); however, an insecticide treatment may be necessary into the R6 (full seed) reproductive stage if populations are increasing rapidly at that time.1 When scouting for soybean aphids, make sure to look for the presence of winged aphids and natural enemies. The presence of winged aphids may serve as an indicator that the soybean aphid population is preparing to migrate to another location, and the presence of high populations of natural enemies such as lady beetles, pirate bugs, damsel bugs, syrphid flies, lacewings and aphid mummies, caused by parasitoids, may be enough to keep soybean aphid populations from reaching the economic injury level.
There are two common methods used to scout for soybean aphids. With the first method, the entire field is evaluated using a “W” or “Z” scouting pattern. A minimum of 38 soybean plants are examined for every 50 acres and the estimated number of soybean aphids on each plant is assessed.1 Once the scouting is complete, the estimated average number of soybean aphids per plant is determined and compared to the economic threshold, which is an average of 250 soybean aphids per plant on at least 80% of the plants. When scouting, be on the lookout for soybean aphid look-alikes and avoid counting them. In my experience, the two insects most frequently mistaken for soybean aphids in my area are the potato leafhopper and the soybean thrips. These insects are tiny like the soybean aphid and are commonly found on the underside of soybean leaves, but they will readily move when disturbed, unlike the soybean aphid.
The second scouting method is called “Speed Scouting,” and it was developed by researchers at the University of Minnesota. Speed Scouting involves randomly selecting plants across a soybean field and checking them for soybean aphids. Soybean plants that are considered infested have a minimum of 40 soybean aphids on them, whereas soybean plants that are considered uninfested have fewer than 40 soybean aphids on them. Once 11 plants have been examined, a decision can be made to either not treat the field and rescout in seven to 10 days, continue sampling another five plants, or to treat the field and recheck the performance of the insecticide three to four days after application. Speed Scouting is my preferred method because it takes less time to reach a decision. You can download the Speed Scouting form and instructions at the following link: https://www.ent.iastate.edu/soybeanresearch/files/page/files/2017_speed_scouting_blank_form.pdf
Once scouting is complete, if the economic threshold is reached or a decision to treat has been made, producers may choose to manage the soybean aphid population with a foliar insecticide. There are many foliar insecticides labeled to manage soybean aphids, and the insecticide used should be selected based on multiple factors such as efficacy, tank-mix compatibility, residual activity, environmental safety (for humans and nontarget organisms), price, preharvest interval, etc.
Pyrethroid-resistant soybean aphids have been found in Iowa, Minnesota, North Dakota and South Dakota.1 To help combat soybean aphids from becoming resistant to insecticides, it is recommended to treat them using the full labeled rate of an insecticide product consisting of a single mode of action rather than tank mixing partial rates of insecticide products consisting of multiple modes of action.1 If it becomes necessary to retreat the field for soybean aphids, use an insecticide that consists of a different mode of action than the first insecticide that was sprayed.
For more information on soybean aphid, please see Channel® Agronomy ADVICE, Soybean Aphid.
1Hodgson, E. and Koch, R. 2018. Soybean aphid field guide (2nd edition). Publ. IPM 0060. North Central Soybean Research Program, Ankeny, IA