How to Scout for Soybean Aphid

Identification. Soybean aphids (Aphis glycines), are the only aphid usually found in soybeans in South Dakota. They are pear-shaped, 1/16th inch long, and range from pale yellow to lime green in color. On late-season soybeans, some aphids may be pale and smaller and found on lower leaves of the plant. Adult soybean aphids can occur in either winged or wingless forms. Wingless aphids are adapted to maximize reproduction, and winged aphids are built to disperse and colonize other locations. In South Dakota we often see a ‘second wave’ of aphids in early August as winged migrants colonize new fields. Winged soybean aphids have a dark thorax (central body segment). Soybean aphids have piercing-sucking mouthparts that are used to siphon plant sap; visible damage is subtle or unapparent until plants are heavily infested, but yield can still be reduced before this point.

Scouting. Check fields from mid-June through late August, with more intense scouting from mid-July through August. Walk a broad U or X pattern through the field and examine at least 20 to 30 plants, spread out over the field. Aphids can occur in “hot spots” but treatment decisions should be based on a broad sample of randomly-selected plants. A speed scouting method is also available that can reduce the time needed for sampling. It is less accurate and tends to lead to a ‘treat’ decision earlier than conventional scouting, but can save time when scouting demands are high (see UMN Soybean Aphid Sampling website)

During flowering, pod, and seed fill (R1-R5) the decision threshold for treatment is when populations are 250 aphids per plant when at least 80% of the plants are infested and the population is still increasing. Once soybeans reach the R6 growth stage (full seed, when a pod on one of the four top nodes has green seeds that fill the pod to capacity), soybeans are much less susceptible to yield loss from aphids (and, therefore, many more aphids can be tolerated). Thresholds for R6 soybeans have not been developed because natural aphid infestations in this late growth stage are uncommon, but research to develop R6 treatment guidelines is underway.

Producers who are near but not at threshold should consider checking the field again before treatment (3-4 days after the initial treatment decision is made). If aphid numbers have decreased, or are still near the threshold and have not increased noticeably, or if many natural enemies such as ladybeetles are present, producers may wish to delay treatment, as populations can sometimes decline naturally before reaching damaging levels.

Should you adjust your soybean aphid threshold for high crop values?

Many producers may wonder what higher crop values mean for the insect treatment thresholds they’ve used in the past. It’s logical to assume that climbing commodity prices mean lower treatment thresholds, but that’s not always the case. For soybean aphid thresholds in particular, 250 aphids/plant is still a valid guideline for decision-making. This has to do with the relationship between the decision (aka ‘economic’) threshold, the economic injury level, and the damage boundary. The economic injury level is the point where the insect injury justifies the control cost. This does vary with commodity value and control costs. The decision threshold is a lower value – not when economic loss is occurring, but when to make a decision take action to keep a pest population from climbing to the economic injury level. It builds in time to react before the population becomes a problem, and is based on how quickly the population can be expected to grow. The damage boundary is the lowest insect pressure/damage level where yield loss can be detected. It is not logical to have an economic injury level lower than the damage boundary, because no economic injury can occur when no yield is being lost. The average damage boundary for soybean aphid is greater than 4000-5000 cumulative aphid days [corresponding on average with a population greater than 485-600 aphids/plant].

When soybean values were lower the economic injury level ~675 aphids/plant, and the recommended decision threshold was 250 aphids/plant to give a 7-day lead time. We can recalculate economic injury levels for more current values. For example at a value of $12/bushel and a $13/acre treatment cost with a yield expectation of 50 bushels, the economic injury level can be calculated theoretically as 356 aphids/plant. However, this is below the damage boundary where yield loss begins – typically in excess of 500 aphids/plant), and thus is not a valid economic injury level [there is no economic injury when yield is not being lost]. Using a conservative damage boundary of 485 aphids/plant, a decision threshold of 250 aphids/plant can still be used and gives an average lead time of 5 days to arrange treatment before the populations reaches 485 aphids/plant. So if treatment can be made in this window, this decision threshold will still prevent economic loss. The data behind these guidelines were collected over three years in 19 locations under a wide variety of conditions, including moisture stress. In most trials in the region, including threshold field trials in South Dakota, we have not seen a yield return when using a threshold lower than 250 aphids/plant. 

Prophylactic or “insurance” treatments at low aphid levels do not provide good value, because treated fields often see pest resurgence, frequently to levels higher than untreated fields. This can occur because the insecticide has eliminated the beneficial predatory insects that help keep pests in check, or because winged migrant aphids have recolonized the field, or both factors working together. A single well-timed application based on scouting information and thresholds is more economical than two poorly timed applications; and in some years even a single application may not be needed. Another problem with insurance treatments in soybean is secondary pest outbreaks of twospotted spider mites. Spider mites are a particular problem when the weather is hot and dry. Spider mite outbreaks often happen in fields that have previously been treated for soybean aphid. This is because many products (particularly most pyrethroids), can actually flare spider mite populations.

Soybean aphid management

Soybean aphid scouting and managementChemical control. Some pesticides labeled for soybean aphids in South Dakotaa

Resistant varieties. Aphid-resistant soybean varieties are available and are an under-utilized tool in aphid management. We have extensively tested lines containing the resistance genes Rag1 and Rag2 in South Dakota. Though resistant varieties will seldom be aphid-free, these genes typically provide a five-fold to ten-fold or greater reduction in aphid populations compared to susceptible checks. Many companies provide aphid resistant varieties in various maturity groups. Two such providers are Syngenta and, for organic producers, Blue River Hybrids.

Natural enemies. There is a diverse community of natural enemies in soybean, which help suppress soybean aphid colonization and population growth. These natural enemies include ladybeetles, lacewings, pirate bugs, and entomophagous (insect-killing) fungi. One of the most important predators of soybean aphid is the multicolored Asian ladybeetle, Harmonia axyridis. In the absence of these natural enemies, soybean aphid population growth is significantly faster (2–7 times). Pesticides often more thoroughly eliminate natural enemies than aphids, which allows the remaining aphids to rebound quickly without their natural checks.