Spraying for Aphids in Soybeans: What You Need to Know

For over a decade, spraying for soybean aphid control has become a standard practice for many growers. However, there’s still plenty of debate about the best way to manage these pests. To make informed decisions on how to handle aphids, it’s important to understand their lifecycle and spread.

The Origins and Spread of Soybean Aphids:

• Soybean aphids were introduced to the U.S. from Japan in 2000 and by 2004 had spread across much of the Midwest.
• Aphids overwinter as eggs on buckthorn plants in wooded areas. When temperatures hit 50°F in the spring, they undergo a few reproductive stages.
• As buckthorn begins to flower (typically in early to mid-June, though timing depends on the weather), aphids undergo another generation change, where some develop wings and begin migrating to soybean fields.
• Once in soybean fields, aphids go through about 15 reproductive cycles, with each cycle taking anywhere from 2 days to 2 weeks, depending on temperature. Since aphids reproduce asexually, their populations can double in as little as 2 days.
• Although most aphids move only a few feet in their lifetime, some will develop wings and can travel up to 6 miles per day as their population increases.

Natural Predators of Aphids:

Soybean aphids have several natural enemies, including:

• Asian lady beetles
• Parasitic wasps
• Entomophthora fungi, which can also kill aphid colonies.

While aphids thrive in warmer temperatures (ideal range: 77-82°F), they can be killed if temperatures exceed 95°F. Natural predators are effective at managing low aphid populations (up to about 25–50 aphids per plant). This is why aphid outbreaks typically don’t become widespread until late July or early August. However, when insecticides are used to control aphids, these beneficial insects are often killed, allowing aphid populations to rebound quickly.

Aphid Damage and Yield Loss:

Aphids use their needle-like mouthparts to pierce soybean plant tissue and suck out sap. This feeding damage can also facilitate the transmission of various plant diseases. The real threat from aphids is the yield loss they cause, especially in drought-stressed areas. Aphid infestations can reduce yields by up to 50%, depending on how early and severe the infestation is.

When to Spray for Aphids:

There is some disagreement on the ideal aphid population threshold for spraying, but most experts agree that spraying should be considered before the R5 or R6 growth stages, particularly in drought conditions. University research suggests it’s not economical to spray unless there are at least 250 aphids per plant. South Dakota State University (SDSU) has introduced the "Aphid Days" concept, where 25 aphids per plant for 10 days equals 250 aphids for one day. This approach helps account for the fact that aphid populations can rapidly recover after spraying.

Aphid Control Methods:

There is one main way to manage aphids:

• Insecticide Application:
  • You can apply either residual or non-residual insecticides. The key is understanding the withdrawal period—some insecticides have a 45-day waiting period before harvest.
  • It’s important to note that resistance to certain pyrethroid insecticides has been documented in some areas.

Conclusion:

While aphids remain a persistent challenge for soybean growers, understanding their lifecycle and the options available for managing them can help minimize damage. By monitoring aphid populations, using insecticides wisely, and considering aphid-tolerant varieties, you can protect your crop and maintain healthy yields.

Resources:

https://ag.purdue.edu/department/entm/extension/field-crops-ipm/soybean/soybean-aphids.html