Published on Monday, April 20, 2020
Soybean aphids (Aphis glycines Matsumara) are a piercing and sucking insect that have been affecting soybeans in the U.S. since the early 2000s. Aphids tend to be a problem in late-planted soybean fields during years with dry conditions and moderate temperatures. The insects themselves are small (1/16 in. long), pear-shaped, and yellow-to-green in color. They have black extensions on the body toward the back legs that are often called “tailpipes.” Winds deposit aphids in fields, so the infestation works from the top of the plant to the bottom. Aphids are most damaging in dry field conditions but shy away from heat, so look for them on the underside of leaves.
The life cycle of soybean aphids is complex. Aphid population can ramp up very quickly due to both asexual and sexual reproduction, producing up to 20 generations per year. Individuals may be winged or wingless. Soybean aphids overwinter as eggs on the buckthorn plant where they reproduce sexually for the first few generations in the spring. In mid-June and early July, the females from the later generations develop wings, enabling them to move on winds, eventually depositing into soybean fields.
The winged females are capable of asexual reproduction and give birth to live, wingless aphids, which reproduce just three to seven days after birth. Populations experience exponential growth under ideal conditions. Environmental stress from weather or deterioration of the host plant will result in the next generation being born with wings. Wings allow the young aphids to migrate to areas of fi elds with better conditions. This wave effect that is prevalent later in the growing season is referred to as aphids “moving in.”
The primary driver for yield reduction from aphid feeding is drought stress on plants created from aphids sucking the sap from the phloem. If field conditions have ample moisture, soybeans can tolerate low to moderate aphid populations with minimal yield impact. Moisture stress increases the sensitivity of the plant to aphid pressure, but the aphids themselves will have a population decline in response to high temperatures.
The second yield impact from soybean aphids is a reduction in the plant’s photosynthetic capability due to honeydew (sticky aphid excrement) accumulation on the leaf and subsequent fungal disease. Sooty black mold forms on the sugary honeydew deposits and spreads quickly under warm, humid conditions. The sooty black mold will cause the plants to have a gray-to-black appearance throughout the canopy. Once the mold begins to cover the leaf, sunlight interception and photosynthetic capacity dramatically decline. In recent years the sooty mold hasn’t been as prevalent as in the past.
A farmer should consider control when the yield reduction from the aphids has a higher value than the cost of a treatment or chemical application. Scout proactively starting in the late vegetative growth stages; tracking waves of aphids in your area can help you know when to scout. Scout the same field twice in one week to gauge whether populations are increasing or decreasing. The prevalence of winged aphids, the intermediary alatoid nymph, or large populations of predator insects is a good indicator that the aphid population is on the decline and may not warrant control measures.
Aggressive treatments and blanket recommendations have allowed some soybean aphid populations in northern Iowa and Minnesota to develop resistance to class 3a (pyrethroid) insecticides. To prevent the spread of resistance, implement a rotation of different affective modes of action. Following a responsible management program makes this pest easy to control with minimal economic risk.
Author: Nate Mayer, CCA
Categories: Agronomy, Agronomy Talk