Soybean Aphid Populations Late in Season

BY Dairyland Seed Agronomy Team

With some reports of aphid populations reaching actionable threshold levels of greater than 250 aphids per plant and considering this pest’s complicated lifecycle with rapid reproduction rate, as well as known resistance to pyrethroid insecticides, we felt an update was in order as the season winds to a finish.

The soybean aphid (Aphis glycines) is a native of Asia and was first documented in North America in Wisconsin in July 2000. Since, it has spread to nearly all soybean producing states. The highest risk for economic yield loss occurs in all states within the Dairyland Seed footprint, where cooler northern temperatures and adequate moisture can sustain rapid aphid development. In addition to geographical considerations, soybean fields at highest risk include those planted late and those that experience hot, dry weather that stresses the crop.

As with all aphids, the soybean aphid has a piercing-sucking stylet mouthpart. There are two types of soybean aphid, wingless and winged adults.

Wingless aphids are 1/16 of an inch long and have a typical pear-shaped body. They are bright yellow-green with dark eyes and black cornicles, like tailpipes, at the end of the abdomen. They have pale legs, antennae, and small appendage on the tip of the abdomen.
Winged aphids have a dark head and thorax. They have two pairs of clear wings that extend well past the end of the abdomen.

To understand how to scout for damage and manage soybean aphid infestation, it is important to understand its lifecycle. The soybean aphid alternates between a winter and summer host. Eggs overwinter on the buckthorn plant and egg hatch is synchronized with buckthorn bud burst in the spring. A few wingless generations are produced before winged adults are formed. Then, spring migrants fly to emerging soybean fields during May and June. Depending upon temperature and moisture, 15 to 18 generations are possible on soybean. During the summer, there is a mixture of wingless and winged adults. Aphid crowding, plant quality, and the presence of natural enemies may prompt winged aphids to be formed. Long distance migration can occur because the aphids move with jet streams. As soybeans mature and day length decreases, winged soybean aphids move back to buckthorn plants to mate and in the fall, deposit eggs, which overwinter. Nymphs and adults feed on plant sap in the phloem of all above-ground plant parts, which removes plant nutrients.

Heavily infested plants may be discolored or wilted. Severe aphid infestations can cause flowers and small pods to abort. Prolonged aphid feeding results in large amounts of cast skins and excreted honeydew on all above-ground plant parts. Honeydew is shiny and sugar rich, and it can promote sooty mold growth. The combination of aphids removing plant nutrients and mold-covered leaves can result in a yield reduction of up to 40 percent when experienced early in the season. Stunting of infested soybean plants can occur if left untreated. Aphids are also able to transmit viruses to soybean plants.

Soybean aphids prefer the newest soybean foliage, such as expanding trifoliates, and therefore, generally tend to colonize late planted soybean. Ants and lady beetles are good indicators of developing aphid colonies. Also look for white shed skins that are found after aphid’s molt. When scouting, look for bloated or fuzzy-looking aphids. They may be infected with a fungal pathogen that wipes out a colony in a few days. Along with bloated aphids, the presence of lady beetles and other natural predatory invaders help reduce population pressures. Higher temps and heavy rain events can also aid in diminishing populations as well.

Late season controls for this pest are limited and insecticidal-resistant populations to pyrethroid chemistries have been documented in some geographies. Insecticides labeled in different states can vary. Please check with your local state pesticide database for a current listing of insecticide products registered for aphids on soybean in your state. See this fairly recent NDSU update on resistant populations and control advice. https://www.ndsu.edu/agriculture/ag-hub/publications/management-insecticide-resistant-soybean-aphids Management strategies this late in the season, especially insecticidal control, may not provide the ROI needed to cover the cost of application. And, may be bumping up against preharvest intervals for those chemistries.