Figure 1. Spotted alfalfa aphid (Photos by Michael Rethwisch)	Figure 2. Cowpea aphid

August 9, 2012

Two species of aphids are being encountered in alfalfa this summer: spotted alfalfa aphid and cowpea aphid. While not usually major pests in Nebraska alfalfa, they are both favored by hot, dry conditions, and thus, their numbers may be higher this year. 

While both aphids can be found causing damage through the fall, especially under dry conditions, the vast majority of what we're seeing are the spotted alfalfa aphids. Both species give birth to live young — sometimes more than five aphids a day — hence their ability to cause damage in a short time. If aphid populations are present or developing, you should monitor populations

Spotted Alfalfa Aphid

The spotted alfalfa aphid is a fairly small (1/8 inch long), pale yellow colored aphid that derives its name from the rows of spots on the dorsal (back) area (Figure 1). It damages alfalfa by sucking out juices from the underside of leaves and injecting a toxin during feeding. Symptoms of feeding can range from puckered leaves to foliage and entire plants being killed by this aphid.

Alfalfa varieties differ in their resistance to this pest, and population thresholds that cause damage differ by both alfalfa variety resistance and amount of regrowth.

Seedling alfalfa stands can be devastated by populations of one to three spotted alfalfa aphids per stem. Thresholds for alfalfa are

• 1-3 spotted alfalfa aphids in seedling alfalfa stands,
• 10 aphids per stem if regrowth is less than 10 inches,
• 30 if alfalfa stems are more than 10 inches, and
• 100 if alfalfa is taller than 20 inches.

With today's higher alfalfa prices, some states have recommended a treatment threshold of 50 spotted alfalfa aphids per stem for alfalfa more than 20 inches tall. Research is necessary to determine if this threshold is accurate under very low moisture conditions such as Nebraska is currently experiencing.

Cowpea Aphid

Cowpea aphids also have been noted the past 10 days. This aphid is shiny black as an adult (Figure 2), has a dull grey color as nymphs, with light colored legs and dark "feet", and black cornicles. This aphid is small and similar to the spotted alfalfa aphid in size. Colonies prefer feeding on newly expanding leaves, unlike spotted alfalfa aphids which are found on leaf undersides.

Cowpea aphids also inject a toxin while they feed and can cause wilting and discoloration.  High populations (more than 100 per stem) can cause severe stunting, dieback, or death. Thresholds for this species are not well established, but existing ones suggest treating when there are

• 5 cowpea aphids per seedling alfalfa stem,
• 40 per stem when alfalfa is less and 10 inches tall,
• 75 per stem when stems are 10-20 inches tall, and
• 100 per stem for alfalfa taller than 20 inches.

When mixed species of aphids are present as is the current situation, use the thresholds for the spotted alfalfa aphid. 

Natural Predators

Ladybeetles (both adults and larvae) have been known to eat many aphids, and alfalfa field scouts need to include their populations relative to spotted alfalfa aphids to determine if insecticide treatments are necessary or if the aphids will be biologically controlled. Insecticide treatments are not considered necessary if there is more than one predator for every 10 aphids.

Michael Rethwisch
Extension Educator, Butler County