There have been several reports of pea aphids in alfalfa in Iowa this week. “I heard of one near Dorchester in northeast Iowa, one near Clarion in north-central Iowa,” says Brian Lang, Iowa State University Extension field agronomist in northeast Iowa.
It takes a lot of aphids to reach the economic threshold to justify applying an insecticide treatment, and populations can be spotty across fields, he says. Below are guidelines from ISU publication IPM-58. “As with most insect thresholds and forages…if it is less than a week to harvest, take the harvest—move it up a bit if possible. If it is more than a week from harvest, consider a treatment. The harvest process kills many aphids, but not all. Whether the population rebounds is anyone’s guess. Weather will change. It may be still favorable for the insect, maybe not. Beneficial insects get a start on a lower population of aphids after harvest. However, typical management for second crop alfalfa growth is to treat for Potato leafhopper at just a few inches of alfalfa growth, which would handle any aphid rebound as well.”
In Iowa at least four kinds of aphids infest alfalfa
Aphids are common insects to see in field crops, especially in alfalfa. In Iowa, there are at least four aphid species that can persist on alfalfa. A recent report of pea aphids near Clarion in north-central Iowa from ISU Extension field agronomist Angie Rieck-Hinz prompted ISU Extension entomologist Erin Hodgson to provide the following information on the ISU website. “Learning to distinguish aphids in alfalfa takes a little practice, but is worth knowing for making sound treatment decisions,” she emphasizes.
In general, aphids are soft-bodied and pear-shaped insects with walking legs. The main diagnostic feature of aphids is a pair of cornicles (tailpipes) on the tip of the abdomen. Sometimes the cornicles are highly reduced (e.g., spotted alfalfa aphid), making correct identification more difficult. In addition, all aphids have a piercing-sucking stylet and feed on plant phloem. Table 1 summarizes the four most common aphid species in Iowa alfalfa.
Table 1: Four most common aphid species in Iowa alfalfa
Common aphids that can be found in Iowa alfalfa
Pea aphid: The pea aphid is found throughout North America and is the most common species in Iowa alfalfa. Adults are ¼-inch in length, and body color ranges from light green to yellow, or pale pink. In addition to their relatively large size, pea aphids can be distinguished from other aphids by the dark bands of color on the antennae. Pea aphids are in alfalfa the entire summer, but reproduction is dramatically slowed down when temperatures exceed 90 degrees F. Colonies prefer to feed on stems and newly expanding leaves. Pea aphid feeding may turn leaves yellow and stunt overall plant growth when present in moderate numbers (50 to 100 per stem).
Blue alfalfa aphid: The blue alfalfa aphid can be found throughout the United States, but is not commonly found in Iowa alfalfa. Often blue alfalfa aphids and pea aphids are intermingled on plant stems, but can be distinguished with a few common characters. Adults are 3/16 inches in length and bluish-green in color. Blue alfalfa aphids can be dull or waxy and have uniformly dark antennae, compared to shiny pea aphids with dark antennal bands. Blue alfalfa aphids are most productive during spring and early summer due to mild conditions; these aphids begin to decline when temperatures exceed 90 degrees. Colonies prefer to cluster and feed on newly expanding leaves, but will move down to stems as the leaves mature and become crowded.
Spotted alfalfa aphid: The spotted alfalfa aphid is found throughout the United States and is occasionally found in Iowa alfalfa. The spotted alfalfa aphid is smaller than pea aphid, reaching ? inch in length. This aphid is also distinguished because it is pale yellow with dark spots covering the abdomen. Unlike the pea aphid or blue alfalfa aphid, the spotted alfalfa aphid can successfully reproduce in warm temperatures (greater than 90 degrees). Colonies prefer to feed on the lower portions of alfalfa, including stems, petioles and leaves. Spotted alfalfa aphids transmit a plant toxin while feeding and can cause early leaf drop, distinctive vein-banding or chlorosis.
Cowpea aphid: The cowpea aphid is common throughout the United States and Mexico and is becoming more common in Iowa alfalfa. This small aphid is less than ? inches in length, and is easily distinguished from other aphids in alfalfa because the adults are shiny black and the nymphs are dull grey. The base of cowpea aphid antennae is white, but gradually darkens towards the tip, and the legs are white with dark “feet.” Colonies prefer feeding on newly expanding leaves, but cluster on leaves, blooms and stems. These aphids are most successful during early spring or late fall, and begin to decline when temperatures exceed 75 degrees. Cowpea aphids transmit a plant toxin while feeding. Moderate infestations can cause wilting and discoloration, and heavy infestations (greater than 100 per stem) can cause severe stunting, dieback or death.
Aphid scouting and thresholds for treatment
Aphids excrete a sugar-rich honeydew that can promote a sooty mold and potentially reduce photosynthesis. Heavily infested plants will be discolored and stunted. Some aphids are capable of vectoring plant diseases via persistent or non-persistent (i.e., dirty needle) transmission. Those species that vector disease are considered more economically important because even low aphid densities can reduce quality and yield.
Although aphids are considered secondary pests in alfalfa, sometimes they surpass treatment guidelines (Table 2).
Scouting for aphids in alfalfa is relatively easy, and can be estimated by sweep netting or direct stem counts. Fields should be scouted weekly, especially in the spring and early summer. Count aphids on at least 30 stems or take at least 20 sweeps per field, and average the number of aphids per stem or per sweep. For large fields, consider sampling multiple areas to ensure coverage.
Aphid management recommendations
There are options to consider before using insecticides. Biological control, the use of resistant cultivars, and harvesting will often minimize aphids to tolerable levels in most cases. Fortunately, there are many different natural enemies to aphids. For those fields with consistent aphids, consider cultivars with at least moderate resistance to pea aphid. Insecticides should only be applied if they exceed treatment guidelines outlined by UC IPM website (Table 2). Use sufficient volume and pressure to ensure contact with aphids on the lower parts of the plant.
Table 2: Spray thresholds for aphids in alfalfa