Scout alfalfa for weevils and aphids

Alfalfa growth is off to the races in many parts of Iowa, and we have heard reports from around the state that insects are very active in alfalfa as well. Field agronomists, especially in southern Iowa, have reported that hay is being cut early due to alfalfa weevil pressure, and they are seeing quite a few aphids as well. If you have not yet done so, consider scouting alfalfa for these pests to ensure that a timely management decision can be made. A sweep net can help you detect both of these pests, but below is more detailed information on how to scout for weevils and aphids.

Alfalfa weevil

Alfalfa weevil adults started becoming active in southern Iowa in late April. By now, eggs have hatched and larvae are active around the state. Adults are ¼ inch long beetles with blunt snouts; thick golden hairs covering the body; and a dark, narrow stripe down the back. The larvae are legless and yellow-green in color with a black head and a white stripe along the middle of the back. Both adults and larvae may feed on alfalfa leaves, but it is the larvae that cause most of the injury. Young larvae typically chew pinholes in terminal leaves, and mature larvae feed between the leaf veins and skeletonize leaves. Heavily infested fields look frosted or silver due to the plant material dying. Typically, injury is worse prior to the first cutting, but larvae may continue to feed on regrowth under windrows.

Once larvae have been observed in a sweep net, sample the entire field to determine plant height and number of larvae. Stop at 5 random locations throughout the field, making sure to get a representative sample of the entire field. At each location, collect six alfalfa stems by breaking them off at the base gently. Measure the collected plants, then shake them into a bucket vigorously to dislodge larvae. Determine the average plant height and total number of larvae found, then use that information to determine whether the threshold has been reached based on control costs and hay value. Look at Tables 1 and 2 in this encyclopedia article. Continue to scout after harvest and on regrowth. If 8 or more larvae per square foot are found or if regrowth is delayed due to feeding, an insecticide may be warranted.

Aphids

There are at least four aphid species that colonize alfalfa in Iowa, and we have heard of at least two species being found right now. In general, you can recognize an aphid because they are soft-bodied and pear-shaped with a pair of cornicles (tailpipes) on the tip of the abdomen. Table 1 shows a summary of the common aphids in Iowa alfalfa, but a more detailed description of the different aphid species can be found here.

Table 1. Common aphids in Iowa alfalfa.

All aphids have a piercing-sucking stylet and feed in the same way. They insert their stylet into plant cells and extract phloem, which often leads to discoloration of plant tissues and stunting. Some aphids may transmit toxins to the plant that can cause vein-banding, dieback, or death, and some aphids could vector plant pathogens. Aphids also excrete honeydew which promotes sooty mold development on leaves, potentially reducing photosynthesis.

In general, most aphids prefer feeding on newly expanded leaves and are more easily found near the growing point (spotted alfalfa aphid prefers feeding in the lower canopy). Populations tend to decline when temperatures are too warm (>90°F). Scouting for aphids could include sweep netting or direct stem counts, but the recommended thresholds are based on stem counts. Count aphids on at least 30 stems and estimate the average number per stem. Consider sampling multiple areas to get a representative sample across the field.

Management

For both alfalfa weevils and aphids, there are a few options to consider before using insecticides to manage the population. Biological control can keep populations in check in most years. Alfalfa weevil larvae have a few natural enemies, and a fungal pathogen provides control in warm, humid conditions. Aphids are readily eaten by many natural enemies, including lady beetles and lacewings. If the field is close to being harvested, early cutting is an effective way to manage populations of weevils and aphids. Harvesting early is preferred to insecticides once alfalfa is at least 16 inches tall.

If the field cannot be harvested early, many insecticides are effective against both aphids and weevils. If an insecticide is being used, especially for aphids, use sufficient spray volume and pressure to ensure contact with aphids on the undersides of leaves and in the lower canopy. Continue to scout the field after application for aphids, weevils, or other pests or for injury to regrowth. Be aware of pre-harvest intervals for insecticides when alfalfa is close to harvest.

Additionally, although Iowa has not yet had confirmed reports of insecticide resistance to either pest, be aware that many western states have documented pyrethroid resistance in alfalfa weevil. Please report to us if a field received an application of a pyrethroid insecticide and control was not achieved.