Depending on your geography, third-cutting alfalfa is either near or has already been completed. Periods of warm, dry weather and some early south winds have led to widespread reports of high bug pressure in alfalfa fields.
    Many types of insects are found in alfalfa. The beneficial insects include pollinators, parasites and predators. Other insects, which feed on the leaves, stems, crowns, seed pods and flowers, can cause economic loss when they become abundant.
    Alfalfa weevil is an early-season alfalfa pest in the Midwest. They are most damaging in the larvae stage as they can skeletonize leaves to the point that only leaf veins or holes remain. This causes stunted plant growth and reduced forage yield and quality. A larva is yellow to green with a black head and white stripe along its back.
    Young larvae feed on the leaf buds and between folded leaflets in the plant terminal. Older larvae feed mostly on open leaflets, but they also feed on the terminal buds. From a distance, the foliage damage appears grayish to white. Mature larvae then drop to the leaf litter and spin silken cocoons. The adult weevils emerge from the cocoons in one to two weeks. The new adults emerging in late spring feed for a short period and then leave the alfalfa field to go to nearby protected areas for a summer resting period, returning to the fields again in the fall. The adults also feed on the leaves but cause less damage.
    The first cutting is at the greatest risk for alfalfa weevil damage. However, in some years such as 2020, the larvae and adults may threaten subsequent cuttings and require treatment.
    The potato leafhopper is perhaps the most serious insect pest of alfalfa in the Upper Midwest. Although it is a native species, PLH cannot survive northern winters, so adults migrate into the Midwest each spring. Leafhoppers can sometimes be confused with other small, soft-bodied insects such as aphids, but leafhoppers have elongated bodies and tend to be very mobile compared to aphids.
    Leafhopper populations frequently increase to damaging levels on the second and subsequent alfalfa crops. They can be especially damaging on new alfalfa seedings, particularly near weedy areas where they can have larger populations. Once PLH colonize alfalfa fields, adults deposit eggs into stems and leaf veins. In warm weather, like this summer, these eggs will develop into adults in about three weeks, so populations can increase quickly.
    PLH have piercing-sucking mouthparts and extract plant juices. Heavy feeding disrupts nutrient flow within plants, causing V-shaped yellow triangles to form at the leaflet tips (hopper burn), but this evidence of damage does not develop until seven to 10 days after feeding begins. As feeding continues, damage gets worse and the chlorotic areas spread toward the base of the leaflet. Once hopper burn is evident, economic loss has occurred.
    Until recently, crop scouting and insecticide application when warranted was the only effective means of PLH management. However, development and marketing of glandular-haired alfalfa varieties appears to be changing the way we manage this pest and may ultimately alter the very pest status of the insect on alfalfa.
    Aphids are soft-bodied, slow-moving insects that live in colonies and suck plant sap from stems, leaves and terminals. Three primary species of aphids feed on alfalfa in the Midwest: pea aphid, cowpea aphid and spotted alfalfa aphid. Infested plants turn yellow and wilt, and their growth can be stunted. Honeydew can accumulate on the leaves and stems with certain species. Predatory insects and parasites, aphid diseases and weather conditions often keep aphid numbers low. However, aphids have a high reproductive rate and can increase very rapidly under favorable conditions.
    The most common aphid species is the pea aphid. The adults are bright green with long legs. They are about one-eighth inch long, making them largest aphid species found in alfalfa. Only the tip of each antennal segment is black, differentiating it from other aphid species. Pea aphids congregate in dense colonies along the stems, terminal shoots and leaves. Often, crops are damaged the most in the spring.
    Each week during the growing season, check for insect activity in at least four or five spots in each field. Pest populations can vary across fields, and your results may be inaccurate if you check only a limited area. Sample away from the field edges and avoid sampling when the fields are wet from rain or dew, which makes it difficult to detect insects.
    Once physical signs of damage are present in an alfalfa field, you may already be behind the eight ball in terms of treatment. Work closely with your agronomy team to develop an effective scouting plan. Your cows and pocketbook will appreciate the improved alfalfa quality and yield.
    Barry Visser is a nutritionist for Vita Plus.