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Scout early, often for alfalfa bugs

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A mild winter, periods of warm, dry weather and early south winds have led to reports of high bug pressure in alfalfa fields.

Many types of insects are found on 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

The alfalfa weevil is an early-season alfalfa pest in the Midwest. They are most damaging in the larvae stage as they can skeletonize leaves so only leaf veins or holes remain. This causes stunted plant growth and reduced forage yield and quality. Larvae are yellow to green with black heads and a white stripe along the 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. 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, larvae and adults may threaten subsequent cuttings and require treatment.

Potato leafhoppers

The potato leafhopper is perhaps the most serious alfalfa insect pest in the Upper Midwest. Although it’s a native species, PLH cannot survive northern winters, so adults migrate into the Midwest each spring. PLH can be confused with other small, soft-bodied insects such as aphids, but PLH have elongated bodies and tend to be 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 in alfalfa fields, adults deposit eggs into stems and leaf veins. In warm weather, these eggs develop into adults in about three weeks, so populations can increase quickly.

Leafhoppers 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.

Historically, crop scouting and insecticide application when warranted was the only effective means of PLH management. However, glandular-haired alfalfa varieties are having an impact on how we manage this pest.

Aphids

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 rapidly under favorable conditions.

The most common aphid species is the pea aphid. Adults are bright green with long legs. They are about one-eighth inch long, making them the largest aphid species found on 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. Damage is often most prevalent in the spring because pea aphids like cooler temperatures.

Scouting

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 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 in terms of treatment. Early cutting can be a management strategy depending on bug severity relative to alfalfa growth. Some bugs find it difficult to survive once vegetative foliage is removed, especially under sunny conditions.

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.

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