Alfalfa weevil

Article author: Blayne Reed
Most recently reviewed by: Pat Porter & John Thobe (2021)

Common Name(s): Alfalfa weevil

Description

The alfalfa weevil, Hypera postica, is in the Order Coleoptera (beetles and weevils) and is considered by many to be the most destructive pest in alfalfa. Damaged alfalfa plants show skeletonizing of leaflets and shredding of the tips of new growth, mostly caused by larvae in the spring. Alfalfa weevil larvae hatch from eggs lain in the stems where, after hatching, they will often feed for several days before emerging from the stem. Once emerged from the stem, the larvae move quickly to the newest growth up the plant. These larvae are often almost white or yellowish after hatching, but quickly become green with a white or often yellowish middorsal stripe and a black head capsule. They are approximately 1/4-inch long when full grown. These larvae are legless, somewhat resembling a plump-bodied maggot in body shape, but they have well developed ridges on the underside of the body that take the place of legs. Larvae begin feeding on the new growth of the plant but will move lower down the plant as foliage is consumed. They will typically only be found in field for about a month in the spring, usually before the first cutting but subsequent, prolonged, or otherwise late oviposition can cause larva hatches later in the spring or early summer.

Once the larvae mature, they spin silken cocoons on the plants to pupate, emerging as adult weevils. Adults feed on leaves, stems, and new growth over the summer before leaving fields for sheltered locations but rarely at the level or density they did as larvae earlier in the season, and most of the damage goes unnoticed. Adults exhibit the typical medium sized weevil snout that protrudes downward with chewing mouthparts on the end that the weevils will use to feed with and bore oviposition sites in young alfalfa stems. The adult weevil is typically between 1/8 and 1/4-inch long, light brown in color with a dark stripe along the center of its back that tappers to the end. To many, the lighter portions of the adult weevil’s back resemble racing stripes. The adult’s coloring changes to darker as they age, and the dark center stripe can be difficult to see with the naked eye on older weevils.

: Alfalfa weevil adult.

: Alfalfa weevil larvae.

Origin and Distribution

Alfalfa weevils are not native to the United States. They were likely accidentally imported from southern Europe first around 1900 with the first economic population noted around Salt Lake City, Utah in 1904. Since then, they have spread to all alfalfa growing regions in the contiguous United States. There are differences in populations of alfalfa weevils that once led to the belief that there were two species of alfalfa weevils, but it is now thought that these differences have more to do with alfalfa production differences depending upon location.

Alfalfa weevil damage. Pat Porter.

Habitat & Hosts

Alfalfa crops are the alfalfa weevil’s primary host, but these pests will attack several species of clover. Adults move in and out of fields readily, and alfalfa undergoing establishment can become unexpectedly infested even if there are no alfalfa fields nearby.

Life Cycle

Alfalfa weevils exhibit complete metamorphosis but have one generation per year. The larvae hatch in the spring. This usually happens well before the first cutting, but after the plants begin the new season’s regrowth. In the various growing regions within Texas, this can occur sometime between February in the south of Texas and late April on the High Plains. The larvae feed on new plant growth and skeletonize plants during this time. Unchecked, large populations can easily ruin the first cutting. If the damage is severe enough from this spring infestation, plant stunting and retarded growth can last the entire growing season. Extended or late egg laying can lead to the direct larval feeding of the second and possibly even subsequent cuttings in extreme situations.

Once the larvae mature, they spin silken cocoons on the plants within the curl of dead leaves that make it to the soil in leaf litter. Adults emerge and feed in alfalfa, usually below noticeable levels for several weeks. During the peak summer, adults leave alfalfa for sheltered sites. With colder temperatures, adults return to alfalfa to hibernate or overwinter below the warm soil near the crown of the alfalfa plants. Adults may become active and begin laying eggs at any point in the winter if temperatures permit, particularly in southern production areas. From the moment the eggs are lain, weevil degree days (WDD) begin to accumulate. This can start between December and April with an initiate temperature of 48°F (8.9°C). At around 155 WDD, eggs may begin hatching and by 325 WDD the peak of the 3^rd larval instar stage should be reached.

Management

If you live in the State of Texas, contact your local county agent or entomologist for management information. If you live outside of Texas, contact your local extension for management options.

Please consult your local Texas A&M AgriLife Extension county office for best sampling and local control methods and the Texas A&M AgriLife Extension guide Managing Insect Pests of Texas Forage Crops https://extensionentomology.tamu.edu/files/2018/03/Managing-Insect-Pests-of-Texas-Forage-Crops-ENTO-064-2017.pdf for action thresholds and labeled chemical treatment options.

Sweep nets or beat buckets are often helpful for field scouting of alfalfa for weevils. Thresholds for the alfalfa weevil are based upon the number of larva per stem, larvae per square foot, or larvae per sweep. Economically significant populations of alfalfa weevil larvae often require chemical treatment. Alfalfa varieties that are of a less dormant type can increase egg lay, as can abundant growth in the fall. Early harvest of the first hay cutting is a common control option if hay quality will not be too negatively impacted or if foliage is excessively thick and insecticide coverage could be an issue. However, stubble must be monitored closely, and needed chemical treatments shortly following this cutting are possible. Alfalfa varieties with low-level host plant resistance are also available.