Alfalfa Weevil
The Alfalfa Weevil (Hypera postica) is a common insect pest of alfalfa throughout much of the western United States, including Wyoming. It is one of the most important early-season pests of alfalfa because the larvae feed directly on developing leaves, which can significantly reduce forage yield and quality if populations are high.
Adult alfalfa weevils are small, oval beetles about 3/16 inch long with chewing mouthparts located at the end of a long, distinctive snout typical of weevils. Newly emerged adults are light brown and often have a dark brown to black stripe running down the thorax and forewings. As the beetles age, the tiny scales that cover their bodies wear away, causing them to darken to a deep brown or nearly black color.
The larvae are creamy yellow to light green and have a distinct white stripe running down the center of the back. Their heads are dark brown to black. Larvae are very small when they hatch—about 1/20 inch long—but grow to roughly 3/8 inch as they progress through four larval stages, known as instars.
The alfalfa weevil completes one generation per year. Adult beetles overwinter in alfalfa fields, leaf litter near field edges, or natural habitats that may be several miles away. In spring, shortly after alfalfa begins to break dormancy, adults migrate into fields and begin feeding on the new growth.
Egg laying in Wyoming typically begins when alfalfa plants reach about 3–4 inches in height. The female weevil chews a small hole into an alfalfa stem and deposits a cluster of eggs—often around ten—inside the plant tissue. Eggs hatch in approximately 4 to 21 days, depending on temperature. After hatching, the small larvae move to the top of the plant and feed on the developing terminal leaves for two to four weeks.
Young larvae are difficult to detect because they feed inside the protective leaf whorls before the leaves fully expand. This early feeding produces small, scattered pinholes in the leaves. As larvae grow larger, they feed openly on the exposed leaf surfaces between the veins of the upper leaves.
Feeding damage increases rapidly as larvae develop through their four instars. Each successive stage consumes four to five times more foliage than the previous one. As a result, the majority of forage loss usually occurs just before the first harvest, when larvae are approaching their final stage and feeding rates are highest. If large populations reach the final larval stage before the first cutting, significant leaf loss and yield reduction can occur.
Once fully grown, larvae move down to the base of the plant or onto the soil surface, where they spin a small cocoon and pupate. Adult beetles emerge from the cocoon about 10 to 14 days later. These new summer adults may feed for several weeks, but typically do not cause significant economic damage.
By midsummer, usually around the time of the second cutting, adult weevils begin leaving alfalfa fields in search of sheltered areas where they will spend the winter. In most years, little to no egg laying occurs before winter. However, research suggests that in some years, a small number of eggs may overwinter and hatch the following spring, contributing to early larval populations.
Monitoring for alfalfa weevils is commonly done using a sweep net. A standard 15-inch diameter sweep net is passed across the top of the alfalfa canopy once plants reach about 10 inches in height. One full sweep consists of a 180-degree arc from one side of the body to the other. To obtain a representative sample, multiple sets of consecutive sweeps—often ten sweeps per location—should be taken from several areas throughout the field. This method helps determine larval abundance and whether management actions may be needed.
