Alfalfa Weevil, Hypera postica
Alfalfa weevil is the most important insect pest of alfalfa in Colorado.
Description and Lifecycle
- Adult alfalfa weevils are small, 0.25-inch, brown-grey beetles, with a dark brown band down the back.
- Adults chew holes in young alfalfa leaves until April, and lay eggs in alfalfa stems. Up to 40 can be deposited in each cavity. Egg hatch occurs two weeks later, and the newly hatched, yellow-green larvae feed within the stem for a few days, and then feed on leaves as they go through their larval life stages.
- At maturity, the larvae are dark green, 0.4-inch long, with a black head and white stripe down the back. Larvae usually be found in the field for one to two months, mostly during May and June.
Mature larvae move down the plant or drop to the ground and spin a fragile, lacelike cocoon attached to debris or the plant. A new adult emerges after 10 to 12 days. In Colorado, adults can be found in the field until early fall, when they move to their overwintering sites.
Clemson University – USDA Cooperative Extension Slide Series , Bugwood.org.
Adult alfalfa weevil.
Frank Peairs, Colorado State University, Bugwood.org
Alfalfa weevil larvae.
Phil Sloderbeck, Kansas State University, Bugwood.org.
Alfalfa weevil damage.
- Alfalfa weevil is the most important insect pest of alfalfa in Colorado and both larvae and adults are pests of alfalfa
- Adult alfalfa weevil are small, 0.25-inch, brown-grey beetles, with a dark brown band down the back
- At maturity, the larvae are dark green, 0.4-inch long, with a black head and white stripe down the back.
- Alfalfa weevil can be controlled via crop management practices, biological control, and chemical control.
- Resistant varieties or early harvest can reduce weevil populations
- Insecticide treatments can be effective, but should be used cautiously to protect pollinators
- Both larva and adult stages of alfalfa weevil cause injury to plants.
- Adults and larvae chew holes in leaves and stems of alfalfa, causing ragged leaf damage and skeletonizing of plants.
Newly hatched larvae will consume the internal stem tissue that they were laid into before moving to the external portions of the plant.
Alfalfa weevil can be controlled via crop management practices, biological control, and chemical control.
- To scout for alfalfa weevil, sweep the crop 20 times in multiple locations with a standard sweep net after 148 degree days (48°F) have accumulated. A simplified economic threshold is an average of 20 larvae per sweep.
- A more precise economic threshold can be determined by collecting 30 stems from the field and carefully counting the number of larvae per stem. This number, in combination with the size of the plants, the value of the hay, and the cost of treatment can be used as an economic threshold using the table found at: https://extension.psu.edu/alfalfa-weevil
- Early harvest can reduce weevil populations if an infestation exceeding economic threshold develops during the bud or early bloom stage
- Several natural enemies, such as lady beetles and parasitic wasps, of the alfalfa weevil can be found in Colorado.
- Insecticide treatments can be effective, but should be used cautiously to protect pollinators and avoid triggering pea aphid outbreaks, particularly when aphid-susceptible alfalfa varieties are grown
- Insecticides should be used only when economic threshold is reached
Timing of cutting can help reduce weevil problems. Early cutting can reduce alfalfa weevil larvae. Insecticide treatments are expensive and can be hazardous to pollinating insects when not used carefully. For these reasons, use insecticide treatments only when it is necessary.