Description. Newly hatched alfalfa weevil larvae are tiny and yellowish green with black heads. Older larvae also have black heads, but transition to more of a green color. Larvae have a distinct white line down the center of their backs and more subtle white lines along each side. Adult weevils are about a quarter inch (5-7 mm) long and light brown with a broad darker stripe extending down their midline. Being weevils, they have a distinct narrow beak or rostrum extending in front of their heads. Their chewing mouthparts are at the tip of this rostrum.
Life History. Adult weevils overwinter and when temperatures warm up in spring they chew holes in alfalfa stems, where they will lay their eggs. These eggs will hatch in about 200-250 Fahrenheit degree days and newly hatched larvae will move to terminal leaves where they will feed causing small holes. Older larvae will feed on unfurled leaves and complete larval development takes about three weeks. Most mature larvae drop to the leaf litter and spin silken cocoons, emerging as adults in 7-10 days. Adults also feed on alfalfa, but do not appear to cause much damage. Adults leave fields when summer temperatures begin to increase and spend warm months in a type of hibernation.
Damage. Alfalfa weevil larvae defoliate plants and their feeding reduces yield, quality, and stand health. Weevil damage is typically concentrated on the first cutting of alfalfa (in most years, weevil larvae are out of fields by mid-June), but the impact of weevils on the first cutting can negatively influence vigor of the second cutting. Alfalfa weevil damage typically occurs as farmers are planting corn, so it can be easy to ignore. A density of one larva in thirty twelve-inch-tall plants has been estimated to reduce yield by about 3 lbs per acre. It is important to realize that the negative impact of alfalfa weevil on yield decreases with plant height; therefore, one larva in thirty sixteen-inch-tall plants translates to a loss of approximately 0.75 lb per acre.
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