Scouting Short Alfalfa for Alfalfa Weevil

By Ashley Dean and Rebecca Vittetoe

The cooler temperatures this spring have slowed down alfalfa growth, but alfalfa weevil could already be active based on growing degree day (GDD) accumulation since January 1. We recommend beginning to scout for alfalfa weevils once 200 GDD have accumulated (base 48°F) south of I-80 or 250 GDD have accumulated north of I-80.

One common question that comes up when we get cold spells during the spring months is “How cold is too cold?”. Alfalfa weevils are cold-tolerant insects. Once temperatures fall below 45°F they become inactive – they simply rest near the base of plants or under residue until it warms up. According to literature, it takes pretty low temperatures to kill alfalfa weevils. Here are a few statistics from the literature:

• 45°F – alfalfa weevils become inactive.
• 21°F – considered the lethal low temperature for larvae; however, scientists believe some can survive much lower temperatures.
• 3°F to 16°F – considered the lethal low temperature for adults, depending on the age of the adult.

Essentially, it is unlikely that any alfalfa weevil adults that were active before this cold spell were killed by the cold temperatures. As temperatures warm back up, we expect alfalfa weevil activity to resume. So, what does that mean for scouting alfalfa?

Scouting may be more challenging this spring. We normally recommend using a sweep net to scout for larvae; however, alfalfa growth has been slow and plants are small (3-4 inches in southern Iowa), and a sweep net is not effective. When scouting in shorter stands, directly look at plants for adults or larvae on terminal leaflets or for injury to new growth. If alfalfa weevil larvae are found, follow the scouting and management guidelines in this encyclopedia article. If larvae are feeding on short alfalfa plants, and especially if alfalfa weevil, fall armyworm, or drought severely impacted stands last year, be more conservative with the economic threshold.