The Alfalfa Weevil and similar looking Egyptian Alfalfa Weevil, are major pests of alfalfa, and other crops, in many areas. Alfalfa weevils overwinter as adults, usually in field trash, or other secluded areas and emerge in spring, (or in late winter in warmer climates.) Soon after the adults emerge, females begin to lay eggs into alfalfa stems. Larvae emerge from the eggs and feed on the alfalfa terminals, before dropping to spin a cocoon and pupate. Adults typically emerge in summer and in most cases, there is only generation per year.

Weevil feeding damage can sometimes be limited by cutting alfalfa plants as soon as most plants reach bud stage. In other cases, insecticides may be needed. Monitoring for adults with insect net sweeps should begin in late winter or spring. Researchers have developed a growing degree day model to help predict when life stages will occur based on that year’s weather.

Calculating the Growing Degree Day Units

The Alfalfa Weevil growing degree day model uses a lower threshold of 50F, and an upper temperature threshold of 87.8F. The Egyptian Alfalfa Weevil uses a lower threshold of 45F, with no upper threshold.

These units can be calculated with daily high and low temperatures, using the sine model to estimate growing degree day units, or the Pest Prophet app can be used to calculate GDD accumulation from hourly temperature data for a specific field.

The first larvae typically emerge at 197 GDD for Alfalfa Weevils and 212 GDD for Egyptian Alfalfa Weevils after the first adults have emerged and have been consistently observed in the field, which is the “biofix” or date at which growing degree day unit accumulation should begin. Pupation will occur approximately at 505 GDD after the biofix date for Alfalfa Weevils, and 585 GDD for Egyptian Alfalfa Weevils.

Setting the Biofix Date

For growing degree day models, there needs to be a “biofix”, a set date after which growing degree day units will accumulate. For Alfalfa Weevils and Egyptian Alfalfa Weevils, this is the date at which adults are first consistently observed in the field in spring, or late winter.  In the Pest Prophet app, this will correspond to the growth stage, ” Adult / Eggs.” The bio-fix can also be re-adjusted after later growth stage are observed, in order to better predict later life stage events.

Sources:

Guppy, J. C., and M. K. Mukerji. 1974. Effects of temperature on developmental rate of the immature stages of the alfalfa weevil, Hypera postica (Coleoptera: Curculionidae). Can. Ent. 106: 93-100.

Integrated Pest Management for Alfalfa Hay. University of California Statewide IPM Project. Div. Agr. Sci. Publ. #4104. pp. 47-50.