HPIPM:Alfalfa Caterpillar
Author: Sue Blodgett, revision Frank B. Peairs[1], Jay D. Donahue & Michael J. Brewer
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The alfalfa caterpillar, Colias eurytheme Boisduval, (also called "alfalfa butterfly") is a sporadic warm weather pest. Natural enemies often maintain this insect below economic levels most years.
Identification (and life cycle/seasonal history)
Adult alfalfa caterpillars (alfalfa butterflies) are medium-sized, with yellow or whitish wings that are solid on the top surface and bordered by black on the lower surface. The wingspan is about two inches. When they are seen flying about, they have probably just emerged from the field where they wintered as pupae. Females lay eggs singly onto alfalfa under one foot high. In about one week, the eggs hatch, and the larvae grow rapidly. They grow to one inch long at maturity about three weeks later. Larvae are green, covered with white hairs, and the larger larvae have thin white stripes along the sides of their bodies. Alfalfa caterpillars are favored by hot dry weather, abnormally slowly growing crops and by low densities of natural enemies.
Plant Response and Damage
Alfalfa caterpillars consume the leaves whole, whereas armyworms skeletonize the leaves. Damage is worst in newly planted fields, where the plants are too small to withstand much defoliation.
Management Approaches
Cultural Methods
In established fields, early harvest avoids serious damage from dense caterpillar populations.
Host Plant Resistance
None, although mature plants are able to withstand a great deal of defoliation.
Biological Controls
Preservation of natural enemies may keep alfalfa caterpillar from becoming economic most years. Besides general predators such as spiders, parasitic wasps and pathogens specific to this pest are common. Bacillus thuringiensis kurstaki is also active against alfalfa caterpillar (the larval stages only). Diseased and parasitized larvae tend to be abnormally colored, and are not as active. Natural enemies are preserved by avoiding unnecessary insecticide applications and by practicing non-chemical controls, such as early harvesting.
Chemical Control
Fields should be checked twice monthly, with more frequent monitoring after the adults are seen flying, especially when the plants are young and weather is hot and dry. Alfalfa caterpillars are best monitored with a sweep net. Diseased and parasitized larvae in high numbers compared to normal larvae indicate good control by natural enemies, so the threshold given below may be adjusted upwards, thus avoiding unnecessary insecticide applications.
An average of one caterpillar per two plants is the threshold for chemical control. However, as noted above, many diseased or parasitized caterpillars indicate natural enemy populations are present, so insecticide use may disrupt future control. Bacillus thuringiensis kurstaki should be considered because only butterfly and moth larvae are killed, preserving alfalfa caterpillar's natural enemies.
Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.
