Alfalfa Caterpillar
Alfalfa Caterpillar
Have you seen a large number of little yellow butterflies lately?

If you were wondering what they were, they are Colias eurytheme, or commonly known as the Sulphur Butterfly or the Alfalfa Butterfly. It belongs to the lowland group of “Clouded Yellows and Sulphur’s” subfamily. 

It is found throughout North America from southern Canada to Mexico, but is absent from the central and southeastern United States.

Although, this cute little butterfly looks harmless, occasionally this species multiplies to high numbers, and can become a serious pest to alfalfa crops, because they are the butterflies of the alfalfa caterpillar. 

The butterflies lay their eggs on the new growth of alfalfa when it is less than 6 inches tall. 

Eggs hatch into green caterpillars in 3 to 7 days. 

Full-grown caterpillars are about 1.5 inches long and are distinguished from other caterpillars on alfalfa by their velvety green bodies with white lines along their sides.

Caterpillar populations usually result from a flight of butterflies into the field when the alfalfa is less than 6 inches tall. 

Extremely large numbers of adults migrating between fields are often present from May to October. 

Factors contributing to economically significant caterpillar numbers are; Slow and uneven growth of the crop, 

Lack of natural enemies such as Hyper-parasites (other parasitoid wasps attacking the natural enemy wasps reducing their numbers) and Hot, dry weather.

There are four to seven generations per year of alfalfa caterpillar, and each generation is closely synchronized with the hay-cutting cycle so that the caterpillar pupates before cutting occurs.   

Alfalfa caterpillars can cause significant damage by consuming entire leaves. The larger larvae are most destructive. In contrast to armyworms, alfalfa caterpillars do not skeletonize leaves and will also consume the midrib.

The most important way to control the alfalfa caterpillar is to preserve natural enemies that parasitize and prey upon this pest. 

Use selective insecticides against caterpillar pests in the summer to maintain natural enemies and minimize subsequent build-up of caterpillars.

Preserve and encourage natural enemies by avoiding unnecessary insecticide applications for aphids or weevils in the spring.

An important parasite of the alfalfa caterpillar is Cotesia medicaginis, a dark brown to black wasp about 0.25 inch long. 

This wasp stings very small alfalfa caterpillars and lays an egg inside. The egg hatches and the wasp larva eats the inside of the caterpillar. 

A parasitized caterpillar dies before it reaches 0.5 inch in length.

Border-strip harvesting is a useful method for preserving the natural enemies of both the alfalfa caterpillar and aphids because it helps retain parasite larvae in the field. 

Border strip harvesting involves leaving uncut strips of alfalfa at various intervals across the field as a refuge for natural enemies. 

Early harvesting of fields infested with economically significant levels of alfalfa caterpillars kills a large number of caterpillars, preserves crop yields, and avoids reducing natural enemy numbers. 

Time this cutting to avoid serious damage, yet obtain satisfactory yield. These biological and cultural controls, as well as sprays of Bacillus thuringiensis, are acceptable for use on an organically certified crop.

If cutting is not practical or not scheduled soon after monitoring, apply an insecticide if there is an average of 10 or more non-parasitized alfalfa caterpillars per sweep. 

For a reference on which insecticide to use, refer to the Pacific Northwest Pest Management website at: https://pnwhandbooks.org/insect/hay-pasture/alfalfa/alfalfa-hay-alfalfa-caterpillar

Although these butterflies are pretty to watch flying around your fields, they can also indicate an alfalfa pest problem in the future, which may affect your farms profit margin.

Sources:

Wikipedia, the free Encyclopedia: https://en.wikipedia.org/wiki/Colias_eurytheme

UC Pest Management Guidelines, Alfalfa, Alfalfa Caterpillar, http://ipm.ucanr.edu/PMG/r1300611.html

Pacific Northwest Pest Management Handbook: https://pnwhandbooks.org/insect/hay-pasture/alfalfa/alfalfa-hay-alfalfa-caterpillar