September 12, 2017
Colorado State University has issued a pest alert for alfalfa caterpillar in fields in the Wiggins and Weldona areas. The yellow or white butterflies of the alfalfa caterpillar lay eggs on new alfalfa growth that is less than 6 inches tall. August is often a time for these butterflies to be seen — but even this late in the season, they can be a problem.
Eggs hatch into green caterpillars in three to seven days, and full-grown caterpillars are about 1.5 inches long. They're distinguished from other caterpillars on alfalfa by their velvety green bodies with white lines along their sides.
There are several factors that contribute to economically significant caterpillar numbers, including slow and uneven growth of the crop; lack of natural enemies; hyperparasites, which are parasitoid wasps attacking there natural enemy, wasps, reducing their numbers; and hot, dry weather.
In a single year, there can be several generations of alfalfa caterpillar. The bugs consume leaves whole, while armyworms skeletonize leaves. Damage is worse in newly planted fields, where plants are too small to withstand much defoliation.
When scouting, the basic threshold of 10 larvae per sweep, or one larva per two plants for standing alfalfa, should be adjusted based on the status of the pest and the crop. If many of the larvae you find seem to be diseased or parasitized, double or triple the treatment threshold.
A newly seeded crop may require a lower threshold of two caterpillars per sweep, or one caterpillar per 10 plants. Infestations on regrowth are more important than on a standing crop, but not as important as on new seedings.
In a regrowth situation, the recommended threshold is between the threshold for a standing crop and for a new seeding.
In its alert, Colorado State University said it had no local efficacy data for alfalfa caterpillar for insecticides, but noted that any of the pyrethroid insecticides should be effective. Higher rates are recommended for larger larvae.
Earlier in the year, biological insecticides (such as Bt) are recommended as the better choice to preserve biological controls, like parasitic wasps, in the area. Chances are, however, that this late finding may have benefited from a full season of natural controls. If the crop is due to be cut soon, it may be best to hold off treatment until after harvest, and determine larvae survival to see if it is a concern for regrowth. You can learn more at wiki.bugwood.org/HPIPM:Alfalfa_Caterpillar.
Source: Colorado State University
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