NoCo Pest Alert

Assefa Gebre-Amlak
Colorado State University Extension
Greeley, CO 80631-2049
Phone: (970) 304-6535 Ext. 2074
Fax: (970) 351-0415
E-mail: Assefa.Gebre-Amlak@Colostate.Edu

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Check Alfalfa Fields for Alfalfa Caterpillars
Wilma Trujillo and Frank Peairs
Colorado State University Extension
Logan and Morgan Counties
Phone: (970) 522-3200 or (970) 542-3540

In recent days we have received a number of phone calls about an increased number of alfalfa caterpillar (Colias eurytheme) in alfalfa fields in the Wiggins and Weldona areas.

The yellow or white butterflies of the alfalfa caterpillar lay eggs on the new growth of alfalfa that is less than 6 inches tall.  The adults of the alfalfa caterpillar can be seen flying over alfalfa fields particularly in August.  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.

          Alfalfa butterfly                                       Alfalfa caterpillar

Factors contributing to economically significant caterpillar numbers are:

  • Slow and uneven growth of the crop
  • Lack of natural enemies
  • Hyperparasites (other parasitoid wasps attacking the natural enemy wasps reducing their numbers)
  • Hot, dry weather.

There are several generations per year of alfalfa caterpillar.  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.

The threshold of 10 larvae per sweep/1 larva per 2 plants for standing alfalfa should be adjusted based on the status of the pest and of the crop.  If many of the larvae that you collect seem to be diseased or parasitized, then you should double or triple the threshold.  If the crop is newly seeded, then consider lowering the threshold to 2 caterpillars per sweep/1 caterpillar per 10 plants.  Infestations on regrowth are more important than on a standing crop, but perhaps not as important as those on new seedings.  Consider a regrowth threshold that is intermediate between the threshold for a standing crop and that for new seedings.

We have no local efficacy data for alfalfa caterpillar, but any of the pyrethroid insecticides should be effective.  Higher rates should be considered for larger larvae.  Earlier in the year, biological (Bt) insecticides would be a better choice in order to preserve biological controls.  However, we likely are dealing with the last generation of the year and have already benefitted from a full season of natural controls.  Also, if the crop is due to be cut soon, it might be best to hold off treatment until after harvest and see if enough larvae survive to be a concern on the regrowth.

For more information on chemical control, please visit the High Plains Integrated Pest Management Guide at


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