Caterpillar Defoliators in Alfalfa

Crop Injury: Many different species of caterpillar consume alfalfa foliage including the alfalfa caterpillar (Colias eurytheme), alfalfa webworm (Loxostege cereralis), fall armyworm (Spodoptera frugiperda), forage looper (Caenurgina erechtea), garden webworm (Achyra rantalis), green cloverworm (Hypena scabra), true armyworm (Mythimna unipuncta), variegated cutworm (Peridroma saucia), and yellowstriped armyworm (Spodoptera ornithogalli). Several species are often present simultaneously. Feeding can appear as holes in leaves or skeletonization and stems may be consumed after leaf tissue is gone. Risk of injury is lowest to alfalfa plants with vigorous growth; risk is highest to seedlings and post-cutting regrowth.

Pest Description: Caterpillars are long, soft-bodied insect larvae with a hardened head capsule, six true legs near the head, and a varying numbers of prolegs near the hind end. Alfalfa caterpillars appear green with a white stripe on each side of the body. They have four pairs of prolegs on the abdomen and appear “velvety.” Alfalfa webworms are yellow to green, turning dark green as they mature, with light and dark stripes along the back. Each body segment has six black spots and feeding occurs on leaves that have been tied together with silk strands. Fall armyworms have a distinctive “Y” marking on head capsules and four bumps in a square shape on the hind end. Fall armyworm caterpillars range in color, and eventually have stripes running the length of the body. Forage loopers have two pairs of prolegs on the abdomen and move like inchworms. They have multiple light to brown stripes on the body and a striped head. Garden webworms have four pairs of prolegs on the abdomen, with dark spots and short hairs along light-green colored bodies. Green cloverworms have three pairs of prolegs on the abdomen and one pair on the last segment. These pale green caterpillars have multiple white stripes and react to disturbances by thrashing their bodies. True armyworms have four pairs of prolegs on the abdomen, each with a dark stripe. Their skin appears grainy and orange stripes run the length of the body. Variegated cutworms range in color, but have a line of lighter-colored, diamond shaped spots running along the back. Yellowstriped armyworms have a conspicuous yellow stripe running along dark-colored bodies, with four pairs of prolegs. Most segments have a dark spot.

Life Cycle: Life cycles vary by species, but all start as eggs laid by adult moths or butterflies. After eggs hatch, emerging larvae feed on foliage as they develop through multiple instars. Larger larvae generally cause the greatest injury. The final instar enters pupation, and adults eventually emerge to repeat the life cycle. The number of generations is dependent on species and geographic location. 

Scouting: Scout fields throughout the growing season, checking for defoliation or using a sweep net to dislodge and collect caterpillars. At the very least, scouting should coincide with time periods of highest caterpillar risk to alfalfa, such as during seedling growth stages and post-harvest regrowth.

Using a Sweep Net

Management: Natural enemies can reduce caterpillar populations. Early alfalfa harvest may be beneficial but stubble should be scouted for continued feeding. Insecticide use may be warranted with large populations of caterpillars which can rapidly consume alfalfa fields. Thresholds vary by species and are often non-existent; check with local extension resources for the most up-to-date economic thresholds. Consider reduced-risk insecticides such as Bt-based formulations. 

Prepared and reviewed by the Alfalfa Pest Management Working Group.