Larval mining causes conspicuous white blotches on the leaflets, which typically are comma-shaped. Blotches and punctures can cause deterioration of foliage quality, loss of photosynthetic area, and defoliation. May. Larvae feed within the growing tips, on the upper leaves as they open, and later on the lower leaves. Plants become skeletonized from weevil feeding and the leaves dry, giving the field a frosted appearance. After cutting, the larvae may feed on the new emerging shoots, severely retarding alfalfa regrowth.
Adults also feed on the alfalfa plant.
Several species of parasitic wasp and a fungus help maintain alfalfa weevil populations. See A Pest Management Program for Alfalfa in Pennsylvania for information on the alfalfa weevil life cycle, identification, sampling techniques, and economic thresholds. Black cutworms can cause extensive damage to new seedings in late May and early June. See Section 4, Corn , for information on cutworm biology. Pea aphid control sometimes is needed, but natural controls usually suffice to keep aphid populations in check.