Be on the Lookout for Variegated Cutworm in Alfalfa
Significant rainfall in most areas of the state has growth in the first alfalfa crop looking good. Some growers may already be looking at an early cutting. Cool wet weather has kept aphid populations in check for the most part this spring and we are approaching the normal peak of alfalfa weevil larval infestations (around mid-April). As we draw closer to the end of one pest season, others may be lurking.
During April, adult moths of the variegated cutworm, Peridroma Saucia, emerge from overwintering pupae and lay eggs in alfalfa fields. Typically, larvae hatch and feed on foliage while growing to a length of .5 to 1 inch before the first harvest is taken. However, feeding by small larvae in the first crop is often not noticeable. The most serious damage by this species typically occurs in early May on new growth for the second crop. Like the army cutworm, the variegated cutworm feeds at night and seeks shelter by tunneling into soil or gathering beneath windrows during daylight hours. Consequently, damage will be most severe beneath windrows where the cutworms have found shelter. In the most severe infestations, the cutworms may delay growth of the second crop by 2-3 weeks (Fig. 1). There is frequently a "ribbon-like" appearance of alternating heavy damage (under windrows) and light damage (between windrows) after baling (Fig. 2).
Figure 1. Field Damage from Variegated Cutworm.
Photo Credit: Department of Entomology, Iowa State University, 2001.
Figure 2. Alternating damage from feeding under windrows.
Photo Credit: Dana Bay, Ellis County Extension Educator.
Larva are gray to black in color with distinctive white to yellow, diamond-shaped markings along the center of the back (Fig. 3).
Figure 3. Variegated Cutworm
Sampling and Control. Detection of cutworms and assessment of population densities is often difficult because of their nocturnal feeding habits. Whenever alfalfa seems slow to "green-up" during March or after first harvest, fields should be checked for the presence of cutworms. Sampling requires sifting through plant debris and the upper 1-2 inches of soil around plants. A minimum of ten, 1-sq. ft. areas should be sampled in each 10-20-acre field area. When numbers of small larvae (up to ½ inch long) exceed an average of 3/sq. ft., an insecticide should be applied. If large larvae (>½ inch) are present, spraying is recommended when numbers exceed 2/sq. ft. Control of cutworms with insecticides may be difficult. It is important that applications be made in late afternoon or evening, near the time when larvae begin to feed, using adequate spray volumes to insure thorough coverage. Recent rain and cooler weather may also aid in suppression. Additional information on control of cutworms can be found in OSU Extension Facts No. 7150 .