Corn Rootworm Life Cycles
All beetles have a similar life cycle. Beetles undergo complete metamorphosis, meaning they have four distinct life stages: egg, larva, pupa, and adult. Corn rootworm species differ in the overwintering stage and number of generations per year.
Both northern and western corn rootworms have one generation per year, beginning as eggs that overwinter in the soil. Once eggs hatch in the spring, larvae transition through 3 instars and feed on the roots of corn plants for 4-6 weeks before pupating in the soil. Adults emerge from the soil 5-10 days after pupation to feed on plants and mate throughout the summer. During late summer and fall, adults lay eggs in the soil, which will overwinter to begin the life cycle the following year. Adults continue feeding but die after several hard frosts in the fall.
Southern corn rootworms may have two or three generations per year and overwinter as adults in protected places or around the base of plants. Beetles become active in the spring and feed on various host plants. Once corn emerges, adults lay eggs singly in the soil close to the base of plants. Eggs hatch 7-10 days later, and young larvae feed on roots for 2-4 weeks before pupating. First-generation adults emerge 1-2 weeks following pupation. The entire life cycle takes 6-9 weeks to complete. Typically, the first generation larvae are found on corn roots from spring through mid-summer, and second-generation adults are found on clover, alfalfa, and other plants from September to November.
Northern and western corn rootworm have altered their life cycles to adapt to crop rotation. These are called variants, and each species has adapted with a different mechanism.
Northern corn rootworm has adapted to crop rotation with extended diapause, where some of the eggs remain dormant for two or more winters. If corn is rotated annually with soybeans or another non-host crop (ex: corn-soybean-corn), the extended-diapause eggs will hatch in two years when corn is planted again. This variant has been observed in Iowa, Minnesota, Nebraska, South Dakota, and Wisconsin.
Western corn rootworm has adapted to crop rotation by modifying its behavior during oviposition. Female adult western corn rootworms that are soybean-variants will lay eggs in both soybean and cornfields. The eggs that overwinter in soybean fields will hatch in a cornfield in the spring. This is a result of nearly universal use of a strict corn-soybean rotation scheme throughout much of the Corn Belt. This variant has been observed in Illinois, Iowa, Indiana, Michigan, and Ohio.