The role of dipteran parasitoids in periodic dynamics of grasshoppers with a 2-year life cycle

Some insects that have multi-annual life cycles exhibit periodic population dynamics, that is, one of two or more temporally isolated cohorts is much more abundant than the others. There are two or three, possible mechanisms capable of enforcing periodic dynamics: intraspecific competition between cohorts, satiation of generalist predators, and specialist parasitoids with a one-year life cycle. Many grasshopper species have a two-year life cycle at high altitudes or latitudes, because eggs require two years to hatch in cool climates. In the interior of Alaska, populations of several grasshopper species are abundant in even-numbered years, and scare in odd-numbered years. Rates of parasitism by dipteran parasitoids (Diptera: Sarcophagidae and Tachinidae) were monitored for 5 years. Parasitism was much lower in the abundant, even-numbered year grasshopper cohort, compared to the scare cohort, providing strong evidence for the role of these parasitoids in enforcing the alternate-year dynamics of grasshoppers in this region.