I came across hundreds of grasshopper cadavers in various stages disintegration clinging to goldenrod stems in a naturalized area this past week. Looking like an odd Halloween decoration, the macabre scene was the work of a fungal grasshopper pathogen, Entomophaga grylli. The name of the genus clearly describes the focus of these fungi: "entomo" refers to insects and "phaga" means "to eat."
Grasshoppers infected with the E. grylli fungus crawl to the tops of plants and die with their heads pointing upward and their legs wrapped tightly around the stalks of the plants. The disease is commonly called "summit disease" because of the grasshopper's peculiar weed-climbing behavior. The cadavers remain attached to the plants for several days until their bodies dry out and fall apart after having been digested and consumed by the fungus. As the grasshopper disintegrates, millions of fungal spores are released into the environment.
The entomopathogenic fungus is actually considered a fungal species complex with members separated into different "pathotypes" based on varying disease cycles and host specificity. E. grylli is subdivided into the North American pathotypes E. macleodii that infects band-winged grasshoppers (subfamily Oedipodinae) and E. calopteni that infects grasshoppers belonging to the genus Melanoplus. A third pathotype, E. praxibuli, was introduced from Australia into North America several years ago and infects both groups of grasshoppers.
E. calopteni is "monocyclic" which means that this pathotype only forms overwintering resting spores after finishing off a grasshopper host. This allows the pathotype to survive from year-to-year; however, the fungus does not spread from dead grasshoppers to living hosts within a summer season. There is only a single summit disease cycle per season. E. macleodii and E. praxibuli are "polycyclic" which means there are multiple summit disease cycles per season with spores released from dead grasshoppers to infect new victims. Overwintering resting spores are produced at the end of the season. These pathotypes can have an ever-increasing plague-like impact on grasshopper populations as the season progresses.
Depending on environmental conditions, summit disease is capable of causing high mortality in grasshopper populations. The disease is an important regulator of grasshopper populations in the Great Plains and northwestern states. Unfortunately, in Ohio, disease outbreaks are usually sporadic and highly localized. The highest level of infections generally occurs late in the season after the heaviest grasshopper feeding damage has occurred.