As with most insects, gypsy moths are host to a suite of natural enemies and these play a pivotal role in the dynamics of the gypsy moth populations. There are two major diseases: a nuclear polyhedrosis virus and a fungal pathogen. The virus, LdMNPV, causes epizootics that are largely responsible for the collapse of gypsy moth outbreaks. Similar viruses terminate the outbreaks of many defoliating Lepidoptera. High mortality from these viral diseases occurs only in dense populations, because transmission of the virus takes place when larvae feed on leaves contaminated by cadavers of larvae that have previously died from the disease. Encounters with cadavers are only likely in dense populations. Transmission of LdMNPV from one generation of gypsy moths to the next occurs primarily by way of external contamination of the egg mass; larvae become infected as they emerge from the mass in the spring. It is not entirely clear how the virus persists at low density, but it does survive in the forest litter for several decades.
The fungal pathogen Entomophaga maimaiga was, until recently, known only in the Far East, especially Japan. In 1989, a dramatic epizootic of E. maimaiga occurred throughout the northeastern United States from Pennsylvania to Maine. In subsequent years, the fungus spread across the mid-Atlantic states and was introduced intentionally by researchers to Virginia and Michigan. It is now established throughout the region infested by gypsy moth in North America. E. maimaiga produces two kinds of spores: conidia and resting spores. The conidia are released from cadavers and are carried by wind currents to uninfected larvae, which they infect by penetrating the cuticle; these conidia are responsible for the rapid spread of E. maimaiga in North America. Late instars produce resting spores that overwinter in the forest litter, where they persist for up to 10 years before germinating to infect new gypsy moths. A. Hajek and colleagues analyzed the DNA of E. maimaiga and showed that the pathogen in North America is identical to E. maimaiga in Japan. How E. maimaiga was introduced into North America is unknown. Since 1989, it has continued to cause high levels of mortality in gypsy moth populations, particularly in years with high rainfall in May and June. A key difference from LdMNPV is that E. maimaiga causes substantial mortality in low-density as well as in high-density populations of gypsy moth. This means that E. maimaiga can prevent outbreaks from occurring, whereas LdMNPV can only cause the collapse of outbreak populations.
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