Various naturally occurring methods exist in reducing Spongy Moth populations. These natural “enemies” of the Spongy Moth have been proven to contribute to the eventual collapse of Spongy Moth populations.

• Nucleopolyhedrosis Virus (NPV) – a naturally occurring virus

Usually the most important factor in the collapse of Spongy Moth outbreaks in North America. The virus is always present in a Spongy Moth population and can be transmitted from the female moth to her offspring. It spreads naturally through the Spongy Moth population especially when the population builds to high levels. Caterpillars are more susceptible to this virus because they are stressed from competing with one another for food and space. Typically, 1 to 2 years after an outbreak begins, the NPV virus causes a major die-off of caterpillars.

• Entomophaga maimaiga – a naturally occurring fungus

Fungal spores that overwinter in the soil will infect young caterpillars early in the Summer. When the young caterpillars die, their bodies produce windblown spores that can spread and infect older caterpillars. Within several days, the cadavers fall to the soil and disintegrate, releasing the spores that will overwinter back into the soil. The fungus is most effective at reducing caterpillar populations, during cooler and wet seasons.

• Extreme Winter Temperatures

Evidence suggests, that egg masses exposed to prolonged periods of extreme cold weather (below -20 degrees Celsius, for more than 2 days) may die off.  Egg masses must be exposed to winter conditions, such as on the outside of tree bark edges, while egg masses laid in crevices or at tree bases, may be insulated from the extreme weather.

• Wildlife

Some birds, mammals and rodents will also feed on the growing larvae – but this type of predation is unlikely to cause a population decline.