CORNELL (US)—Efforts by land managers to release pathogens to control the spread of gypsy moth caterpillars are ineffective and unnecessary because the pathogens are already chasing down the migrating moths on their own.

The finding is important because gypsy moth populations “can develop unpredictably and erratically, with caterpillars eating all the leaves off of most of the trees,” says Ann Hajek, professor of entomology at Cornell University.

“We were pretty surprised, no one knew how long it took the pathogens to chase their hosts,” Hajek says.

Gypsy moths migrate slowly westward—slowly because the females don’t fly. By tracking the edges of the migration, where population densities are low, researchers are able to investigate how long it takes their viral and fungal pathogens that attack the caterpillars to catch up.

Land managers gather fungal spore-containing caterpillar cadavers and spread them to try and control new populations of gypsy moths. The virus (Lymantria dispar nucleopolyhedrovirus), infects gypsy moth caterpillars and is used in a spray by the U.S. Forest Service to control the moths in environmentally sensitive areas.

Hajek and colleagues studied “leading edge” populations of moths and pathogens in central Wisconsin in 2005-07. They set pheromone traps west of the migrating population and then traveled east to lay traps to catch the flying males.

Once their traps caught more than 74 moths each in one year, there was a more than 50 percent chance of finding the fungus in that area in the following year; when more than 252 moths were trapped in a year, there was more than 50 percent chance of finding the virus the next year.

“Our data show that the fungus spreads into lower density leading edge populations sooner than the virus, but the virus eventually colonizes the populations too,” Hajek explains.

Fungal spores actively shoot out of the moth cadavers and disperse in the environment, thereby spreading quickly; the virus spreads from one caterpillar to another, and possibly via parasitoid flies and predators, which is a slower process, she says.

Hajek says the efforts of land managers to release the pathogens along the leading edges of spreading moth populations are ineffective and unnecessary because there appears to be no association between the release of pathogens nearby and presence of the pathogens among the moths.

“These results suggest that the pathogens are dispersing on their own and land managers don’t need to release them in leading edge gypsy moth populations, because they’ll get there on their own anyway,” says Hajek.

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