Things are only getting worse for the formerly innocent earthworm.

After being cited by a University of Minnesota forestry professor as a threat to northern forests, the earthworm — long a symbol of earthy richness and fishing-hole fun — is now being blamed in a study for possibly helping to increase greenhouse gas emissions.

In a paper published recently in Nature Climate Change, two U.S. scientists, along with one from the Netherlands and one from Colombia, analyzed 57 studies of earthworm behavior and determined that they likely increase greenhouse gas emissions by turning over soil. The study also asserted that when other tiny creatures feast on the worms’ droppings, they, in turn, also emit carbon dioxide and nitrous oxide, which trap heat in the Earth’s atmosphere.

“My guess is, on balance, they’re emitting,” said Lee Frelich, director of the University of Minnesota’s Center for Forest Ecology, when asked about the study.

Frelich does not view earthworms kindly. He has published extensive research over the years outlining how earthworms, which are not native to North America but have spread slowly and steadily since European settlement, pose a threat to the northern forests by eating the leaf litter off the forest floor. That allows the forest floor to harden, hold less water and get warmer. The forest floor gets comparatively bare, which ultimately allows invasive tree and shrub species to outcompete the more cool-suited natives.

Shallow-rooted trillium are already showing declines, said Frelich, who in 2011 was one of four authors of a paper dealing with the effects of “global warming and ‘global worming’ ” on plants.

The Nature Climate Change study suggested that increased emissions from soils might be self-correcting by nurturing more robust plants, which in turn would pull carbon dioxide from the atmosphere into the soil. Frelich said this theory misses the point of some of his research showing a decline in plant life following colonization by earthworms.

Earthworms spread well on their own, but are aided when people toss out their fishing bait, Frelich said.

The Minnesota Department of Natural Resources notes that this is illegal, because it is releasing an exotic species into the wild.