Gallflies used to combat invasive knapweed in Montana–which does an estimated $14 million of damage statewide–have recently been shown to hurt native plant growth through indirect food chain “ripple effects.”
The flies, originally introduced in the 1970s, provide an attractive source of extra calories for native deermice, which leads to a deermouse population increase. Since the deermice are natural predators of native seeds, the increased population means more hungry rodent mouths eating native seeds that would have otherwise taken root and grown.
This finding suggests that the use of gallflies to control knapweed may end up harming the native grasses and shrubs they are meant to protect. These results introduce a new and important consideration in the complex battle against invasive species.
“Biocontrol practioners have known for along time that things eat their biocontrols,” commented Dean Pearson of the Rocky Mountain Research Center, lead author of the study. “Nobody has really looked at what effect that has on an ecosystem until now.” Pearson’s study appeared in the September issue of the journal, Ecological Applications.
Biocontrols are an increasingly attractive option to deal with widespread invasive plants such as spotted knapweed. “When it comes to widespread weeds, biocontrol is often the best line of attack,” explained Lynn LeBeck, Executive Director of the Association of Natural Biocontrol Producers. LeBeck went on to explain that biocontrol agents are rigorously tested before being introduced into ecosystems, but warned that, “When you release something into the environment, no matter how much work you’ve done, you can’t be sure of what’s going to happen.”
The use of one organism to control another has a checkered history. “Biocontrol has had a past and a present,” explained Barry Rice of the Nature Conservancy. “The past was littered with failures,” he continued. The cane toad in Australia and the mongoose in Hawaii are two classic examples of organisms brought in to control one kind of pest but that ended up “jumping ship”, damaging others.
Early failures such as these taught resource managers that vertebrate animals are too adaptable to limit themselves to the target organism. Invertebrates, however, like flies and beetles, can be much more predictable in what they consume. This realization, along with improved science, led to some dramatic biocontrol success stories, such as the use of imported Australian beetles to nearly eliminate Klamath weed in California in the 1940s and 50s.
Modern biocontrol agents are heavily tested for “host-specificity”, a measure of how likely the creature is to start eating things other than the target species. Most agents are not, however, tested for what things might eat them.
In Montana, Pearson started to suspect that deermice might be eating gallfly larvae when he and his team noticed piles of “shucked” knapweed seedheads. Gallfly larvae burrow into the seedhead, providing a tasty and nutritious gooey center to an otherwise unattractive food source for the mice. The added calories allowed more mice to survive the winter—when food is otherwise scarce—ultimately leading to a near tripling of their population. In 2006, Pearson announced that this population explosion had led to an increase in hantavirus, a potentially fatal-to-humans pathogen, normally kept in check by smaller mouse populations.
Pearson’s new finding shows that, in addition to the unforeseen rise in hantavirus, the gallfly may indirectly harm the establishment of native plants it was originally introduced to protect.
Pearson and his team looked at patches of grassland that had been treated with herbicide to destroy knapweed and compared them to patches where the knapweed was left in place. They measured the rate at which native seeds were removed from these patches, taking care to control for birds, other mammals, and insects that might also eat the seeds. The researchers found that areas without the knapweed—and by extension without gallflies—had lower seed removal rates. This correlated with observations of smaller deermouse populations.
Spotted knapweed has taken over large areas of grassland throughout the American West. Originally from Europe and Asia, it’s believed to have hitchhiked in alfalfa bales accompanying ranchers moving west. The purple-flowered invader squeezes out grasses, ruining previously productive foraging land. In Montana alone, over two-million acres are infested, causing both ecological and economic damage due to lost foraging opportunities.
Biocontrols are just one of many ways to combat the weed. Herbicides, pulling, and fire can also be effective over small areas, but Rice cautioned that any method will have consequences for the environment. “Biocontrols can do invasive species management on a landscape level that you can’t do with other methods,” he explained, “…but there could be a weird feedback effect, a surprise effect that no one knew about.”
Pearson believes that he has discovered one such feedback effect and suspects that there may be others. He concluded, “My guess is that problems like this are rampant.”
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