The tiny fly, only a few millimeters long, lays its eggs in blueberries and other soft fruit. A few days later, the fruit begins to wrinkle, mold, or get spots. Split open the softened flesh and inside are maggots.
The spotted wing drosophila is just one of many foreign pests that have invaded Vermont to the detriment of farmers, foresters and sportfisherman. Others include the hemlock woolly adelgid, which threatens to wipe out Vermont’s shady hemlock groves, and the zebra mussel, which has infested Lake Champlain, clogging pipes and rocky shores with their sharp shells.
Additional pests loom on the horizon, poised to invade Vermont in the near future: the spiny waterflea, a crustacean that alters aquatic food webs and could be carried into Lake Champlain through the Champlain Canal; the Asian clam, which fuels algae blooms in Lake George in New York; and the emerald ash borer, an iridescent green beetle that could wipe out Vermont’s ash trees.
The invasion of new pests is aided by the global activities of humans who move food, products and recreational vehicles from place to place.
Vermont’s changing climate threatens to exacerbate the problems associated with globalization.
“Climate change certainly contributes to the continued invasion of lands and waters by invasive plants and animals,” said Paul Marangelo, a conservation ecologist with the Vermont Chapter of The Nature Conservancy.
Not only is Vermont’s climate getting warmer, which allows cold-blooded pests to do better here, but the changes in water availability — lots of rain in the spring, drought in the summer — stresses trees and lowers their ability to defend themselves.
According to a team of researchers led by Jeffrey Dukes of the University of Massachusetts, in a paper published in the Canadian Journal of Research in 2009, insect metabolism roughly doubles with every increase of 10 degrees Fahrenheit. As an insect’s metabolism increases, it grows faster, eats more, moves around more, and mates more often. In fact, fossilized leaves show that in warmer periods in the Earth’s past, insects ate more than in cooler periods. The Northeast is expected to see a temperature increase of 5 to 10 degrees Fahrenheit by the end of the century.
That’s not to say that insects will uniformly become more populous and voracious. According to Dukes’ team, there is some evidence that some insects that spend the winters under the leaf litter will suffer from the loss of snow cover. However, the odds appear to be in the insects’ favor.
Until recently, Vermont was relatively safe from hemlock woolly adelgids, for example. Winter temperatures killed them off or at least reduced their survival rate. As Vermont’s winters get warmer and shorter, more hemlock woolly adelgids are surviving the winter and reproducing more often. Scientists predict that Vermont may have just a handful of living hemlock groves left by the year 2070.
Marangelo says the Asian clam is also an increasing threat. Currently found in Lake George in New York, this clam is thought to be limited by colder waters. Now that lakes and ponds are warming, the clam is creeping northward. As it does so, it changes the ecology of the lakes it infests. Released from natural predators into a new environment, the Asian clam reproduces quickly and reaches densities of 8,000 clams per square meter. Aside from clogging boat machinery, the prolific clams release fecal matter into the water, feeding algae blooms. State and local governments around Lake George are currently spending about $500,000 a year to manage this species, according to Marangelo.
Marangelo says there have been no systematic efforts until recently to measure the abundance of non-native species in Vermont. “We’re very much in reaction mode,” he said. Nonetheless, there are plenty of anecdotal observations that indicate invasion is on the rise.
“I think everyone can remember a time when they didn’t know what poison parsnip was,” said Marangelo. “We’ve witnessed the explosions of populations such as zebra mussels and there are a number of other examples like that.”
Marangelo said it is easier to quantify invasions in lakes because they are more like closed systems. According to the 2012 State of the Lake report produced by the Lake Champlain Basin Program, non-native species in Lake Champlain have increased from less than 10 before 1920 to 49 in 2011.
Not only is the number of non-natives in Lake Champlain increasing, but the rate of invasion is increasing as well. For much of the 20th century, there were one to three new arrivals each decade; since the 1970s there have been four or more new arrivals in Lake Champlain each decade. The 1990s alone saw twelve new species show up in the lake.
That’s not to say they are all due to climate change—increased commerce and boat traffic certainly played a part. But invaders do well when the ecosystem they arrive in is already under stress and therefore vulnerable.
The Northeast Climate Impact Assessment, a report that details the current and predicted effects of climate change on the agriculture, economy, and ecology of the Northeast, has bad news for farmers, too. Plants grow better when the air is enriched with carbon dioxide. However, some studies suggest that when plants put on more growth in response to that enrichment, the nutrient concentration of their tissues declines — causing insects to eat even more to get the same amount of nutrients.
The assessment also has bad news for pest control. Southern farmers spray their crops with pesticides more frequently than northern farmers. Floridian farmers, for example, spray their sweet corn 15 to 32 times a year while New York farmers spray zero to five times a year. Part of the reason is southern farmers have more pests to contend with. Plus, higher temperatures reduce the efficacy of common pesticides like pyrethroids and spinosad.
Because there are so many uncertainties involved, scientists can’t easily provide clear links between climate change and the spread of most invasive pests.
“It’s hard to describe it as a sole contributor in a lot of cases,” said Marangelo. “It will be more of a helping factor for some of these species.”
Vern Grubinger, a professor at UVM Extension, echoes Marangelo’s emphasis on generalities. “We’re very concerned about climate change and we have data to show it’s happening. I’m not aware of a clear connection between those changes and invasive pests.” That is, you can generalize that “warmer temperatures are a concern for a lot of the pests because we do count on winter temperatures to kill them off.” But, said Grubinger, many pests only recently arrived in the country, or they are moving toward Vermont from the north or the west, not the south, or some years their populations boom and other years they crash and it’s not always clear why.
Sandy Wilmot, a forest health specialist with the state Department of Forests, Parks, and Recreation, agrees. “We have a lot of pests we are concerned about. Some of them really aren’t responding in particular to climate change, though, they’re just coming.”
One example she cited is the emerald ash borer, which kills ash trees. “It is moving here, it’s going to change our ash population, but with or without climate change I think that would happen,” said Wilmot.
Though there’s a lot of concern in the state and a lot of work to monitor and eradicate invasives, there’s still a lot to learn. “There’s not a ton of information available,” said Wilmot. “The best thing we can do is wait and observe and try to take actions that improve situations without going too far in a direction that we don’t know anything about.”