Every day, pest management professionals face off against alien invaders in protecting structures and property.
Many of these from-elsewhere pests — Formosan termites, Argentine ants, Norway rats — are now common foes, while newer invaders, like the tawny crazy ant and brown marmorated stink bug, have experts scrambling to find effective control solutions.
Unfortunately, many more invasive threats exist. Experts, in fact, are tracking certain species that aren’t here yet but that could significantly harm public health, the nation’s food supply, and built/natural environments should they get established.
And those are the potential invaders they know about. Surprise introductions also can devastate, like the emerald ash borer, which is not a serious pest in its native Asia but has killed hundreds of millions of ash trees in North America since it was discovered in Michigan in 2002.
The economic impact is considerable: One report estimated that invasive plants, animals, fungi and pathogens cost the U.S. $120 billion a year. The ecological repercussions — loss of species and loss of habitat — are even greater, said scientists.
“There’s really not a sector that could not be impacted in a severe way by some kind of invasive organism,” said Helen Spafford, an entomologist who specializes in invasive species.
WHY SO SUCCESSFUL? Alien invaders share some common traits and behaviors, which give them an edge in expanding their range. Here are reasons they thrive:
1. They’re good hitchhikers.
And with increased global trade, they have more opportunities to travel long distances to new locations in a short amount of time.
“As those organisms get moved around, they have an opportunity to establish if they survive the journey. A lot of times they’ll fail. But, with enough opportunities eventually somebody’s going to make it,” said Spafford.
In marine environments, alone, the risk of introducing invasive species could increase three- to 20-fold by 2050 as global shipping traffic increases, according to a 2019 study by McGill University, Montreal, Quebec. Bio-invasions will result from organisms being discharged in ship ballast water and hitching rides on ship hulls.
Alien insects and seeds also stowaway in shipping containers and more ship traffic may increase the risk of these introductions as well.
Even a domestic species can hitch a ride on a vehicle or get moved in soil, firewood or potted plants and become invasive elsewhere in a country. An example is the odorous house ant, which is native to the continental U.S. but is wreaking ecological havoc in Hawaii. Likewise, the mountain pine beetle, a native of the western U.S., jumped the Rocky Mountains with human help and now threatens the pine forests of Minnesota.
2. Their biology and behavior help.
“Most invasives tend to have very high rates of reproduction and (exhibit) very little maternal care. They’re just laying large egg masses and moving on,” said Mike Toews, an expert on invasive row-crop pests who co-directs the Center for Invasive Species and Ecosystem Health at the University of Georgia.
By comparison, native species lay smaller egg clusters or single eggs and often take steps to protect them, such as how a spider guards an egg sac, he said.
Some invaders even can reproduce asexually, which may help them overcome the challenge of finding a mate in a new area. Examples are the woolly adelgid, a native of Asia that threatens Eastern hemlock trees, and the Asian longhorned tick, which first was reported in the U.S. in 2017 and now is found in nine states.
Invasive ants have adaptive behaviors that help them succeed in otherwise hostile environments, said Rob Morrison, a research entomologist with the USDA Agricultural Research Service in Manhattan, Kan. For instance, a chemical on the cuticle tells an ant who belongs to its family and who does not. “Some of the species that have been introduced, they’ve lost that ability (to recognize the nest-mate cue) so you get colony after colony and they all think they belong to the same group,” said Morrison. This helps them take over an area instead of fighting among themselves.
3. They have no natural enemies in their new environment.
“They’ve basically been able to escape from all their predators and parasitoids and things that would otherwise eat them in their native range and so that takes the lid off their population and allows them to increase,” said Morrison.
4. They prefer disturbed habitats, which humans excel at creating.
We level forests, build homes, plant crops, mow grasses, and engage in activities that degrade the air and water, which changes and weakens the native ecosystem.
Even man-made structures can help invasive pests like fruit flies and brown marmorated stink bugs (BMSB) survive. “Essentially they’re taking advantage of human-built structures in which to overwinter,” said Brent Sinclair, a biologist at Western University in London, Ontario.
Invasive pests further disrupt an environment once they get established, as they tend to “piggyback” on one another, Sinclair added. If emerald ash borer kills the ash trees, then invasive European buckthorn or phragmites, a non-native wetlands grass, move in. This makes it harder for other native organisms to survive.
Overall, more than 400 of the 1,300-plus species protected under the Endangered Species Act are considered at risk at least partly due to displacement by, competition with, and predation by invasive species, stated the U.S. Fish and Wildlife Service. “Basically, we see these huge ecological cascades,” Sinclair said of the compounding effect of invasive species. “In terms of the challenges that the natural world faces, invasive alien species are one of your four horsemen of the Apocalypse,” he said.
5. They like the warming temperatures.
Because of climate change, more areas will become favorable to introduced species. In fact, within 60 years, the climate of North American cities will be more like those of cities 500 miles away and mainly to the south, according to a 2019 University of Maryland study. The climate of Toronto will become more like that of Secaucus, N.J.; Los Angeles like Las Palmas, Mexico; and Charlotte, N.C. like Tallahassee, Fla. (See the interactive map: umces.edu/futureurbanclimates.)
“As it becomes more tropical in New Jersey, then suddenly the weird insects that came in on the boat with your furniture made in Vietnam are maybe more likely to be able to survive,” said Sinclair.
Milder, shorter winters will help invaders establish and warmer, longer summers will allow them to spawn multiple generations to rapidly grow their populations.
HOW DO WE STOP THEM? The first goal is to prevent potential invaders from entering the country. Toward this end, researchers are constantly tracking the worldwide locations of sure-to-devastate species, predicting how and where they might gain entry (using computer modeling) so preventive actions can be taken.
Armed with these insights, USDA employees inspect high-risk shipments of products and commodities at ports of entry. Infested cargoes are quarantined and treated using fumigation, heat, cold or irradiation. Within the country, states and provinces may ban the transport of soil and firewood to reduce the spread of invasive pests, such as the red imported fire ant and wood-destroying beetles.
Still, some pests make it through these defenses. Hopefully, they can be detected early while their population numbers are low and confined to a small area.
“The more eyes we have out there looking the better,” said Spafford. She urged PMPs to look for invasive organisms at clients’ properties as “they’re in an ideal position to know what’s there normally and to recognize what is out of the ordinary.”
Be careful not to assume an organism’s identity because invasive species may look similar to native ones or to established invaders. BMSBs, for instance, were first believed to be a native species and that’s part of the reason why control efforts took so long to get rolling, said Morrison.
An invasive species can be eradicated if it is confined to a small area. “That’s the strategy right now that’s being taken for the spotted lanternfly,” said Rob Venette, a research biologist with the USDA Forest Service in St. Paul, Minn. He heads the Minnesota Invasive Terrestrial Plants and Pests Center.
A native of China that threatens fruit, ornamental and woody trees, the spotted lanternfly was detected in Pennsylvania in 2014 and has since been found in New Jersey and Delaware. If eradication efforts don’t work, the strategy will shift to a slow-the-spread program, Venette said.
When invasive insects cover a widespread area, scientists use biological control as a management tool. “You just can’t spray pesticides on a large scale without creating other problems,” Venette said.
Biological control typically involves introducing a natural predator from the pest’s native land it to its new range. Tiny parasitoid wasps have been released to help control emerald ash borer and BMSB; they attack the pests’ eggs and larvae and are beginning to show promising results, said researchers. Not all invasive species, however, have biological control solutions.
On occasion, a natural predator may show up on its own. Several years ago, a Japanese wasp appeared that virtually eliminated the kudzu bug, an invader from Japan that significantly damaged soybean crops in the Southeast. “We still have a few kudzu bugs but nothing that’s an economic pest,” said Toews.
Unfortunately, scientists expect invasive species to cause bigger problems in the years ahead. Unless some “pretty dramatic changes” are made to how goods are moved around the world and how potential new pests are detected before reaching borders, issues with invasive pests will continue, said Spafford.
Likewise, “sleeper pests” — the alien invaders already here but undetected — will become problematic as environmental conditions turn in their favor, she said.