Plants and animals are disappearing at an increasing rate, being out-maneuvered by foreign pathogens, insects, and plants with no natural enemies. Researchers in the College of Agriculture and Life Sciences are hot on the trail of these alien invaders.

 

Harrowing For Hardwoods: The Asian longhorned beetle, identified by Richard Hoebeke (pictured) in 1996, destroys many valuable native species of hardwoods.

The New York State insect is essentially no more. Once among the most common ladybugs in the eastern United States, the nine-spotted lady beetle has not been seen since 1984. This comely reddish-orange beetle with four spots on each wing and a shared one in the middle has been displaced by a voracious cousin with seven spots, imported by the millions from Europe in the 1970s as a biological control agent.

The idea made sense at the time. Entomologists had observed that lady beetles eat aphids, so thought that a more aggressive species would be only that much more effective in controlling these common crop pests. They were, so much so it turned out, that the imported beetles monopolized the food source and apparently starved the natives out.

The disappearance of the tiny nine-spotted lady beetle would hardly seem a great loss to any but the proudest New Yorker, yet the introductions of invasive species-which multiply unchecked by natural enemies-have caused entire landscapes to vanish.

In the early 1900s, one in every four deciduous forest trees was an American chestnut. "Today there is hardly a person alive who remembers what they looked like," says George Hudler, a professor of plant pathology and author of the entry on invasive pests and pathogens of trees for the Encyclopedia of New York State.

In merely 40 years, the chestnuts disappeared, decimated by a fungus that was inadvertently introduced when Japanese chestnuts (with inconspicuous disease symptoms) were planted in the New York City Zoological Park.

It took the same amount of time for Dutch elm disease to topple the thousands of stately elms that once lined the streets of America's cities east of the Mississippi.

And today, crows, jays, and many of our most loved feeder birds are dying in droves, infected with the African West Nile Virus.

Food crops also are vulnerable to nonindigenous pathogens-fungi, oomycetes, viruses, bacteria, phytoplasmas, and nematodes-causing $30 billion in losses annually. In Florida, periodic outbreaks of the Asiatic citrus canker, a fruit-blemishing bacterium, trigger immediate quarantines disrupting the flow of fresh fruit and necessitating the destruction of well more than 1.56 million commercial trees and 600,000 backyard citrus trees.

Karnal bunt, a fungal disease of wheat first discovered in India in the 1930s, is working its way up from Mexico through Arizona and neighboring areas of California. Although this disease causes little yield loss, it has major impact on American wheat exports as many countries of the world restrict the importation of wheat carrying teliospores of karnal bunt.

Then there are those old fungal standbys like Phytophthora infestans, which caused the Irish potato famine of the 1840s. It's still here today, causing damage worth $3 billion each year in the developing world and $1 billion in the developed world.

All told, ecologist David Pimentel, professor emeritus of entomology, estimates that the more than 50,000 nonindigenous species in the United States cost $123 billion per year in economic losses. In addition, more than 40 percent of the species on the U.S. Department of the Interior's endangered or threatened species lists are at risk primarily because of non-indigenous species.

The unintentional importation of nonindigenous species is nothing new. As early as 1700, plant pathogens were introduced to the New World when trading ships taking goods from here to Europe filled their empty holds with dirt as ballast for the return trip, off-loading it at American ports. A slightly different form of the same practice continues today.

 

Loosestrife on the Loose: Bernd Blossey has had outstanding success in controlling purple loosestrife with biological control using beetles.

The much-feared Asian longhorned beetle, which destroys many species of valuable hardwoods (including the maples, whose resplendent red and yellow leaves are a marvelous vision each fall), arrived in the United States nestled in wooden crating material used to stabilize the contents of bulk freight containers. Pallets and dunnage (rough-cut timber with the bark still on it) from 3 million of these containers is routinely dumped outside warehouses each year, releasing their unintended cargo of wood-boring insects undetected.

The explosion in international trade and travel in the last two decades has resulted in an exponential increase in invasive species. Data from port inspection records on ships coming from China alone tell the tale. In 1984 China ranked 21st on a list of interceptions of beetles found in wood crating. By 1990 it was fourth, according to senior extension associate E. Richard Hoebeke, an insect taxonomist in the Department of Entomology, who made the definitive identification of the Asian longhorned beetle in 1996.

Hoebeke has spent the last 25 years on the lookout for new bugs. Although his strongest suit is beetles, Hoebeke is one of a handful of people in the country skilled in identifying a broad range of insects. He conducts his own field surveys throughout the Northeast and the maritime provinces of Canada, as well as tackling the identification of insects that stump U.S. Department of Agriculture and U.S. Forest Service inspectors. In the past four years alone Hoebeke has identified eight species new to North America. One had the potential to become a serious pest.

"Ninety-nine point nine percent of the time, conditions usually aren't right for invasive insects to take hold," Hoebeke says. "They perish because they don't find favorable habitat or mates or host plants."

Although it's estimated that if the Asian longhorned beetle were to become established in American cities, this one species alone could kill 1.6 billion street trees at a cost of $607 billion, plants produce the most disastrous effects. They can cause entire ecosystems to collapse.

"Introduced plants come in and change the landscape, they occupy space where native plants would grow providing ecosystem services to a whole range of other species," explains Bernd Blossey, assistant professor of natural resources and director of the Ecology and Management of Invasive Plants Program.

Ecosystem services, although essential, are typically unseen and often unknown, until disrupted. Amphibian populations, for example, are on the decline worldwide. But why? In novel research, Blossey and his students are showing how invasive plants disrupt food webs.

"Some of our data show that American toads suffer as much as a 50 percent increase in mortality when tadpoles develop in purple loosestrife versus in cattail wetlands," Blossey says.

Invasive plants also set "ecological traps" for native organisms. Black swallowwort, an invasive that's exploding along roadsides, is taxonomically close enough to native milkweeds that its scent entices monarch butterflies to lay their eggs on its leaves, yet different enough that it can't provide the nutrition monarch larvae need to develop.

Understanding the ecosystem effects of invasive plants is essential to designing biological control programs that keep invasives in check without unknowingly introducing another organism that, like the seven-spotted lady beetle, will itself spin out of control.

Predicting the behavior of introduced species is tricky because they go through, what Blossey calls, a "lag phase." He's trying to figure out exactly what happens in this time between the arrival of a new species and evidence of its destruction of ecosystem services. There seem to be genetic changes that Blossey thinks may occur because plants without natural enemies have excess "currency" to put into some other activity. Growing bigger is one. Blossey found this increased size to be the case with introduced purple loosestrife versus its European native counterpart.

Floras of the Cayuga Basin, written in the mid 1800s, shows that loosestrife, whose tiny seeds had hitched a ride in the fibers of cheap wool shipped from England to mills in New England, was seen in upstate New York, but only occasionally. Then all of a sudden it got out of hand. By the late 1960s, this beautiful plant of temperate wetlands had spread through nearly all of temperate North America. Today, this spiky hot pink flower thrives in every state except Florida!

Biological control of purple loosestrife has been one of Blossey's most outstanding successes since the program was established in 1995. He spent a decade conducting host specificity testing on a combination of beetles (Galerucella calmariensis and G. pusilla) and weevils (Hylobius transversouittatus and Nanophyes marmoratus) imported from Europe where the plant, thanks to them, never grows out of control. Blossey wanted to be certain these organisms behave the same way here, that they eat only purple loosestrife without adversely affecting native plant and animal communities.

 

Tree Threat: Plant pathologist George Hudler says that New York State has the dubious distinction of being the main port of entry in North America for tree diseases and insect pests.

With the help of collaborators in 35 states and more than 2,000 wetlands, Blossey's combination has succeeded in reducing the abundance of loosestrife in many wetlands to 5 to 10 percent of its former level, a point at which it is biologically benign. Native plant communities, birds, frogs, and other organisms not seen in many years have returned to these wetlands. Biological control, in practice now for 100 years, can work.

But time is of the essence.

"The more we uncover about ecosystem effects of invasive plants, the scarier it becomes," Blossey says. "While you may recognize that once-common native plants disappear, the decline of insects and other species is silent, nobody is watching out for them."

- Metta Winter

What Can We Do to Subdue Invasive Species?

There's much each of us can do to prevent the spread of invasive species. Above all be curious, Hoebeke says. If you see an insect that seems unusual on a street tree or in your own backyard, follow through and take a sample to your county Cornell Cooperative Extension office for identification. Asian longhorned beetles were discovered to have spread from Manhattan to New Jersey by a man who, while helping a friend move into a new apartment complex, noticed the 1.5-inch-long shiny black and white spotted beetle making spectacular 400 yard hops from tree to tree. Home gardeners should be conscientious about where their plants come from and stick to local natives, Blossey advises. Avoid all plants advertised in catalogs or stores as "hardy everywhere," "very easy to grow," or "has invasive tendencies." Instead plant only those hardy in your immediate area. "You open catalogs and see that wholesalers are bringing in plants from Asia and South Africa every single year," Blossey says. "These might just turn out to be tomorrow's invasive species."unseen and often unknown, until disrupted. Amphibian populations, for example, are on the decline worldwide. But why? In novel research, Blossey and his students are showing how invasive plants disrupt food webs. "Some of our data show that American toads suffer as much as a 50 percent increase in mortality when tadpoles develop.

Plant Diagnostic Network

Protecting the United States from accidentally or purposely introduced plant pathogens and pests is the mission of the Plant Diagnostic Network, which is funded by the U.S. Department of Agriculture and collaborates with federal and state agencies. Cornell coordinates the network's activities in the Northeast, and Rosemary Loria, chair of the Department of Plant Pathology, directs the Northeast division. The network's mission is carried out through early detection of introductions and characterization of the scope and potential damage of those introductions.