More than 13,000 foreign killers were turned loose in Connecticut this summer, an airborne army of assassins unleashed to take down a very special enemy.
The killers are known as "parasitoid" wasps. Their only target is the emerald ash borer, an insidious alien invader that could end up destroying large sections of Connecticut forest if left unchecked.
Whether these tiny "biocontrol" warriors will succeed is anybody's guess.
"These wasps are kind of the best hope we have, but it's hard to predict the long term," says Claire E. Rutledge, a scientist at the Connecticut Agricultural Experiment Station.
Rutledge was in charge of releasing thousands of the predatory (but stingless) wasps in Prospect and Middlebury between May and September. Those two towns were ground zero for Connecticut's emerald ash borer (or EAB) invasion.
Repeated scientific studies have shown that the only creatures these wasps go after are EABs, and they've been cleared by the U.S. Environmental Protection Agency for use in states where those little green beetles are busy murdering our ash trees.
History has demonstrated that bringing in the wrong sort of biocontrol agents can create as many problems as they solve. ("Bufo" or cane toads, introduced from Puerto Rico into Hawaii in the 1930s to control the sugar cane beetle, offer one infamous example. Turned out the toad's skin was highly toxic and killed lots of native wildlife.)
Rutledge says a lot's been learned since those bad old days. One critical lesson is to only bring in something that is "host specific," a predator that's evolved to eat or kill only the invasive species you're trying to eliminate.
This isn't our first biocontrol insect war in Connecticut, and there have been some spectacular successes.
The birch leafminer is one such victory. This type of tiny sawfly lays eggs in birch tree leaves, and its larvae eat those leaves. In 1974, Connecticut introduced a different type of parasitoid wasp dedicated to going after the birch leafminers, and by 2007 it had basically completed its task. "It's done such a great job that it's pretty much extirpated the birch leafminers," she says.
Which brings us back to the hopes to do the same for the emerald ash borer.
A native of Asia, these wood-boring beetles are believed to have hitched rides to the U.S.A. aboard solid-wood packing materials around 1990. They weren't actually identified until someone spotted them destroying ash trees in Michigan in 2002. Now they've spread to 19 states and Canada.
EABs are deadly: It's estimated that more than 30 million of Michigan's ash trees have died from these pests. (Ash trees make up 4-15 percent of Connecticut's woodlands.)
When EABs were first detected in the U.S., researchers headed for the beetle's native Asian lands to look for its natural foes. In China, they found a couple of different types of EAB enemies, and tests have shown these wasps apparently have no taste for any species of insects native to North America.
(The key there is "apparently." As Rutledge warns, while scientists are "really pretty sure" about the safety of using these wasps, "We can't predict the whole world.")
The technical name for one of these EAB hunters is Tetrastichus planipennisi. This particular breed lays its eggs inside the emerald ash borer's larvae. Each wasp mommy lays 150 eggs inside the EAB larvae, and those little guys eventually kill their host.
These wasps are being raised in a federal lab in Michigan, says Rutledge.
Another anti-EAB wasp type released in Connecticut, the kind of kinky-sounding Oobius agrili, is generally used in fewer numbers because it lays only a single egg inside a beetle's larvae. That makes it harder to raise in the lab since you need more EAB larvae to get more Oobius agrili.
Rutledge says Connecticut experts won't have much idea how effective the wasps can be until research is done to find out how many of the little predators have survived our harsh winter season. But they have proven they can take the cold weather in other northern states so chances are they'll do fine here.
Connecticut officials would love to release lots more of these EAB killers, but so would environmental agencies in many of the other states fighting to save their ash trees. Federal officials say 14 states are now using these wasps, and Rutledge expects the competition to get them from the Michigan growing facility will be fierce.
So far, the parasitoid wasps look like the best chance to save our ash trees.
Emerald ash borers were first spotted in Connecticut in Prospect, in July 2012. The little green buggers have now spread to at least eight other Connecticut towns.
Just last week, federal experts in the Animal and Plant Health Inspection Service issued an expanded EAB quarantine order that prohibits anyone from moving ash logs and lumber, ash nursery stock, and other hardwood firewood out of Fairfield, Litchfield and Hartford Counties. (The original quarantine order only covered New Haven County.)
The reason for the quarantine is that shipments of wood products, particularly firewood, are the way EAB spreads fastest.
"The infestation grows at about one mile a year," explains Rutledge, "but it grows a lot faster if it's in the back of a pickup truck."
In China and other parts of Asia, native ash trees have evolved defenses to limit the damage from emerald ash borers. Our North American ash forests have no such natural anti-EAB systems.
"We're fighting an uphill battle," says Rutledge, "because the trees aren't helping."
The war to stop the emerald ash borer is going to last for a long time, and it may be years before we know how successful our wasp assassins can be. "Biocontrol is a slow burn," says Rutledge. "You have to wait a while to see if it's working."