Stink bugs: the scientific battle against an insect invasion

Millions of stink bugs are emerging from their hibernation, a sign of spring that strikes fear into the hearts of farmers across the US. Could a tiny wasp be the ultimate weapon in the battle against the foreign bug invasion?

Anybody who has dealt with an infestation of stink bugs knows what a nuisance these armoured insects can be. They cluster in the thousands and release a foul odour when squashed or threatened.

And for farmers, they can be devastating.

Unlike many insects that feed on a small group or even a single plant, the brown marmorated stink bugs, which originated in Asia, eat pretty much anything. More than a hundred different crops have been recorded in their diet so far.

The devastating effect of the bugs' appetite became apparent to farmers in 2010 when swarms destroyed fruit crops across several states in the mid-Atlantic region. The attacks cost the apple industry alone an estimated $37m (£24m).

Hunt for a killer

Matt Buffington, a research entomologist at the US Department of Agriculture (USDA), says the stink bugs arrived in the US sometime between the late 1990s and 2003 to find an "enemy-free space".

"That means it can expand its population infinitely because there is nothing keeping it in check," he says.

"Birds haven't learned to eat it - possibly because it tastes gross - and native insect predators that would normally eat something like a stink bug don't recognise it."

The stink bug killer? A parasitoid wasp - in close-up (inset) - attacks stink bug eggs and may be the best hope for ending the bug's crop destruction

In Asia, brown marmorated stink bugs are naturally controlled by parasitoid wasps. This genus of wasp, called Trissolcus, contains around 70 different species, many of which have yet to be properly described.

These wasps could prove to be the bulwark against the invasion - but identifying the most effective type to take on the Asian stink bug in the US could take years.

That is why Dr Buffington is working at the Smithsonian's National Museum of Natural History in Washington DC, home to the National Insect Collection and its 36 million specimens. The collection offers his best chance of finding the right wasp.

"We consider it a national treasure - there's nothing like it in the world," says Dr Buffington. "It's the single reference centre for US agriculture."

There are wasps native to the US that prey on indigenous stink bugs but are ineffective against the Asian variety.

In China and other Asian countries, stink bugs and wasps have evolved together over millions of years. Importing a Trissolcus species from Asia may be the solution.

But researchers have to be sure the same wasp will not attack other insects, such as native stink bugs that are not a threat to agriculture.

"Otherwise we could be investing millions of taxpayers' dollars in a biological control programme that is either going to be ineffective or potentially disastrous for North American agriculture," says Dr Buffington, "because we might release the wrong wasp."

Stink bugs lay batches of 25-30 eggs at a time. Female parasitoid wasps then attack the bugs' eggs by laying one of their own inside each stink bug egg. The developing wasp completely devours the embryonic stink bug before hatching.

Stink bugs are stored on sticks in the lab

To have any effect, tens of thousands of wasps would need to be released into carefully selected areas to enable them to establish numbers large enough to control stink bugs. And any release would have to be sanctioned by the USDA.

But wasps are not the only possible answer. At the USDA's Appalachian Fruit Research Station in West Virginia, researchers are learning about the stink bugs themselves in the hope of developing other natural controls.

"There isn't a single solution," says Tracy Leskey, head of the stink bug task force, an alliance of government agencies and university scientists.

"First we have to understand the basic behaviour of the stink bug, its biology and ecology. We need to know what plants it feeds on and how far it can move."

Know your enemy

To measure the insects' flight range, stink bugs are tethered to miniature windmills that revolve as they fly. By counting the laps, Dr Leskey's team have discovered that most cover a distance of one to two miles - but that some can travel more than 20.

"We call them super-fliers," she says.

A scientist uses radar to track stink bugs' flight across an orchard

Stink bugs are also monitored by radar using miniscule radio transmitters that are glued onto their backs. That data will help scientists track the food they eat.

Experts say pesticides are not a sustainable solution because the problem is so widespread. In the last decade or so, brown marmorated stinkbugs have established themselves on the east and west coasts of the country. And although they do not harm people, they can be a nuisance - hundreds of thousands can congregate in warm houses to hibernate through the winter.

"We're still trying to work out why they do that, what attracts them," says Dr Leskey.

Insects recognise each other using chemical markers and it is possible that areas laced with pheromones may keep stink bugs away from crops. Other options might include tempting them away with other tasty plants.

Whatever the solution, researchers know they have to act fast. The 2010 stink bug invasion was the worst on record. But stink bugs have already been found earlier than usual in Maryland - and 2013 could be another bad year for farmers.