Invading wasps at Haleakala National Park, which usually make nests the size of a football, have grown nests "the size of a '57 Buick," according to a new study.

Research just published in the Proceedings of the National Academy of Sciences shows a fascinating interplay in which the invaders are being shaped by their new environment, just as they are drastically changing the native ecosystem. Not only do the aliens — western yellowjacket wasps, Vespula pensylvanica — take advantage of the lack of cold winters to grow huge nests, they have taken to eating vertebrate meat such as geckos and native shearwaters, as well as other insects.

University of California-San Diego grad student Erin Wilson studied the yellowjackets at Haleakala and Hawai'i Volcanoes national parks in 2006 and 2007. The yellowjackets have been a problem in the parks for years, but their new diet and numbers were a surprise.

In a telephone interview, Wilson said yellowjackets like high, lonely places.

They are hard to find, which is why the size of the nests — up to 600,000 individuals compared with a few thousand in a usual nest — escaped attention.

Along with Argentinian ants, the yellowjackets are among the most dangerous alien arthropod invaders in the park.

"It's not just what they're killing," Wilson said. "They're also collecting great amounts of nectar, drawing down the resources for anything else that might want to feed on it, whether it's native insects or birds like the Hawaiian honeycreepers."

The wasps do not attack and kill vertebrates. They scavenge the protein-rich remains of dead animals. But even that could help unbalance the native ecosystem by usurping the food supply of native scavengers like the pueo (a native owl).

Human intervention can reduce the impact, unlike with the ants, which are so far beyond control. Poisoning or digging out the nests can ease the pressure on native species, and Wilson's research found that the native species are able to make a comeback in areas freed of the wasps.

The yellowjackets are not aggressive toward humans unless the nest is disturbed. "The hard part is to find them," she says.

Wilson is concerned about the native solitary bees and wasps, particularly the yellow-faced bee, which not only have their nectar taken but are attacked in the air by the yellowjackets and in the soil by the ants.

The vast numbers of yellowjackets are changing the ecology of Hawai'i's 'ohia forests and subalpine shrublands, Wilson said.

The adults mainly consume nectar but need protein to feed to their larvae. Besides lizards and birds, they go for tree lice and juicy caterpillars and spiders and a wide range of other arthropods. "They're just like little vacuum cleaners," she said.

Being flexible as to dinner can help explain why some species become invasive, the report says, but less attention has been paid to how a new environment can alter the invader's behavior. "It's a fascinating system," Wilson said.

She is lead author of the paper, which was part of her doctoral dissertation.

Now that Wilson has her Ph.D., she has several grant applications pending and hopes some of them bring her back to the Islands.

"Some of the stimulus money is trickling down, finally," she said Monday.