The stinging nettle caterpillar, which has established itself in Kipapa Gulch, appears to have spread to some neighboring Mililani residential areas, the state Department of Agriculture said.

"It's been building up populations in (Kipapa Gulch), and it looks like it's spreading to the edge of Mililani right now," said Neil Reimer, head of the state agriculture department's plant pest control branch.

Reimer said officials have set up survey traps for the adult male moth in Mililani and Mililani Mauka and found moths in areas bordering Kipapa Gulch this year. So far, only moths have been found but it's an indication that caterpillars could also be in the area, Reimer said.

Officials will work with residents in neighborhoods where moths and caterpillars are found, he said. Insecticide sprays combating caterpillars, such as the nontoxic Bt, Bacillus thuringiensis, are effective in controlling them, Reimer said.

Nettle caterpillars, first spotted on the Big Island in 2001, were discovered at a nursery in Kipapa Gulch about a year ago. Officials knocked down its population to "extremely low levels" in the nursery and other areas but found that it had already been well established in the surrounding gulch vegetation, Reimer said.

And that poses a problem for Mililani.

"What we're going to do is work with the residents in Mililani so that ... they can control it," Reimer said. "It's easy to control with sprays, but the problem is it's going to keep coming into the area from Kipapa Gulch and from the surrounding wild areas around Mililani."

But officials haven't given up on heavily infested areas.

Reimer hopes to soon release a natural enemy of the nettle caterpillar: a tiny parasitic wasp found in Taiwan. The wasp Aroplectrus dimerus — about the size of a gnat — lays its eggs on the nettle caterpillar, and when the larvae hatch, they feed on the caterpillar. Tests have shown the wasp will attack only the nettle caterpillar, which means the wasp population should fall as the caterpillar numbers decline, Reimer said.

Officials hope to obtain federal approval to release the wasp by the end of the summer, Reimer said.

State and federal agriculture officials are also researching other tools to combat the nettle caterpillar, such as using pheremones to disrupt mating, he said.

The inch-long nettle caterpillar is white with black bands and has bristly stinging hairs that when touched can cause welts, blisters and rashes in humans. Stings may also cause allergic reactions.

The caterpillar has been found primarily on the undersides of palm plant leaves, grasses and lilies. The adult moth is smaller than a dime.

Residents may report nettle caterpillars by calling 643-7378.