Experts are releasing tiny stingless wasps in certain areas across the state to attack prickly pests that are killing plants.

Entomologists from the Department of Agriculture’s Plant Pest Control Branch released about 300 stingless wasps, species Aroplectrus dimerus, which look like gnats, in Kipapa Gulch yesterday. The wasps are natural enemies of nettle caterpillars that are infesting tall grass and plants.

Renato Bautista, an insect specialist with the branch, said the pesky critters are known to attack coconut leaves, ti leaves, areca palms and other plants, causing financial losses at island nurseries.

It was the second time scientists released the stingless wasps in the state. The first was in Waimanalo two weeks ago.

"The caterpillar is a really serious problem," said Plant Pest Control Branch Manager Neil Reimer. "We’re kind of catching it before it becomes a real problem in Mililani."

The use of biological control to reduce nettle caterpillars is one approach entomologists are taking to reduce invasive pests in Hawaii with host-specific insects. While scientists prefer to use biocontrol methods over non-biocontrol methods to attack pests because of their economic and sustainability factors, Reimer said it doesn’t work in every situation.

"It’s not going to eradicate," he said. "It will knock it down to a level where it’s no longer a problem."

Scientists spent six years doing research to ensure the wasps brought over from Taiwan will attack only nettle caterpillars and not other insects or plants. Experts will head to the Big Island next week and Haiku, Maui, by the end of the month to release the wasps in those locales.

In September 2001, the nettle caterpillar or Darna pallivitta was first discovered on rhapis palms at a Panaewa nursery on the Big Island. The caterpillar population proliferated primarily in Hilo, prompting experts to use chemical sprays to control the infestation.

But the problem continues, with the Plant Pest Control Branch receiving at least five complaints a day from Big Island residents stung by the insects.

The caterpillars grow up to an inch long and are covered with stinging spines. Contact with plants infested by the pests can cause rashes, welts and blisters in humans, similar to a Portuguese man-of-war sting.

To kill the caterpillars, the wasps lay eggs on the caterpillar. Its larvae then eat away at the caterpillar, creating tunnels through its body.

Six years after the pests were spotted on the Big Island, the caterpillars made their way to a Kipapa Gulch nursery on Oahu, traveling on plants flown in from the Big Island in 2007. Reimer said the caterpillars in Kipapa Gulch now are encroaching on residential areas. Experts released the wasps near homes yesterday to kill them before they travel up the grassy hillside.

Other biocontrol projects entomologists are working on involve the fireweed or Senecio madagascariensis, a toxic plant that poses a danger to livestock in pastures, primarily on the Big Island and Maui. Experts spent six years researching a solution to kill the invasive weed. The natural enemy, Secusio extensa, a caterpillar found in South Africa, eats away at the toxic weed.

Reimer said fireweed is a widespread problem on ranches on the Big Island and Maui. Officials are in the process of obtaining state and federal permits to release the caterpillars in the area.

A successful biocontrol project was held a year and a half ago when entomologists brought in another stingless wasp, Eurytoma erythrinae, from Tanzania. The wasps were brought in to prey on the Erythrina gall wasp, or Quadrastichus erythrinae, which was killing native wiliwili trees at an alarming rate in 2005. The project helped the wiliwili population recover.