HILO, Hawai'i — Scientists want to release on the Big Island an insect that attacks strawberry guava in an attempt to curb the invasive plant that has already crowded out native species on thousands of acres of forests.

Conservationists have been hacking down or bulldozing stands of strawberry guava, or waiawi, for years, but the plant spreads aggressively as birds or pigs eat its fruit and distribute its seeds. It is found in thick stands on every island.

Originally brought to Hawai'i in 1825, strawberry guava is very difficult to remove once established. It sends up new shoots from cut stumps, and trees toppled by bulldozers quickly sink new roots into the ground, Johnson said.

It has no natural enemies in Hawai'i, it is spreading in the national park, "steadily advancing in the forests where it occurs," he said. Even releasing the scale isn't expected to kill thick stands, at least not right away, Johnson said.

"The best we can really hope for at this point is to stop strawberry guava in its tracks," he said.

Scientists aren't certain how quickly the scale will spread, but they expect the process will be relatively slow. The males can fly, but can't spread the species on their own. The eggs and newly hatched nymphs can be distributed by the wind, but scientists don't know how far they will move or how quickly.

Newly hatched nymphs crawl to new tissue on the plant, which triggers a process where the tree forms what's called a gall to isolate the insect. The galls look like cones or bubbles on the leaves, and each one has an insect inside.

When the tree forms galls to protect itself, that drains off energy the plant would otherwise devote to growing and producing fruit, and the plant weakens.

Johnson said he is proposing that the impact of the scale be monitored in the national park and the Big Island for perhaps two years before releasing it on other islands. As for the overall effect, "We're looking at decades for the impact to unfold," he said.

He said scientists in Florida are also studying the scale as a possible method for controlling strawberry guava infestations there.

This is the first time this scale has been released for this purpose, but the state has a very long history of biological control efforts, Johnson said. Examples include the use of moths to chew the insides of prickly pear or panini cactus, and a number of insects that were introduced to attack the lantana weed, which was much more common years ago.

There are some people who have planted strawberry guava as an ornamental plant or have considered it as a potential alternative crop, but Johnson said it does not have a high commercial value in Hawai'i.

As for commercial-grade guava crops, Johnson said years of lab tests and observations of the scale in Brazil demonstrated the scale does not attack the commercial guava varieties.

Extensive tests have also been done on a variety native plants such as 'ohi'a, and the scale does not attack them, he said.