Pesky African wasps that destroyed more than 1,000 of the city's wiliwili trees will likely cost taxpayers $1 million next year.

The dying wiliwilis have kept workers so busy that trimming and maintenance of other trees has fallen behind, said city parks director Lester K.C. Chang.

"We have to catch up before we get into a real safety and liability issue," he said.

The work will require a $1 million increase to the city's $7.8 million urban forestry budget, which pays for tree planting and maintenance, he said.

Erythrina gall wasps kill trees by attacking the leaves, and have destroyed thousands of wiliwilis across Hawai'i over the past few years.

The trees are found only in the Islands, and are often used as windbreaks or to mark property boundaries. They were especially common on O'ahu's North Shore until the wasps began wiping them out.

Scientists fear the insect could push the wiliwili into extinction.

The wasps are the size of grain of sand, and are not harmful to humans. But they also attack nonnative tropic coral and coral trees.

The Erythrina gall wasp was first identified in 2004 by a Korean scientist who examined specimens taken from Singapore, Mauritius and Reunion.

Officials suspect the tiny insects arrived in Hawai'i in a shipment from Taiwan, which has also struggled with coral tree damage caused by the wasps.

The city has been under extra pressure to maintain trees properly since 2005, when a city-owned Norfolk Pine severely injured a 13-year-old girl in Manoa. Her parents later sued a company the city had hired to inspect the 80-foot-tall tree, which had been badly damaged by termites.

Reach Johnny Brannon at jbrannon@honoluluadvertiser.com.