Trapping of nearly 20,000 insects at ports and timber processing yards has failed to show up any species new to New Zealand which might threaten forestry.

Forest health officials say the first season of high-density beetle trapping, over last summer, used 380 traps to catch 19,801 insects.

The Ministry of Agriculture and Forestry national adviser on forest pest surveillance, Mark Ross, said the results of the Agriquality New Zealand trapping programme were pleasing.

"Results indicate that New Zealand remains gypsy moth-free and we have no new beetle species to worry about."

A further trapping programme is planned for next summer.

The traps were designed particularly to catch species of bark beetles and wood borers.

Some bark beetles carry fungi on their bodies which spread diseases, such as Dutch elm disease or pine pitch canker, in addition to causing damage by their feeding.

Pitch pine canker has not been found in New Zealand, but has decimated radiata pine in its native habitat in California.

Other species, such as the burned pine longhorn, cost timber exporters more than $500,000 a year in extra fumigation costs to sterilise timber for the Australian market.

Because it is attracted to lights, ships cannot be loaded at night during summer, when the longhorn is flying.

Bark beetles prefer to feed on recently felled trees and logging debris, but when populations increase they also attack healthy trees, and many species can attack and kill pines.

While New Zealand's $2.6 billion forest exports sector is dominated by plantings of exotic conifers, such as radiata pine, it has so far been relatively free of destructive species from the trees' original habitat.

Beetle attacks can cause serious problems. The first recognised sign of beetle attack is often yellowing or reddening tree crowns.

Less prominent symptoms of attack can include white, sawdust-like "boring dust" at the base of trees, or "pitch tubes" projecting from the bark.

Other traps placed over the summer failed to catch Asian gypsy moths, whose caterpillars are a major forest pest in China, Russia, Korea, Japan and the United States. The caterpillars are known to defoliate more than 500 plant species, with oak, willow and birch the favoured hosts.

So far, it has not established in New Zealand because gypsy moth egg masses and larvae have been intercepted at ports on ships, containers, and used-car imports from Japan.