The state Department of Agriculture in November rejected five containers full of Christmas trees that arrived from the Mainland because they were infested with tiny burrowing insects called midges.

The containers are among a total of 240 with Christmas trees that are expected to arrive this season.

The last time the department sent containers back to the Mainland was four years ago.

Entomologist Darcy Oishi, of the Agriculture Department's Plant Quarantine Branch, located the midges during inspections Nov. 12 and 19 in containers of mixed Douglas fir and noble fir trees from Oregon.

He said he was able to determine that they were one of two closely related species: needle midges and cone midges. Midges are small, fly-like insects when adult, but during their immature stages these midges burrow in plant tissue and can damage the plants.

When he recognized them as potential invaders into the Hawaiian environment, Oishi sealed the containers, and the Department of Agriculture ordered them shipped back to the Mainland.

Agriculture officials inspect containers of Christmas trees for potential infestation, and while they are watching for a range of alien species, the primary concern is yellow jacket wasps, he said. While Hawai'i has some types of yellow jackets, there are other species not found in the Islands that can be extremely aggressive and dangerous both to humans and to wildlife, he said.

Mainland Christmas tree growers occasionally treat their shipments with pesticides, but Oishi said the containers are often packed so tight that there is little penetration. And in the case of larval midges, they can be protected inside the plant tissues and not susceptible to many kinds of pesticides.

Oishi said that this year, many containers of trees from Oregon — the source of most of Hawai'i's yule trees — have contained more than the usual amount of frogs and salamanders. Generally, they can be tracked down and removed without requiring the rejection of the shipment.

"They're normally pretty anxious to get out of the container," he said.

It remains possible for wildlife to be in trees that are sold to Hawai'i consumers. Oishi recommended that anyone finding unusual insects or other wildlife in their holiday trees call the Plant Quarantine Branch at 832-0566 .

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.