Cankers on walnut trees infected with the thousand cankers disease eventually grow together, weakening and then killing the hosts.
Cankers on walnut trees infected with the thousand cankers disease eventually grow together, weakening and then killing the hosts.

Environmental conditions influence seasonal flight behavior of a small beetle that spreads a deadly fungal disease to walnut trees, according to new University of California, Davis, research.

The research may lead to better control of thousand cankers disease, which is found throughout much of the United States, according to a news release.

Yigen Chen and Steve Seybold continually trapped the tiny insect, about a third of the size of a grain of rice, oduring three years along Putah Creek in Davis. They recorded how temperature, light intensity, wind speed and air pressure influenced when the beetles took flight.

"Understanding the walnut twig beetle's seasonal flight cycle and factors that govern its flight are critical first steps in the early detection of invasive species prior to implementing pest eradication or integrated pest management programs," Chen, a research entomologist with the UC Davis Department of Entomology and Nematology, said in the release. The work appears online in the journal Plos One.

The walnut twig beetle, Pityophthorus juglandis, does little damage itself to walnut trees, said Seybold, who is with the U.S.Departmet of Agriculture Forest Service Pacific Southwest Research Station in Davis. He also is an affiliate of the UC Davis Department of Entomology and Nematology.

But when coupled with the fungus, Geosmithia morbida, the beetle causes thousand cankers disease.

The beetle creates numerous galleries beneath the bark, resulting in fungal infections and canker formations.

The earliest symptom of the disease is yellowing foliage that progresses to branch mortality and decline of the tree crown.

Other symptoms are numerous small cankers on branches and the trunk, and holes and other evidence of tiny bark beetles.

When male beetles begin to burrow, they emit pheromones that signal a food source to other beetles.

As the disease advances, trees decline and eventually die.

Seybold and colleagues' earlier work involved identifying the pheromones and synthesizing them to be used in lures and traps. The chemicals also could be used to disrupt normal behavior, which could be key to helping control them.

During the environmental studies, the researchers divided the season flight into three phases: emergence, January-March; primary flight, May-July; and secondary fight, September-October.

They discovered that more females than males flew in response to the male-produced pheromone. During the spring and summer months (May to September), daily flight patterns showed a minor peak in mid-morning and a major peak at dusk, with about three-fourths of the beetles caught between 6-10 p.m.

New knowledge of when the beetles fly and the effects of weather and lighting should help in deciding when the synthetic pheromones could best be used, the researchers say.

Walnut twig beetle is native to the Southwest and Mexico, but it has been detected throughout much of the United States. In 2013, it was reported in northern Italy.