Drosophila parasitoid found

A female wasp attacks a spotted wing drosophila pupa

PhotoS COURTESY OF PETER SHEARER AND PRESTON BROWN, OSU

Scientists at Oregon State University have identified a parasitoid of the spotted wing drosophila, raising hopes that in the future this natural enemy might help control the pest.

It’s a small wasp from the pteromalid family, reports Dr. Peter Shearer at OSU’s Mid-Columbia Agricultural Research and Extension Center at Hood River, but the species has not yet been identified. The wasp has been found in cherries, cotoneaster, and snowberry and has been reared on spotted wing drosophila in the laboratory.

The spotted wing drosophila (Drosophila suzukii) was first found in California in 2008 and began showing up in Pacific Northwest orchards a couple of seasons ago. It has shown a predilection for sweet cherries and, unlike its close relative the common ­vinegar fly, it attacks fruit on the tree before it is fully mature as well as mature and ­unharvested fruit.

To the dismay of cherry growers and entomologists, the new species is not susceptible to the GF-120 (spino­sad) bait, which has become the standard treatment for cherry fruit fly in the Pacific Northwest.

Lab tests

Shearer said OSU applied for funding from the Agricultural Research Foundation in Oregon to look for natural enemies of the spotted wing drosophila. Last fall, scientists went out and collected 2,700 berries and fruits of various kinds that appeared to be infested with spotted wing drosophila. They took the fruit and berries back to the lab and put them in rearing containers.

In most cases, just the spotted wing drosophila emerged from the fruit, but both the spotted wing drosophila and the wasp emerged from the cherries. In lab tests where wasps were exposed to spotted wing drosophila pupae, the wasps parasitized them.

The female wasp pierces the drosophila pupa with her long ovipositor and lays one egg per pupa. It’s not known yet how many eggs the wasp can lay in a lifespan. It takes about 30 to 33 days for a wasp to develop and emerge from the pupa as an adult from the time the egg is laid. The female wasp can also kill a pupa by stinging it and sucking out the body fluids, a process known as host ­feeding.

Shearer said there’s a difference between drosophila pupae from which a drosophila fly has emerged and one that has been parasitized. When the fly emerges, the head of the pupal case opens up like a cargo plane and the adult fly crawls out. When the pupa is parasitized, the adult wasp chews a hole in the pupal case through which it emerges. This difference will enable ­scientists to tell if drosophila have been parasitized or not when they find pupae in the field.

Shearer said more would be known about the parasite once the species has been identified. Scientists will be able to do a literature search and find out what its normal host is and where it came from. Although the spotted wing drosophila has natural enemies in Asia, where it originated, Shearer thinks it likely that the wasp is endemic to the United States. It might be a parasitoid that goes after other insects and targeted drosophila when the fly arrived.

It’s not known yet if it also attacks the common vinegar fly, nor is it known how much help the wasp might be in controlling drosophila. The wasp’s life cycle takes about three times as long as that of the drosophila, but Shearer said parasitic wasps normally have a longer developmental time than their hosts. “Depending upon how many offspring it can produce, it may or may not be able to keep up with spotted wing drosophila,” he said.

It was first thought that the spotted wing drosophila might not be able to survive the cold winters of the Pacific Northwest, but Shearer said that even as late as last November, they found cherries still hanging on the trees that were infested with the pest. Both male and female flies were also trapped in Hood River at the end of January in freezing weather and in traps where the vinegar bait was frozen.

Oregon State University has hired a doctoral student from Thailand to research ­natural enemies of the spotted wing drosophila, including the pteromalid wasp.

Dr. Vaughn Walton, horticultural entomologist with OSU in Corvallis, said graduate student Oraphan “Nikki” Kernasa has a master’s degree and was a lecturer at ­Kasetsart University, Bangkok, one of the bigger universities in Thailand. She is a specialist in biological control and has worked with the spotted wing drosophila in ­Thailand. At OSU, Kernasa will study the biology of the pteromalid wasp and do some foreign exploration of ­natural enemies of spotted wing drosophila.

Spotted wing drosophila originated in Japan, but is now widely distributed across east Asian countries, including China and Thailand. Very little is known so far about its natural enemies there, Walton said, though scientists are looking into it. Researchers with the U.S. Department of Agriculture and the University of California are also involved in the effort to identify natural enemies in Asia and bring them through quarantine in the United States so they can eventually be released.