16 Sep 2022

Drosophila control with Samba wasp launches in advanced phase

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Michigan State University (MSU) researchers specializing in fruit crop pests have begun testing the effectiveness of a new biocontrol agent that could help reduce damage caused by the Drosophila suzukii (SWD in English) on crops.

This invasive pest has been a major production challenge for berry and cherry growers in recent years since it arrived in the United States. The samba wasp(Ganaspis brasiliensis) is a small parasitic wasp that lays eggs in SWD larvae.

After years of evaluation and permit reviews, the U.S. Department of Agriculture (through its Animal and Plant Health Inspection Service) and the Michigan Department of Agriculture and Rural Development have approved the release of this wasp in regions where it threatens fruit production.

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One of the MSU researchers, Juan Huang, went to the USDA lab in Delaware in April and received training on the breeding process. She returned to MSU with 100 of these insects to begin mass breeding. Sufficient numbers of wasps are now in the colony and were recently released for cherries at the Northwest Michigan Horticulture Research Center in Traverse City, Michigan, and on August 18 for blueberries at the Southwest Michigan Research and Extension Center (SWMREC) in Benton Harbor.

Researchers will evaluate the impact of this new species on Drosophila suzukii populations and their potential for establishment in Michigan's climate. This program offers the hope of improving integrated pest management practices on Michigan fruit farms and reducing their establishment potential in Michigan's climate. This program offers the hope of improving integrated pest management practices on Michigan fruit farms and reducing insecticide sprays for this pest.

HOW IT ACTS AGAINST DROSOPHILA SUZUKII

"This little wasp flies around looking for berries already infested with drosophila larvae," explained MSU entomologist Rufus Isaacs.

Isaacs, lab manager Jackie Perkins and integrated fruit and vegetable pest management trainer for Southwest Michigan growers Michael Reinke released three vials of samba wasps in a test plot at blueberries Elliott on Aug. 18.

Ganaspis brasiliensis, an antagonist of Drosophila suzukii
The samba wasp (Ganaspis brasiliensis) (photo FEM)

"Many of these will be in the wild habitat around farms," Isaacs said. "Their job is to find a blueberry infested with SWD larvae. They insert their egg layer into the berry, which contacts the inside of the SWD larva, and then lay the egg in the fruit, inside the drosophila larva . It then develops inside, killing the larva, and comes out as a wasp in about a month."

Issacs said the wasps will mate. Each wasp is expected to lay about 60 to 80 eggs.

"We have already monitored the percentage of native wasps in this field this summer," he said. "Now we have made these releases and we will come back here to see if we can recapture this emerging population from Drosophila suzukii here and at the other sites where we are making the releases."

It took years of evaluating many types of wasps to figure out which would give the best results.

"This one was selected and then went through all the necessary procedures to get tested, including USDA-APHIS approval," Isaacs said. "This wasp species has already been detected in Washington State, so they are a little bit ahead of us. This way we hope everyone can catch up."

BREEDING AND LAUNCHING

MSU researchers are releasing the wasps into Trécé Inc. traps, which provide a dispersal point for the wasps to move into the bushes at blueberry.

The traps are hung in the bush part of the plant, placed far enough away so that workers do not drop them when passing with equipment.

When Reinke pulled the plug on the first wasp vial, a Drosophila conveniently landed on the trap, providing proof of concept.

"We can be a bit ceremonious," Perkins said. "The breeding process is quite complicated. You have to have fresh fruit. You have to have an insect colony (SWD) to access the fruit. You have to transfer the wasps using an aspirator-type device. Each individual wasp is sucked up with an aspirator and transferred to the infested fruit. They are left on the fruit for 5-7 days. Once they lay eggs in that larva, we transfer them to the next stage of the infested fruit."

MSU student Andrew Jones handled most of this process, and the specific conditions, such as humidity, were a learning process for all project participants.

"It's the end of a long journey to get here, but it's also just the beginning to see how they work," Isaacs said.

A NATIONAL EFFORT

Collaborating researchers have recently released the wasps at selected locations on the West Coast, hoping they will become established and help control SWD.

"In Michigan we are doing it here and in Traverse City with cherries, and other researchers across the United States are conducting experiments in other states and in other settings," Isaacs said. "The group of entomologists across the country will compare and see where it establishes well and where it reduces SWD populations. It will take years to measure the long-term benefits of wasps."

The wasps are being released at some unmanaged sites, where they will not be sprayed, to give them a better chance of establishing a population. Release at commercial sites could begin in the fall, Isaacs said.

He acknowledged that it will be a long process to bring wasps to growers on a regular basis.

"We are still trying to improve breeding methods to make them easier than they are now," he said. "On farms, the release will be more likely in woodlands and wild habitat, where there is this reservoir of Drosophila suzukii coming to farms, partly because growers are spraying to protect fruit from Drosophila and these (wasps) are very sensitive to pesticides.

"Right now we are thinking of an early season release to kick off biocontrol of this pest in the spring," he said. "Then releases would go on throughout the summer, especially in wild areas, to try to reduce the invasion that occurs at the edge of fields."

Although there are commercial insectaries that breed biocontrol agents, Isaacs said it is not yet decided whether a commercial distributor will provide the program.

WIDESPREAD APPLICATION

The samba wasp is capable of attacking any fruit in which Drosophila is present. As the parasites move to different hosts, "the wasps will hopefully follow," Isaacs said.

"It is a very active research area. Some labs are already planning a wintering study of these bees. We'll see if they can survive the Michigan winter." "We have good habitat in the western part of the state," Reinke said. We have good snow cover that will protect them during the winter."

Isaacs said original funding for much of the discovery work came from Specialty Crop Research Initiative (SCRI) funds earmarked for California, Oregon and Washington.

"We had more recent SCRI grants that helped us continue the biocontrol evaluation and all the work that needed to be done to get to the point of having an approval for release," he said.

The Michigan Department of Agriculture funds the program through a specialty crop block grant, as does the Michigan Blueberry Commission. Project GREEEN funding is paying for work to increase the wasp population.

Source: FruitGrowersNews
Photo: A vial of samba wasps prepared for release in a field at blueberries. (Gary Pullano)

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