Frutas y HortalizasSep. 1, 2011
Nueva plaga poco conocida complica a productores y exportadores de fruta
Se ha detectado una plaga no muy conocida, llamada s/potted wing Drosophila,/ que amenazaría la calidad de las frutas en la parte noroeste de los EE.UU. y Canadá. Esta plaga se alimenta de fruta que está madurando, y podría llegar a complicar las exportaciones de frutas de los Estados Unidos a otros mercados.
Cooperative Extension Agent Mike Bush hunches over the table, peering through a magnifier at seven tiny, black specks. Collected from a plastic trap holding a mixture of vinegar and soap, the specks are fruit flies. But none of them on this recent morning are from the species that everyone in the soft fruit, berry and grape industries is looking for: Spotted wing drosophila (pronounced druh-SOFF-uh-luh).It’s the newest pest to hit the Northwest and one that researchers, field representatives and growers want to find out much more about. “There’s a lot we don’t know about this bug in the Pacific Northwest,” said Bush, who first found the fly last year in the trap in his backyard. What they do know is that this species of drosophila poses risks to fruit and berries up and down the West Coast and could complicate fruit sales in some overseas markets.
Drosophila is not considered a threat to pears and apples because of their thicker skins. What makes this bug different from other species of fruit flies is its preference for ripening fruit, not overripe or damaged fruit. Researchers are beginning to believe the pest could pose a bigger threat in later-maturing areas like north central Washington and British Columbia. That’s because drosophila populations don’t get big enough to become an infestation until late fall. This much is for certain: Once the female has poked through the skin to lay her eggs, fruit and berries rapidly degrade, which is bad news for the necessary quick transfer of soft fruit to the grocery. “When you are looking at some soft fruit, the turn-around time from harvest to be on the store shelf is quick,” said Doug Walsh, an entomologist at the Washington State University research center at Prosser who is collaborating with other researchers up and down the coast. “Something like this that will degrade fruit quality is the biggest issue.”
Spotted wing drosophila, anywhere from 2 to 3 millimeters long — less than half the length of the common house fly — first appeared in the Watsonville, Calif., area, south of San Jose, in 2008 and has become established throughout California, Oregon and Washington. The pest, which arrived in the U.S. from Asia, gets its name from the single dark spot on each wing of the male. Western researchers have launched a number of drosophila projects, thanks to a federal grant designed for specialty crops like those grown in the Northwest. Walsh and Betsy Beers, a Wenatchee-based WSU entomologist, are trying to devise a better trap to catch the fly. An extensive trapping program is under way, with more than 600 placed throughout Central Washington, including the Columbia Basin. Private pest consultants are trapping and Bush has enlisted the help of the county’s Master Gardeners to serve as an early warning system for home gardeners and commercial growers. “That’s the reason the Master Gardeners are so important. They are the eyes and ears out there for this pest. It’s about awareness,” Bush said.
Walsh agrees that backyard fruit trees represent the pest’s threat to commercial growers. “The point to hit home here is that in Oregon and Washington, growers both organic and conventional that have followed pest- control recommendations have not had a problem with this pest,” he said. Commercial growers are advised to use chemical sprays to control the fly. Tim Smith, a WSU regional tree fruit specialist in Wenatchee, also is working on the issue. He said researchers were bracing for large drosophila populations this year based on trapping results from 2010. So far, that hasn’t happened. The thinking is that the severe below-freezing temperatures in November may have killed all but the hardiest flies. Drosophila overwinters as an adult. As a result of the likely extensive winter kill, trap counts are lagging behind last year at this time. Smith said the early trap counts have been almost a nonevent. The first positive catch was in
early June in the Tri- Cities, which is good news. Higher populations are expected, however. Walsh said positive trap finds have occurred in the same places as last year. Drosophila generally produces as many as eight generations a year. It’s not until the fifth or sixth generation that enough flies are present to pose a threat, he said. Walsh said populations may not be large enough early in the summer to pose a huge threat to local cherries. Even juice and wine grapes generally are past harvest before populations shoot up toward the end of October.
While researchers look for pieces of the drosophila puzzle, those who work on the marketing side are dealing with questions from trading partners. “It has become an important issue for some export markets that the insect is here,” said Mike Willett, vice president for scientific affairs for the Northwest Horticultural Council. The council staff deals with trade issues on behalf of the Northwest fruit industry. Willett said Australian officials are concerned that the fumigation protocol required for import of cherries may not be effective against drosophila. The issue is slowing down efforts to gain access in Australia for stone fruits. Mexico also is asking questions about the pest. New Zealand, another market for Northwest fruit, does appear to accept fumigation as a solution for the pest, Willett said. On the ground, researchers continue to try to unmask the drosophila’s secrets. “We are going to keep our guard up and continue to monitor it,” Smith said.