Spotted-winged drosophila (Drosophila suzukii) is a major pest of fruit and berries that arrived in the United States in 2008. Researchers in China point to spent brewer’s yeast as the source for a promising protein bait for monitoring and managing the pest. (Photo credit: Flickr/Oregon Department of Agriculture, CC BY-NC-ND 2.0)

By Ed Ricciuti

The smelly sludge left over from making beer may be a new tool for fruit growers to control spotted-winged drosophila (Drosophila suzukii), a scourge of fruits and berries, according to researchers in China.

Ed Ricciuti

The possibility is good news for beer brewers, too. Disposing of the dregs remaining after beer production can be a major headache, not to mention costly. Moreover, improper discharge of the gunk at the bottom of the barrel can run a brewery afoul of the Clean Water Act and other environmental protection regulations.

Known as spent brewer’s yeast, this microbial protein from the fungus Saccharomyces cervisiae has been turned into a few marketable products, but they have seen low uptake in the United States. One use is in compost. Spent yeast also has been tried with mixed success as an ingredient of animal feed and food spreads for humans. Yeast spreads are especially popular in the United Kingdom and some Commonwealth nations. Indeed, one yeast spread is so popular Down Under it has been called the “peanut butter and jelly” of Australia. By and large, however, spent-yeast disposal costs can subtract substantially from a brewer’s bottom line, so any new alternative use is a welcome development.

In a study published in October in the Journal of Economic Entomology, a team of scientists in China, led by Pumo Cai, Ph.D., of the Fujian Agriculture and Forestry University, show that protein bait derived from spent brewer’s yeast is much more attractive than traditional baits used on the spotted-winged drosophila, a major pest of soft-skinned fruits such as cherry, strawberry, blackberry, and blueberry. Unlike other Drosophila flies, which deposit eggs in fruit already softened by decomposition, the spotted-winged drosophila has an ovipositor with serrated edges, enabling it to puncture intact fruit. That adaptation, together with its high fecundity and ability to thrive in multiple habitats, makes it an especially scary threat to fruit and berry production. Not only does the egg-laying process damage the fruit, but the larvae that hatch from the eggs consume it.

Long a pest in its native Asia, the spotted-winged drosophila has cost United States growers hundreds of millions of dollars since it was first seen here in 2008. Finding an attractant that specifically targets the fly for monitoring populations and luring it to insecticides is complicated by its exceptionally broad diet. Most attractants draw large numbers of other insects as well as the spotted-winged drosophila. Apple cider vinegar (ACV) or synthetics of similar composition are the attractants most commonly used. Chinese farmers also use a mix of sugar, a traditionally aged “mature” vinegar, and wine (SVW).

Use of yeast as an attractant makes sense because studies have shown that protein is essential to egg development in the group of flies to which the spotted-winged drosophila belongs. The Chinese researchers compared brewer’s yeast protein bait with ACV and SVW baits and found it was by far the most attractive. The yeast protein not only attracted more flies but also drew in flies of both sexes and over a wide age span. Potentially, say the scientists, the bait could be used to remove reproductively immature female flies from the field before they start to mate and lay eggs in crops.

Another finding of the research could help integrated pest management practitioners deploy baits more effectively. It found that the baits attracted the most flies between 8 a.m. and 10 a.m. and between 2 p.m. and 4 p.m.

Like most insects, the spotted-winged drosophila observed in the research used their antenna to sense odor arising from the bait under testing. However, more than 13 percent of flies from which antenna had been removed also found the bait. The research suggests that perhaps other organs, such as palps on the maxilla, also have olfactory functions.

The researchers stress that the spent yeast attractant has yet to be examined in crop fields. It is also not known if the odors arising from fruit could overcome those from the yeast. Before it is truly employed, the attractant may have to be tweaked but it does hold promise.

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.