The State of Integrated Pest Management for Spotted-Wing Drosophila

By John P. Roche, Ph.D.

Spotted-wing drosophila (Drosophila suzukii), is an invasive fruit fly species that causes about $500 million in economic damage to fruit crops in the U.S. each year. A native to southeast Asia, it arrived in the U.S. in Hawaii in the 1980s and in the continental U.S. in California in 2008. It is now widespread through many parts of the U.S. and the world.

Several characteristics make spotted-wing drosophila (SWD) difficult to control. It has a high reproductive rate and strong dispersal abilities, and, unlike most fruit flies, female SWD can pierce the skin of undamaged soft-skinned fruits such as cherries and berries to lay eggs. Also, SWD are highly flexible in their behavior, physiology, and development, and this allows them to quickly adapt to new environments.

In a new review article published last week in the Journal of Economic Entomology, Vaughn Walton, Ph.D., of Oregon State University and a multi-university team of experts have created a comprehensive look at how SWD management strategies are evolving to address these challenges. Walton and colleagues provide a detailed, in-depth analysis of the current state of control of SWD and the most promising future directions in the species’ integrated management. They detail ways SWD can be eliminated, ways its reproduction can be disrupted, and ways it can be repelled from fruits.

They focus on developing a combined approach utilizing a range of different interacting strategies, which is known as integrated pest management. IPM combines tools such as focused chemical control, biological control, habitat manipulation, adjusted cultural practices, and the use of resistant plant varieties.

“When SWD arrived in California in 2008 and Oregon in 2009,” Walton says, “I thought it was a finicky little insect that could be quickly eliminated.” Instead, SWD spread throughout the country, caused immense economic damage to the fruit industry, and proved to be a remarkably challenging pest to control. But Walton says he wants to highlight some of the positive news about SWD management to give the industry hope.

One of these positive stories is biocontrol. “Biocontrol is not the only solution,” Walton says, “but it is a significant part of the solution.” Walton and his colleagues are testing many possibilities for biocontrol species.

Another positive story, he says, is mathematical modeling. With modeling, IPM can be optimized and insecticide use can be minimized. “Chemical control is something we would like to eventually get away from,” Walton says.

A third success story is the training of scientists that this multi-university research program has made possible. “Over the years,” Walton says, “this research initiative has produced a ton of knowledge and has trained many postdocs. Thus, we have been able to educate a new generation of scientists expert at the control of invasive insects.”

An effective management program for SWD depends on knowing where and when SWD are present. Thus, sampling for the presence of the flies is critical. SWD are usually sampled with simple traps using apple cider vinegar or yeast and sugar.

SWD display pronounced seasonality. Walton and colleagues explain that, in the northern U.S., the first adults are found in late June to early July. In warmer areas of the country, adults can be found throughout the year, often with a population peak in the fall. In hot, dry areas, sampling shows population peaks in the spring and fall. In the summer, 90 percent of SWD individuals are eggs, larvae, and pupae, and only 10 percent are adults. As a result, the researchers write, insecticides that target adults in the summer are inefficient. However, several insecticides used against SWD, including malathion, target immature stages of the fly and thus could be effective. In the winter, adult SWD often overwinter in wooded habitats, so managers could successfully target overwintering flies in those locations. Understanding these seasonal changes in SWD is essential for designing effective IPM.

The majority of control efforts for SWD currently rely on chemical control. Insecticides can be very effective, but they have drawbacks. They can harm non-target insects. If they are applied only to crops, pests can then repopulate the crops from surrounding areas. And with some insecticides, with repeated exposure, insects can evolve insecticide resistance, making given insecticides ineffective. Therefore, it is important to rotate use through different classes of insecticide so that resistance does not develop. SWD has a short generation time and a high reproductive rate, characteristics that would be expected to increase the rate at which insecticide resistance develops. But other than data on insecticide resistance in the berry industry in California, SWD resistance has not yet been observed in most farms sampled in North America.

Walton says that he and his colleagues are most focused now on developing biological control and behavioral control of SWD. There are three levels of implementing biological control: protecting natural enemies of the pest, increasing populations of natural enemies that are locally present, and introducing populations of natural enemies from the invasive species’ native region. A huge advantage of biological control is that once established, it can often self-sustain and thus reduce costs. Types of biological control include the use of predators of the pest, parasitoids of the pest that kill larval or pupal stages, and entomopathogens such as fungi, microsporidia, nematodes, viruses, and bacteria. One promising parasitoid species researchers are working on is Ganaspis brasiliensis, which preys on SWD larva in Asia. A recent study found that the species parasitizes over 47 percent of SWD larva in China and up to 75 percent of larva in Japan. Walton and colleagues are exploring whether Ganaspis brasiliensis can be introduced into communities in North America to reduce SWD populations. However, biocontrol can be negatively affected by chemical control. For example, some insecticides used against SWD can be damaging to parasitoids that control SWD.

Behavioral control of SWD aims to interfere with the flies’ ability to find host plants, to feed, or to lay eggs. Tactics include using odors to repel SWD, such as hops, sweet alyssum, or essential oils; using stimuli that attract SWD to traps; and using arrestants that disrupt feeding behavior, such as sucrose.

When asked about how best to inform industry about practices being developed in research, Walton says they rely on traditional agricultural extension service communication, providing information at field days, utilizing social media, and hosting informational webinars each year. “But scientific publications themselves are a vitally important piece of communication initiatives,” Walton says. “They provide rigorous, peer-approved information that is an extremely important resource.” And, Walton points out, scientific publications give investigators the opportunity to share what mistakes were made, so that science and management can evolve and improve. “Scientific publication is one of the most important services we provide to our industry,” Walton says.

The U.S. Department of Agriculture grant that funds part of Walton and colleagues SWD research stipulates that they work with industry influencers, and they have been doing this from the beginning. They bring technologies to industry and seek feedback on how well the technologies work in actual practice. “Federal funding is allowing us to listen to and serve our clients—the growers,” Walton says.

As the Journal of Economic Entomology paper details, many promising control strategies are being developed for this challenging and uniquely adaptable invasive species. With continued advances, researchers can hope that populations of SWD can be controlled and the damage they cause reduced.

John P. Roche, Ph.D., is an author, biologist, and educator dedicated to making rigorous science clear and accessible. Director of Science View Productions and Adjunct Professor at the College of the Holy Cross, Dr. Roche has published over 200 articles and has written and taught extensively about science. For more information, visit https://authorjohnproche.com/.