Spotted-Wing Drosophila: New Seasonal Activity Model Will Aid IPM Efforts
The invasive fly known as spotted-wing drosophila has been in the United States more than a decade, and it continues to threaten a variety of soft-fleshed fruit crops. However, a new analysis of 7 years’ worth of trapping data offers perhaps the best model yet of the pest’s seasonal activity patterns—a key tool for growers and integrated pest management professionals to know when and how to target their management efforts.
Beginning shortly after the arrival of spotted-wing drosophila (Drosophila suzukii) in Michigan in 2010, entomologists at Michigan State University set up traps to monitor the presence of the fly near blueberry crops and have been maintaining them ever since. In a report published in September in Environmental Entomology, the researchers use data from more than half a million flies caught over 7 years to delve into the environmental conditions that drive the flies’ activity, timing, and abundance.
“Our results suggest that spotted-wing drosophila activity is predictable and that environmental conditions can be used in temperate regions to provide regional risk warnings as a component of strategies to manage this invasive insect pest,” says Heather Leach, a master’s student in entomology at Michigan State at the time of the study and lead author on the report. (Leach is now an extension associate at Penn State University.)
Leach and colleagues in the lab of Rufus Isaacs, Ph.D., used data from the spotted-wing drosophila (SWD) traps, which were checked weekly nearly year-round from 2011 through 2017, as well as from weekly samples of blueberries checked for SWD infestation beginning in 2014. They analyzed this info in conjunction with local daily environmental data on ambient temperature, soil temperature, precipitation, humidity, and soil moisture. Combined, it all works to paint a picture of predictable patterns of SWD seasonal activity in temperate regions.
“Winter temperature conditions and the population from the previous year are the strongest factors in determining the population of SWD in the spring,” Leach says. “Cold winters or populations that were low in the preceding year indicate that SWD is likely to have a smaller and slower population growth in the spring. This can then influence the level of management needed, particularly for crops that ripen early—e.g., June-bearing strawberries, cherries, and early-season blueberries.”
The findings can be built into applications that growers and IPM professionals use to assess risk of SWD infestation in a given season and determine the necessity, timing, and intensity of any potential management efforts.
Spotted-wing drosophila is native to Asia but has emerged as an invasive pest in North America as well as South America and Europe in recent years. It is a particularly pernicious pest because, as described in a 2018 Entomology Today post on most-wanted invasive insects, SWD “is attracted to not-quite-ripe or ripe fruit, and it is able to tear the fruit open with serrated hindquarters, which allows it to deposit eggs in ripening fruit.” What’s more, Leach says her team’s analysis provides strong evidence that SWD can overwinter in temperate regions, rather than retreating to warmer zones and reinvading.
“Our models to predict first spring capture and spring activity were fairly simple—hinging on winter temperatures and the preceding year’s population,” she says. “We were surprised at how much the preceding year’s population influenced the first capture, spring activity, and the maximum population in the following year. This information provided support for overwintering by this pest in our region, and potentially local overwintering, within or adjacent to the blueberry crop.”
And yet, how the diminutive fly is physically capable of surviving winter conditions in Michigan remains a mystery.
“Very little is known about how and where they overwinter, what resources they use in the spring, or the potential bottleneck effect their population may experience during the winter—with a significant reduction in the population and only some individuals surviving to the following year, which could have big implications for insecticide resistance in the population,” Leach says. “A lot of great work is currently being carried out on this very topic, which will help growers understand the risk and potentially manage overwintering habitat around their crop to reduce susceptibility of early-season pressure.”