Hot Competition: Climate Change, Invasive Fly Displace a Native Blueberry Pest
By Timothy W. Schwanitz
As the climate warms and temperatures continue to rise, a tiny vinegar fly called spotted-wing drosophila is causing major headaches for U.S. blueberry farmers. This invasive fly is outcompeting and replacing the native blueberry maggot fly as the primary pest of blueberries. Both insects lay their eggs in ripening blueberries. Their larvae feed on the fruit as they develop, causing the berries to spoil faster. Plus, there is an additional “yuck” factor for consumers that find maggots in their fruit.
A group of researchers from Rutgers University have investigated how climate change might affect the competition between these two major blueberry pests, with their findings published in April in the journal PLOS ONE.
The New Pest is a Better Pest, Which is Worse for Blueberry Growers
Before the introduction of spotted-wing drosophila (Drosophila suzukii), the primary pest in New Jersey blueberries was the blueberry maggot fly (Rhagoletis mendax). While a major concern for blueberry growers, some of the blueberry maggot fly’s characteristics allowed for effective management: For example, the flies overwinter as pupae, and the adult females require one-to-two weeks after emerging before they can begin laying eggs. This feature of their life cycle means that early spring monitoring can help predict where a lot of females will emerge before they have had a chance to infest blueberries. By contrast, spotted-wing drosophila flies overwinter as adults and can lay their eggs in fruit as soon as it turns from green to pink. Also, unlike blueberry maggot fly, spotted-wing drosophila can have multiple overlapping generations in a growing season, giving this invasive fly a greater potential for rapid population growth.
Spotted-wing drosophila outcompetes blueberry maggot fly due to life cycle, as shown by increasing spotted-wing drosophila trap catches and decreasing blueberry maggot fly catches. The researchers in the present study were curious as to what effects climate change might also have on this interaction. They did not find a significant impact of temperature directly on blueberry maggot fly populations so far, though this might change as seasonal temperatures deviate more from the historical norm. What they did find, however, is that warmer summers, with their longer growing seasons, and milder winters, with their more favorable overwintering conditions, have a positive effect on spotted-wing drosophila populations.
Climate change thus creates conditions that favor larger spotted-wing drosophila populations. These larger populations, in turn, are bad news for the blueberry maggot fly, since its numbers have already been in decline as a result of this competition.
This new pest has upended pest management regimes and required frequent insecticide applications to protect blueberry harvests. With the invasive fly’s future looking bright, new management innovations will be required to keep its populations under control.
The Value of Long-Term Datasets and Collaborative Studies
All of these findings were made possible by long-term population monitoring. Traps for blueberry maggot fly have been put out in blueberry fields across New Jersey for decades as part of extension work at Rutgers led by Dean Polk, retired fruit IPM agent at the New Jersey Department of Agriculture and Natural Resources and a co-author on the new study. Moreover, population monitoring traps have also been put out to track spotted-wing drosophila since 2012, a year after it was first detected in the state. James Shope, Ph.D., lead author on the study and a research associate in the Rutgers Department of Environmental Studies, notes that “to establish trends and evaluate how these trends may change or how things may be deviating from ‘normal,’ a long-term dataset is essential. Often it feels like long-term record keeping may not be yielding any immediate results, but these are the kind of data that make studies like this possible!”
Having fewer data points might have led the authors to mistakenly conclude that these correlations were due to chance. By contrast, more data points will give their model of annual first-catch dates greater accuracy and predictive power. Continuing to refine it over time will help blueberry growers get a better sense of how large spotted-wing drosophila populations might be in a given year and how early in the growing season spotted-wing drosophila may start affecting the blueberries.
This study also emphasizes the value of collaborative work. Cesar Rodriguez-Saona, Ph.D., senior author on the study and a professor and extension specialist in the Department of Entomology at Rutgers, noticed an interesting trend in historical scouting data for these two pests: “The blueberry maggot fly was caught later and later each year, while spotted-wing drosophila earlier and earlier,” he says. After sharing these results with Shope, the two worked together with colleagues in agricultural extension to use climate parameters to predict population changes in these major pests.
Numerous other berry pests might be affected by climate change. “We are looking into how other New Jersey pests, such as cranberry pests (Sparganothis fruitworm, blunt-nosed leafhoppers, cranberry toadbug), may be influenced by climate change and what we can expect in terms of their impacts in the future. The analysis would follow a similar process to this study but hopefully provide relevant data for New Jersey cranberry growers,” Shope and Rodriguez-Saona say. Improved modeling could help growers more accurately predict pest outbreaks, thereby helping them to apply pesticides only when needed.
“The contrasting role of climate variation on the population dynamics of a native and an invasive insect pest”
Timothy W. Schwanitz is an insect neuroethologist currently pursuing a Ph.D. in the McBride Lab at Princeton University. He majored in English and entomology during his undergraduate studies because of his broad interest in both books and bugs. Email: email@example.com.
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