High Adaptability Allows Invasive Fruit Fly to Thrive in New Environments
By John P. Roche, Ph.D.
Spotted wing drosophila (Drosophila suzukii) is a fruit fly that originated in Asia and was first detected as an invasive species in the United States in Hawaii in 1980. It was discovered in the continental U.S. in California in 2008, but because it is so small and hard to detect, it could have been in North America far earlier. It also became an invasive species in South America and Europe and now has a global range.
Part of this species’ success in expanding its range is due to its effectiveness at exploiting new habitats, communities, and ecological niches. A recent study reported in May in the Journal of Insect Science details one important reason why spotted wing drosophila is so effective as an invasive species: It has a high degree of adaptability.
Spotted wing drosophila lays eggs in small fruits such as cherries, blueberries, and strawberries. One of the traits that makes it problematic to agriculture is that females have a serrated ovipositor that allows them to cut through the skin of intact fruit to deposit eggs. This is destructive on two levels. First, because laying eggs in intact fruit can damage fruit that would be untouched by other species of fruit flies. Second, because eggs and larvae are inside of fruits, they are hard to see, making controlling them harder when fruits are being transported. Other traits that make it a problem pest are its ability to disperse quickly, reproduce rapidly, and feed on a wide range of fruits. The species causes an estimated $500–700 million in damage in the U.S. annually, and it can wipe out entire crops.
Variability in a population, which helps this invasive species thrive in new environments, occurs on two levels. The level that we most often think about is variability within a population that is shaped by natural selection. In natural selection, alleles of genes that provide their bearer with an advantage in terms of survival and reproductive success tend to increase in frequency in the population. This allows populations to become better suited to their environment over multiple generations. But members of populations can also change their phenotype, or set of characteristics, within their lifetimes in something called phenotypic plasticity.
In phenotypic plasticity, the germlines of the individuals involved do not change, but the genetic code of the organism has a blueprint allowing the organism to adjust facultatively to different environments or to fluctuations within an environment. Phenotypic plasticity offers the advantage of providing adjustments to the environment that are very fast. Also, with invasive species, the variability upon which natural selection can act is often lowered by genetic bottlenecks caused by the small initial population size of a colonizing species. Phenotypic plasticity can provide variation that adjusts to local conditions, even in very small populations subject to genetic bottlenecks.
In their review article, Catherine Little, a Ph.D. student at Memorial University of Newfoundland and Labrador, along with Memorial University colleague Thomas Chapman, Ph.D., and Kirk Hillier, Ph.D., of Acadia University, comprehensively compiled what is known about phenotypic plasticity in spotted wing drosophila and examined the astounding extent to which the species can change its traits in response to the environment.
Spotted wing drosophila (SWD) has a robust and varied ability to express phenotypic plasticity. Plasticity is found in its morphology, its development, and its behavior. One example is that, when temperatures are colder, SWD development slows, resulting in larger wings. These larger wings allow the flies to fly faster and disperse faster and farther, permitting them to increase expansion of their invasive range.
If temperature fluctuates while SWD are developing, they have an enhanced tolerance to cold as adults. Acclimation to low temperatures upregulates more than 1,500 genes, including those for cellular signaling, metabolism of carbohydrates, and ion transport. Low temperatures downregulate more than 1,300 genes, including genes for egg development. Exposure to cold stimulates cold tolerance through mechanisms that include accumulating amino acids and carbohydrates that protect against low temperatures.
In subtropical habitats such as Southeast Asia, SWD are active throughout the year. In temperate habitats such as the central and northern United States, however, adult females that have mated spend the winter months as a dormant winter morph. Acclimatization to cold temperatures and short daylengths improves survival in these winter morphs, which are larger and more resistant to cold than non-winter morphs. Females who overwintered as winter morphs have enhanced reproductive success and improved life spans compared to non-winter morphs. Winter morphs can survive in long periods of low temperatures. Female winter morphs can also live through long periods of low humidity better than summer morphs can.
Diet can also trigger patterns of phenotypic plasticity in spotted wing drosophila. A difference in the availability of fruit changes wing morphology, which in turn changes flight ability. With a poor diet, adult SWD show lowered selectivity when choosing mates and a decreased ability to attract mates.
The development of SWD proceeds from eggs, to larvae, through three instar stages, to adults. Research has found that SWD larvae that feed on certain fruits, such as raspberries, cherries, and blueberries, develop more quickly than those fed standard diets. Investigators have also found that larvae feeding on blackberries and raspberries have an enhanced ability to survive competition than larvae that eat other fruit types.
Little observes, “The most important conclusion in our study was that D. suzukii biology, physiology, and behavior are dependent upon the regional climate, local environmental conditions, and available feeding and oviposition resources.”
When asked how the specifics of what we know about phenotypic plasticity might help inform control of this species, Little says, “There is likely no single one-size-fits-all pest management solution for this species across all crop systems or across all geographic areas. But changes in physiology for D. suzukii are often tied to changes in behavior. Understanding behavior of winter morph flies can open new opportunities for controlling this invasive species before local populations reach damaging levels each year.”
As for possible directions for future work, Little says, “We suggest that research into D. suzukii behavior and resource use during seasons prior to when fruit crops are at risk could be useful in limiting potential D. suzukii population growth.”
Co-author Kirk Hillier adds, “As a chemical ecologist, I think there are also many directions to pursue in looking at attraction and repulsion technologies.”
We’ve seen that spotted wing drosophila have pronounced phenotypic plasticity. Since the capacity for this phenotypic plasticity must be coded for by their genes, an intriguing additional research question arises: Why did the capacity for such a high degree of plasticity evolve in this species?
Because of its pronounced capabilities to adapt to new and changing environments, spotted wing drosophila is a formidable invasive species and a challenging agricultural pest. In addition, two general factors happening regionally and globally could be further improving conditions for spotted wing drosophila and will tend to necessitate continual adjustment of control measures: climate change and the extinction of native species. Because of its difficulty to avoid detection and its ability to destroy entire crops, additional research and tests of control strategies are critical for this adaptable and damaging insect.
Journal of Insect Science
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.