Differentiating Drosophila suzukii morphs and their flight capabilities

Abstract

Insects depend on external heat sources to regulate their body temperatures, and this affects their ability to survive in cold dry regions compared to their counterparts in warm humid environments. Insects have adapted to withstand cold winters by 1) overwintering by acclimating to the cold-stress, or 2) by dispersing to escape the cold. However, there are many challenges to determining which winter survival strategy an insect adopts. An example of an insect that researchers have not determine which main winter survival strategy is utilized is the invasive vinegar-fly, Drosophila suzukii (Matsumura), also commonly known as spotted-wing drosophila. First detected in California in 2008, D. suzukii rapidly expanded its geographic range and can now be found in 48 states, including Minnesota. Researchers have hypothesized that D. suzukii survives cold climatic conditions by producing dark colored winter morphs, which enable the flies to absorb more thermal radiation from the sun compared with lighter morphs. However, thus far, there has been no confirmed sighting of an overwintering winter morph emerging in the spring in Minnesota or any other Midwestern state. An alternative hypothesis is that adult flies are capable of long-distance dispersal. My doctoral dissertation is focused on determining the source (overwintering or migratory) of D. suzukii populations by investigating the morphs phenology and their flight capabilities. First, I developed a quantitative technique to differentiate the two morphs to facilitate identification of overwintering populations. I mimicked winter and summer conditions in the lab for creating winter and summer morphs. Subsequently, I collected quantitative data by measuring the flies’ wings and legs and subjected the data to a predictive analysis approach. This enabled me to identify quantitative characters for distinguishing between winter and summer morphs. Second, I observed and characterized D. suzukii flight behavior on a flight mill and in a flight chamber. Each of these techniques has advantages and disadvantages and flight behavior can vary considerably between the two techniques; yet researchers have typically examined flight behavior of their target species using only one method. I compared the flight behavior of winter and summer morphs of males and females using both techniques. Finally, to examine the potential for D. suzukii long-distance dispersal in the field, two studies were conducted. First, three 7.3m poles were set up in an area where D. suzukii populations are known to be present with clear sticky cards placed on each pole at five different heights: 1, 2, 4, 6, and 7.3m. Second a fixed-wing plane/un-manned aerial vehicle (UAV) was flown at varying altitudes between 30-122 m with an insect net to catch a D. suzukii. This study was to determine if D. suzukii could reach altitudes that would allow the fly to disperse with the aid of the wind. The results of my dissertation will provide information on differentiating D. suzukii in a qualitative matter, explore the propensity and duration of flight in a controlled laboratory environment, and examine the potential for long-distance dispersal in the field. The development of creating a morphometric scale will assist in standardizing how D. suzukii morphs are identified. Information of the flight behaviors can assist in furthering the understanding of this pest’s biology and improve management programs. If the D. suzukii overwinters in an area, growers could reduce pest pressure the following year through removal of potential overwintering habitats; if a D. suzukii migrates to warmer areas, growers could time management practices based on predictions of arrival/return of the pest in spring.