SWD adults collected in the later fall and early spring show physical or morphological differences from SWD adults collected in summer. Winter morphs are larger, darker in color, and have longer wings than summer morphs. Both male and female SWD show this physiological change. Individual flies do not change their body size and color. Rather, as the season advances, the flies that emerge begin to develop the larger, darker characteristics of the winter morph. It may be that these physiological changes are a survival adaptation to winter conditions.
Project researchers collected SWD adults from the field on August 14, October 9, and December 12 2012. These dates fall in late summer, autumn, and winter respectively for Oregon’s latitude. Equal numbers of males and females were evaluated for melanization or the degree of darkness of the abdomen. In August, 100% of females and males assumed the lighter summer morph. In October, 50% of the females and 70% of the males assumed the summer morph. By the December observation, 100% of the females and 90% of the males assumed the winter morph. The data collected for wing length corresponds to the color data – as the season advances, the longer wing form of the winter morph predominates.
In a controlled experiment, summer morph and winter morph SWD females were placed in cold chambers and held at 1, 5, 10, 20, or 28 °C (34, 41, 50, 68, or 82 °F) to determine the longevity of each morph at these temperatures. The winter morph females held at 82 °F and the summer morph females held at 34 °F both survived at most about 45 days, as did the summer morph females held at 82 °F. The winter morph females held at 34 °F survived at most about 145 days. Thus, the larger, darker winter morph females survived far longer at the coldest temperatures than any other morph and temperature combination.
Although the well-adapted summer morph females lived only about 45 days at 82 °F, their rate of egg laying increased under hotter conditions. Under cooler summer conditions with the daily high temperatures at about 68 - 70 °F, females laid about 15 eggs per hour. Under warmer conditions with the daily high temperature at about 82 °F, females laid about 32 eggs per hour. Under both conditions, egg laying activity began at about 10:00 AM, showed a sharp increase at about 3:00 PM, and gradually tapered off to zero at about 8:00 PM. This finding has implications for microclimate manipulation and for behavioral manipulation.
A new test has been developed that could tell you within two hours whether larvae found in a fruit or berry shipment bound for export markets is SWD. This test results in no false positives, and significantly decreases holding periods for shipments.
Joanna Chiu, a molecular biologist and entomologist at the University of California, Davis has developed a test to confirm SWD infestation. The larvae and pupae of Drosophilids are virtually indistinguishable from one another by visual inspection alone, and this test will eliminate this problem.
This test to identify SWD does require the capacity to perform standard PCR followed by DNA gel electrophoresis, putting this technology within reach of larger packing houses and standard research laboratories. The SWD genome project has allowed Chiu to develop reliable PCR primers specifically for SWD. The test has proven reliable in identifying flies captured from Washington, Oregon, California, Maryland, Hawaii, Japan, South Korea, and Italy.
PCR stands for polymerase chain reaction. The primers bind to genomic DNA to enable the chain reaction that amplifies and increases the amount of genetic material in a sample. This process creates sufficient genomic DNA to visualize and interpret the results of a gel electrophoresis.
Other possibilities opened up by the completion of the SWD genome include:
Image Caption: Results from gel electrophoresis confirm that the sample was SWD. The strong bright band in lanes 1 through 14 at 221 base pairs indicates SWD. If the sample were from any other insect species, including any other Drosophila species not SWD, there would be no band at all at 221 base pairs. The very faint internal control band at 1248 base pairs confirms that the test is valid. This test is easy to interpret, and uses any PCR machine. It will work on any SWD tissues, including larvae, pupal cases, or adult tissues.
This work was funded by the Washington Tree Fruit Research Commission. It leverages the characterization of the SWD genome, which was funded by USDA-NIFA-SCRI project number 2010-51181-21167.
Find specific details for the method of this test:
Murphy KA, Unruh TR, Zhou LM, Zalom FG, Shearer PW, Beers EH, Walton VM, Miller B, Chiu JC (2015). Using comparative genomics to develop a molecular diagnostic for the identification of an emerging pest Drosophila suzukii. Bulletin of Entomological Research.