Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2019
Publication Date: 6/3/2019
Citation: Wakie, T., Yee, W.L., Neven, L.G., Kumar, S. 2019. Modeling the abundance of two Rhagoletis fly (Diptera: Tephritidae) pests in Washington State, U.S.A.. PLoS One. 14(6). https://doi.org/10.1371/journal.pone.0217071.
Interpretive Summary: Apple maggot and western cherry fruit fly are the two key quarantine pests of apples and cherries in Washington State (WA). These flies are a threat to the tree fruit industry due to quarantine related market restrictions. Though both fly species are present in WA, there is little quantitative information on where they are found, how they are abundant, and what environmental factors affect the abundance of these flies. To answer these questions, Personnel at the Temperate Tree Fruit and Vegetable Research Unit, USDA-ARS, Wapato, WA designed a research project in 2015 and 2016 that involved surveying, pest identification in the laboratory, and statistical analyses. Results indicate that apple maggot is abundant in western WA and less abundant in central and eastern WA. Winter mean minimum temperature, winter degree days below 5°C, frost-free period, and elevation were the top four environmental variables that explained the abundance of apple maggot, indicating that fly populations are constrained by cold weather and higher elevation. In contrast, western cherry fruit fly is abundant in central and eastern WA and less abundant in western WA. The abundance of western cherry fruit fly was mostly explained by environmental factors representing drier conditions, supporting the claim that the fly is well adapted to the drier environments in the central and eastern WA. Higher number of host plants were positively associated with higher number of flies for both species. For areas with zero or very low apple maggot abundance, establishment of Low Pest Prevalence or Pest Free Areas (ALPP or PFAs), which would allow movement of apples without cold treatments, are recommended. The study approaches used here have potential applications for monitoring pests by regulatory agencies such as USDA-APHIS.
Technical Abstract: Well-adapted and abundant insect pests can negatively affect agricultural production. Here we model the abundance of two Rhagoletis fly (Diptera: Tephritidae) pests, apple maggot fly, Rhagoletis pomonella (Walsh), and western cherry fruit fly, Rhagoletis indifferens Curran, in Washington State (WA), U.S.A. using biologically relevant environmental variables. We tested the hypothesis that abundance of the two species is influenced by different environmental variables, based on the fact that two species evolved in different environments and that R. pomonella is an introduced pest to WA while R. indifferens is native. We collected data on fly and host plant abundance at 61 randomly selected sites across WA over two seasons. Fly abundance data were collected using yellow sticky traps baited with ammonia odor. We obtained land-cover, climate, and elevation data from online sources and used these data to derive relevant landscape variables. We modeled the abundance using generalized linear models. We showed that host abundance and land-cover type are important in explaining the abundance of these flies. For R. pomonella, elevation, winter minimum temperature, winter growing degree days below 5°C, and frost-free period were the top significant variables in explaining abundance. In contrast, for R. indifferens, variables related to annual and summer moisture deficit were the top significant variables explaining abundance. Furthering support for our hypothesis, detailed pest abundance maps indicated R. pomonella was more abundant in western WA where it is relatively wet whereas R. indifferens was more abundant in central WA where it is drier. Our results identify pest abundant sites and can be used for understanding adaptation of insects to non-native and native habitats, evolutionary forces shaping adaptations, and for fly quarantines and management.