Submitted to: Annals of the Entomological Society of America
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/10/2010
Publication Date: 3/5/2011
Citation: Yee, W.L., Sheets, D., Chapman, P.S. 2011. Analysis of Surstylus and Aculeus Shape and Size Using Geometric Morphometrics to Discriminate Rhagoletis pomonella and Rhagoletis zephyria (Diptera: Tephritidae). Annals of the Entomological Society of America. 104(2):105-114. Interpretive Summary: Apple maggot fly is an important quarantine pest of apples in the Pacific Northwest. Accurate identification of apple maggot fly is essential in protecting the apple industry because it is the basis for quarantine and control measures. However, the presence of the almost identical snowberry maggot fly, which does not attack apples, makes quarantine decisions and control efforts difficult. Personnel at the Yakima Agricultural Research Laboratory in Wapato, WA, are determining if surstylus and aculeus shape can be used to improve discrimination of the two species of flies compared with older methods for discrimination. We found that surstylus shape discriminates males of the two species with 99.8% accuracy and that aculeus shape discriminates females of the species with 85.3% accuracy, improvements over use of older criteria. The results are important because they indicate that surstylus shape analysis can reduce errors in identifications which can lead to unnecessary control measures or increased threats of flies invading apple orchards.
Technical Abstract: Rhagoletis pomonella (Walsh) and Rhagoletis zephyria Snow both occur in the Pacific Northwest of the U. S. and are frequently confused with one another due to their morphological similarity. The apple maggot, R. pomonella, is a threat to commercial apples in the Pacific Northwest, whereas R. zephyria attacks snowberry and is not a threat. Configuration of the surstyli in males is used to discriminate between species, but this character shows overlap. In this study, we reexamined surstyli configurations in the two species. We then used geometric morphometrics to test the hypotheses that shapes of surstyli and aculei of the two species differ and that combining aculeus shape and size measures can improve discrimination. We found that all R. pomonella had a nearly parallel surstyli configuration, whereas R. zephyria included specimens having either parallel or divergent configurations. Using canonical variates analysis, multivariate analysis of variance, and an assignments test, we found that surstylus shape classified 99.8% of males correctly to species. Aculeus shape accurately classified 85.3% of females to species. Combining aculeus shape and length increased classification accuracy to 94.5%. Within species, surstylus and aculeus shape did not discriminate among fly populations from different host fruit and/or collection sites. Use of surstylus shape would benefit regulatory agencies that depend on accurate identifications of R. pomonella for quarantine and management measures.