by
Martin G. Kelly
Department of Biology
Buffalo State College
by
Martin G. Kelly
Department of Biology
Buffalo State College
Hawthorn trees grow throughout North America and they produce a small fruit which is eaten by a small fly larva. In 1864, apple growers discovered an unknown maggot had started feeding on apples. Through the years, hawthorn and apple maggot flies have progressively become more distinct.
Below I present evidence taken from the original scientific literature for you to consider and evaluate. From what may be incomplete and ambiguous data, I hope you will be able to answer what appear to be two simple questions:
The apple maggot fly (Rhagoletis pomonella, Walsh) is native to eastern North America. It originally bred in the large fruits of hawthorn trees (Reissig, 1991).
Figure 1: Apple maggot flies
(male—left, female—right)
Apple maggot flies are about 5 mm long. The tip of a female’s abdomen is more narrowly pointed than a male’s.
Hawthorn and apple maggot flies are assigned to the same taxonomic species (Rhagoletis pomonella) (Bush, 1966; Bush, 1969; Bush, 1975: cited in Berlocher and Feder, 2002).
Figure 2: Taxonomic Tree of Rhagoletis
(adapted from Berlocher and Bush, 1982)
R. pomonella belongs to a set of four fly species that cannot be physically distinguished.
Figure 2 displays the conventional classification of Rhagoletis. The abbreviated categories are: tribe (T.), closely related genera (R.G.), genus (G.), derived species groups (D.G.), and species group (S.G.). The classification bar at the bottom of the figure arranges the nodes of divergence from the most inclusive taxonomic category (the tribe, T) to groups of closely related species (species groups, SG).
Figure 3: Apple spoiled by apple maggot larvae.
Both the hawthorn and the apple are woody plants belonging to the same taxonomic family (the Rose Family) (Newcomb, 1977).
Figures 4a and 4b: Photo and diagram of Crataegus marshallii.
The apple is the most widely grown fruit in North America (Seelig and Hirsh, 1978).
Figure 5: Empire red apples.
Figure 6 below is a general representation of the timing of fly emergence (solid and dashed lines) and fruit ripening (colored filled-in curves). Adult flies emerge from the puparium to reproduce before fruits are mature (Reissig, 1991). Apple fruits ripen approximately 1 month earlier than hawthorn fruits, but there is overlap at the end of the apple fruiting season and the beginning of the hawthorn fruiting season (Feder and Filchak, 1999; Feder et al., 1994; Filchak and Feder, 1999; Smith, 1988: cited in Berlocher and Feder, 2002). The female fly then lays fertilized eggs into the ripe fruit (Reissig, 1991). Maggots (larvae) hatch from the egg, eat fruit, grow, and pupate (Reissig, 1991).
Figure 6: Emergence of Rhagoletis pomonella (adapted from Bush, 1969)
In Figure 6, the taxonomic term “race” is used to identify the maggot flies that reproduce on apples or the maggot flies that reproduce on hawthorns.
In 1864 maggot flies were first noted to infest apples grown in New York’s Hudson River Valley (Bush, 1969).
Figure 7: Blemishes indicate burrowing larvae.
Apples are bigger fruits than hawthorns.
The larger fruits of apple trees provide 5.5 times more depth (based on diameter) to developing maggots than do hawthorn fruits.
The larger fruits of apple trees provide 220 times more food (based on volume) to the growing and developing maggot than the smaller fruits of hawthorns.
In your groups, address the following and list the evidence used to make your decisions.
Image Credits:
Figure 1—Photo of Rhagoletis pomonella from UC IPM Pest Management Guidelines: Apple, UC ANR Publication 3432. Used with permission from University of California Statewide IPM Program (http://www.ipm.ucdavis.edu), J. K. Clark, photographer.
Figure 2—Jim Stamos, artist, adapted from Bush, 1969.
Figure 3—Photo of apple maggot larvae burrowing in apple is used with permission from University of California Statewide IPM Program (http://www.ipm.ucdavis.edu), J. K. Clark, photographer.
Figure 4a—Robert H. Mohlenbrock @ USDA-NRCS PLANTS Database / USDA SCS. 1991. Southern wetland flora: Field office guide to plant species. South National Technical Center, Fort Worth, TX.
Figure 4b—USDA-NRCS PLANTS Database / Britton, N.L., and A. Brown. 1913. Illustrated flora of the northern states and Canada. Vol. 2: 320.
Figure 5—Photo of Empire apples is courtesy of © New York Apple Association (http://www.nyapplecountry.com).
Figure 6—Jim Stamos, artist, adapted from Bush, 1969.
Figure 7—Photo of damaged apple is used with permission from University of California Statewide IPM Program (http://www.ipm.ucdavis.edu), J. K. Clark, photographer.
Date Posted: 12/03/03 nas
This file is also available in Adobe Portable Document Format (PDF). 
Originally published at http://www.sciencecases.org/maggot_fly/maggot_fly.asp
Copyright © 1999–2010 by the National Center for Case Study Teaching in Science. Please see our usage guidelines, which outline our policy concerning permissible reproduction of this work.
