Symbiotic association of bacteria with the apple maggot, Rhagoletis pomonella (Walsh) (Diptera: tephritidae)

From 159 individuals representing all life stages of the apple maggot, a total of 193 bacterial cultures were isolated. Bacteria were isolated from all of the 39 adults and 30 pupae examined, but only 48 of the 70 larvae and 8 of the 20 eggs. Most of the isolated bacteria were similar to species widely distributed in soil, water, and on plant surfaces. The results of a numerical taxonomic study of the bacteria showed that the largest group of bacteria with a mean intra-group similarity value of 0.7 consisted of 67 cultures, one of which was a named culture of Enterobacter aerogenes; such bacteria were isolated from only 41% of the apple maggots examined. Other groups of bacteria with a mean intra-group similarity value of 0.7 consisted of bacteria resembling Erwinia herbicola (0.7 taxon VIII), Pseudomonas fluorescens (0.7. taxon V), and Pseudomonas sp. (0.7 taxon III). Such bacteria were isolated from 11%, 11%, and 7%, respectively, of the insects examined. The American Type Culture Collection, culture number 23460, reported to be the obligate symbiote of the apple Maggot (13), was included in the numerical taxonomic analysis and found to be an achromogenic strain of Serratia marcescens, which is known to be an insect pathogen. Only one bacterial culture isolated from the apple maggot was closely related to S. marcescens. The results of four experiments in which apple maggot larvae were reared in apple, either axenically or with the bacterium A.T.C.C. 23460 (S. marcescens), showed that larval survival was reduced in the presence of the bacterium by an . average of 35.7%. The period required by larvae to complete development, and weights of resulting pupae was not influenced by the presence of the bacterium. Larvae reared on an artificial medium supplemented with either heat-killed cells or living cells of the bacterium A.T.C.C. 23460 did not survive as well as larvae reared axenically on a similar medium. Larvae reared on artificial medium supplemented with culture medium filtrate of the bacterium A.T.C.C. 23460 survived nearly as well as the axenically reared larvae. The length of time required by larvae to complete development was the same in all treatments. Apple maggot larvae reared on apple either axenically or in the presence of Pseudomonas sp. or Erwinia sp. survived equally well, and developed within a similar period of time. There was no difference in size of resulting pupae. In experiment no. 2 and experiment no. 4, adults of larvae reared in the presence of the bacterium A.T.C.C. 23460 produced an average of 12 times as many eggs as the adults of the axenically reared larvae. In experiment no. 6, adults of the axenically reared larvae produced as many fertile eggs per female as the adults of the larvae reared with either Pseudomonas sp. or Erwinia sp. However, the adults of the axenically reared larvae were found to harbour a bacterium similar to Erwinia herbicola. This bacterium was not isolated from apples in which the larvae had developed, indicating that contamination occurred after larval development had been completed. It was concluded that no species of bacterium is frequently associated with all life stages of the apple maggot. The presence of bacteria was neither essential nor beneficial for larval survival. The presence of bacteria in the digestive tract of apple maggot adults resulted in an increase in fecundity. However, a species-specific relationship was found not to exist. It is doubtful that the apparent stimulation of fecundity by bacteria is of any ecological significance since large numbers of bacteria and other microorganisms undoubtedly enter the adult alimentary canal with the food.