The results of systematically-conducted, mid- to late-season observations and experiments on large populations of apple maggot flies, Rhagoletis pomonella (Walsh), in apple and sour cherry trees revealed the following sequences in courtship behavior. When environmental conditions are favorable, sexually mature males and females fly to the host fruit, the site of assembly for mating and the site of oviposition. Either sex may initiate the flight which takes it to the particular fruit occupied by a member of the opposite sex. Such flights by males are often in response to the visual stimulus of a female (or male) moving about on a fruit, while such flights by females seem to be primarily in response to the fruit as a potential oviposition site. Once on the same fruit, a male and a female locate one another apparently solely through vision, particularly movement. They walk to within 1–3 cm of one another, but there is no tactile contact until such time as the male attempts copulation by jumping onto the back of the female from this distance.
The position from which the copulatory jump is made is variable. Sometimes it is made from a face to face position, with one or both flies having waved its pictured wings at the other. Occasionally it is made from the side or from a flight from a nearby fruit directly onto a female’s abdomen. Most often however, it is made when a male is stimulated by the forward movement of a female, approaches her from the rear, and jumps onto her abdomen from the rear without the female having seen the male. Most copulation attempts, and especially most successful attempts, are initiated while the female is engaged in some phase of oviposition behavior. Males attempt copulation with other males just as often as with females, strongly suggesting that at least up until the time of tactile contact, males are unable to distinguish between the sexes. The fact that a number of male and female apple maggot flies was observed in copula with R. fausta flies in sour cherry trees suggests that neither sex may be able to discriminate too well between members of its own species and members of other species whose wing and body patterns are similar in appearance However males were usually, although not always, able to distinguish stable flies from apple maggot flies prior to tactile contact.
We discuss the known and possible roles of various visual, chemical acoustical, and physical-tactile cues involved in the courtship behavior and suggest that the most important factor insuring reproductive isolation in apple maggot flies at the pre-copulatory stage may be the selection of the proper host plant for oviposition and hence for assembly for mating.