Rhagoletis juglandis

R. juglandis is univoltine, similar to its close relative R. suavis.[8] Experiments have shown that physical host fruit stimuli such as color and shape enhance oogenesis in the first egg maturation cycle; this does require sufficient nutritional value of the fruit.[9] Female flies do not completely develop eggs for oviposition until 7 to 14 days after emerging from the soil. Copulation and oviposition occur in the morning or late afternoon and evening. The female searches for a suitable spot for oviposition; when one is found, she forces her ovipositor several millimeters into the husk of the walnut. She then moves her body around in a semicircle to lacerate the tissue of the inner husk and create a cavity for the eggs. The female oviposits eggs below the surface of the husk of walnuts in batches of about 15 eggs. Oviposition takes several minutes. The spot of oviposition darkens after one or two days; then, the egg cavity can be detected with the naked eye.[2]

The larval stage takes 3 to 5 weeks. The larvae hatch from eggs after 4 to 7 days and immediately tunnel into the inner portion of the husk of the walnut. After maturation, as completion of development usually occurs after the infested walnut falls off the tree to the ground, the larvae exit the decaying husk and go onto the soil, in which they burrow and transform into the prepupal stage within 8 to 24 hours. The complete transformation to pupae takes a few days.[2][10]

Larvae spend the winter in the pupal stage in the soil at around a depth of 1 to 4 inches.[2] A study examined the relationship between individual pupal size and the depth of soil larvae were found for Rhagoletis juglandis and Rhagoletis suavis[11]. Larger pupae were found in deeper soil, most likely because burrowing deep into soil has a high energy cost. Flies in deeper soil depths had increased mortality; they were less likely than flies who burrowed to a shallow depth to emerge from overwintering.[11] Threats to the larvae in shallow depths include predators like ants and parasites like wasps from the genus Coptera.[11][10] Deeper soil, however, costs the larvae energy and leads to a higher risk of mortality.[11]

In mid to late summer, adult flies emerge from soil prior to ripening of walnut husks and emergence declines in number by September.[2][11]

Courtship behaviors are generally rare in the genus. However, R. juglandis is unique in its courtship signaling behaviors. Male flies demonstrate low-frequency wing vibration, accompanied by airborne infrasound. These bouts of vibration typically last from 5 seconds-15 minutes. Male flies raise their wings during these displays, turning the edges upward.[12]

R. juglandis participate in a resource-defense mating system.[12] Females are inherently aware of resource-based cues and males follow females in order to obtain more opportunities to mate. For instance, females have been shown to have innate responses to ripeness cues. Males then respond based on encounter rates with females at different levels of ripeness. Experimentally, females were shown to land more on green models than yellow or brown ones, and experienced males followed the same patterns. There were more mating interactions on green models.[13]

In addition, researchers have found that flies seem to internalize changes in sex ratio in the surrounding environment and respond accordingly. The effect of male density on copulation is stronger than female density. Individual flies copulate for longer in these male-biased environments.[14]

Walnut Tree, typical site of infestation

Superparasitism is the use of hosts that already contain a brood from the same species of parasite. R. juglandis females drag their ovipositors on the husk of the walnut after oviposition, which suggests that they have released a marking pheromone, a behavior typical of the Rhagoletis genus. However, the flies reinfest the same walnuts, and even the same oviposition sites, created by individuals of the same species. This occurs even when there are still uninfested hosts available.[15] In large, less-infested walnut fruits, adult fly size has been found to be larger and larval survival is higher.[16]