Rhagoletis juglandis Cresson is a specialist that deposits its eggs into the husks of developing walnut fruit. Like other walnut infesting flies in the R. suavis group, R. juglandis actively superparasitizes its larval hosts. However, little is known regarding the degree to which hosts are reused and the ecological context under which host reuse occurs. This field study examined the pattern of host utilization by R. juglandis and how fruit variables such as volume and penetrability affect the degree that hosts are reused. Fruit on four of five study trees were synchronously infested and within 2–2.5 wk all fruit on these trees were infested. Fruit on a fifth tree were significantly less penetrable than those found among the other trees in the study and this may explain why fruit on this tree were rarely used throughout the season. Walnut hosts were commonly multiply infested and reuse of hosts occurred in as few as 1–2 d after first infestion. Infestation levels within fruit appeared to stabilize 4–5 d after fruit were first used. Fruit volume was positively correlated with both the number of punctures on hosts and the infestation levels within hosts that had been infested for either 1–2 or 4–9 d. Large fruit were infested more quickly than small fruit, although this trend was stronger on some trees than others. Finally, despite a size-penetrability correlation among two of the five trees, penetrability itself did not explain either which fruit were preferentially used throughout the season or the infestation levels within fruit.
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