Oviposition Threshold for Flight in an Inter-Reproductive Migrant Moth
2013
Rhainds, Marc | Kettela, Edward G.
The present study evaluated an oviposition threshold for flight in a moth species with an unusual inter-reproductive migration strategy, the spruce budworm (SBW), Choristoneura fumiferana (Clem.) (Lepidoptera: Tortricidae). Females typically lay some eggs in their natal patch then ascend by flight above the plant canopy to the atmospheric boundary layer where they can get carried several hundreds of kilometres way. Estimates of residual fecundity (= proportion of eggs remaining in the abdomen of females relative to potential fecundity) were calculated based on the relationship between the wing length and dry weight of 985 field-collected females. The females were sampled using three procedures, light traps deployed in the tree canopy or forest clearings, or by fogging trees with insecticides; the experiment was replicated at nine plots. The data were interpreted under the realistic assumption that females captured at light traps had to fly, whereas samples collected by fogging trees included females that were capable or not capable of flight. The lower residual fecundity of females captured at light traps compared with those collected on fogged trees revealed that females have to lay some eggs before flight. Very few females captured at light traps had laid fewer than 50 % of their egg complement, indicative of an oviposition threshold for flight corresponding to 50 % residual fecundity. The threshold appeared robust as it did not vary much over the flight season or the location of the light traps, and it was consistent for different sets of equations related to residual fecundity. The importance of the oviposition threshold for flight on the nature of spruce budworm population dynamics remains to be investigated.
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