The effect of barometric pressure changes on flight initiation of female Trichogramma evanescens Riley and Trichogramma pretiosum Westwood (Hymenoptera: Trichogrammatidae) was studied in a flight chamber over a 6-h period. Six different barometric pressure regimens were tested for their impact on flight initiation of both species: constant 975 and 1,025 mbar (stable regimens), and 50 mbar increase or decrease in pressure between 975 and 1,025 mbar either within 1 (rapid increase or decrease) or 6 h (slow increase or decrease). Both Trichogramma species did not respond to stable or slow changes in barometric pressure, but rapid barometric changes significantly reduced the flight initiation of females. The effect of rapid barometric pressure changes was almost immediate and independent of its direction, whether it was increasing or decreasing. The reduction in flight initiation was lifted rapidly when the barometric pressure became stable again, immediately for T. pretiosum and 1 h later for T. evanescens. These results are in agreement with the hypothesis of an adaptive response to rapidly changing climatic conditions and associated higher risks of mortality during flight for small insects like Trichogramma. The higher flight initiation observed for T. evanescens females also agrees with field results where they displayed higher dispersal capacity than T. pretiosum females under adverse climatic conditions. The effects of barometric pressure on parasitoid foraging behavior deserve more attention because this climatic variable may play a significant role in their population dynamics.
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