Control of olfactory-induced behavior in alfalfa seed chalcid (Hymenoptera: Eurytomidae) by celestial light.

The behavioral response of female alfalfa seed chalcids is dependent on the polarization of sky light when exposed to olfactory stimuli between 1100 and 1500 hr. Females made no orientation flights to or landings on eight unbaited targets, but when half the targets were baited with the host-plant odor of hexyl acetate, they did make orientation flights to and landed on baited targets. Female preference for baited targets disappeared when natural sky light was plane polarized (at right angles to natural sky light polarization at 1200 hr). When natural sky light was passed through a diffuser filter to completely depolarize the light, females made numerous orientation flights but displayed no preference for baited targets. Any alteration of the natural wavelengths of sky light between 345 and 425 nm with various Kodak Wratten filters that excluded certain wavelengths produced similar results wherein females had no preference for baited or unbaited targets. The hypothesis that chalcid response was controlled by the polarization characteristics of sky light was confirmed when females again preferred baited targets in tests that were conducted 6 hr after sunrise, but they displayed no preference for baited targets when exposed to the mixture of polarized and nonpolarized light present in the sky 1-2 hr after sunrise or 1-2 hr before sunset. The possibility that chalcid response was due to a circadian rhythm was eliminated by exposure of insects to different sequences of light-dark regimes prior to the olfactory test.