EFFECTS OF MOISTURE, GRAVITY, AND LIGHT ON THE BEHAVIOR OF LARVAE OF THE WHEAT STEM SAWFLY, CEPHUS CINCTUS (HYMENOPTERA: CEPHIDAE)

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Abstract
Until mid- or late July, most larvae of the wheat stem sawfly generally move into the upper internodes of the host stems. Then, as the stems mature, the larvae migrate down to the basal internodes and cut the stems at about ground level. The migration is initiated in response to the visible and infrared light transmitted through the ripening stems. The direction of the migration, toward the basal internodes, results from the orientation of the larvae to the vertical axis of the stems reinforced by a positive larval response to gravity. The larvae cut the stems at ground level because they are shielded there by the soil from visible and infrared radiation. The location of cutting is also influenced by the preference of the larvae for drier sites in which to cut. The date of cutting is directly related to the loss of moisture from the stems, but not to a precise level of stem moisture. The larvae may form their cocoons in response to desiccation.