RELATIVE IMPORTANCE OF SEMIOCHEMICALS FROM FIRST AND 2ND TROPHIC LEVELS IN HOST FORAGING BEHAVIOR OF APHIDIUS-ERVI

The responses of female Aphidius ervi to odors from a host food plant (Vicia faba), host aphids (Acyrthosiphon pisum), nonhost aphids (Aphis fabae), and aphid-plant complexes were investigated in a specially de signed wind tunnel and a Y-tube olfactometer. In single-target (no-cho ice) and two-target (dual-choice) experiments, plant volatiles played a crucial role in the host foraging behavior of A. ervi. The odor from the A. pisum-plant complex elicited the strongest responses by A. erv i females, followed by the Odor from plants previously damaged by the feeding of A. pisum. There was a significantly weaker response to odor from A. pisum in the absence of the plant and to undamaged plants. Si milarly, mechanically damaged plants and plants infested with the nonh ost aphid A. fabae did not elicit strong responses. A plant that had b een damaged by A. pisum and subsequently washed with distilled water w as as attractive as an unwashed, previously infested plant. Aphidius e rvi probably overcomes the reliability-detectability problem by select ively responding to herbivore-induced, volatile, semiochemical cues em itted by the first trophic level and by distinguishing between the vol atiles induced by host and nonhost aphids.