Relative importance of semiochemicals from first and second 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 designed wind tunnel and a Y-tube olfactometer. In single-target (no-choice) 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. ervi 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. Similarly, mechanically damaged plants and plants infested with the nonhost aphid A. fabae did not elicit strong responses. A plant that had been damaged by A. pisum and subsequently washed with distilled water was as attractive as an unwashed, previously infested plant. Aphidius ervi probably overcomes the reliability-detectability problem by selectively responding to herbivore-induced, volatile, semiochemical cues emitted by the first trophic level and by distinguishing between the volatiles induced by host and nonhost aphids.