TURBULENCE, TREES AND SEMIOCHEMICALS - WIND-TUNNEL ORIENTATION OF THEPREDATOR, RHIZOPHAGUS-GRANDIS, TO ITS BARKBEETLE PREY, DENDROCTONUS-MICANS

Insects apparently responding to the visual stimulus offered by a tree or other object may also be responding to the variety of physical eff ects caused by the obstruction, including turbulence, a reduction in w ind velocity and eddies. The relative importance of prey odour associa ted with the bark beetle Dendroctonus micans (Kugelann) (Scolytidae), prey-host-tree odour, silhouette, and physical barrier to the wind, in the orientation and landing behaviour of a predatory beetle, Rhizopha gus grandis Gyllenhall (Rhizophagidae), were investigated. R.grandis r esponded very positively to the frass of its prey when presented on a 'tree' but not to the same quantity of frass presented alone. Frass on black plastic uPVC pipes was significantly more attractive than frass on real host tree logs, suggesting that host-tree volatiles do not en hance the attractiveness of prey frass. The beetles responded to 'tran sparent' Mylar three-dimensional cylinders in the same way as they did to the black plastic pipes. Beetles were also tested with frass and t wo-dimensional stimuli offering a visual stimulus or physical barrier alone or in combination. Frass with a two-dimensional visual stimulus was no better than frass alone: few insects landed. Significantly more insects were attracted to the same two-dimensional visual stimulus wi th a hidden upwind barrier producing turbulence, but a similar number were attracted to the frass in front of a hidden barrier alone. As man y plants are similar in size to the logs used in these experiments, co mparable effects might influence the behaviour of other phytophagous i nsects, their predators and parasitoids.