TEMPORAL CLUMPING OF BARK BEETLE ARRIVAL AT PHEROMONE TRAPS - MODELING ANEMOTAXIS IN CHAOTIC PLUMES

The sequence of arrival of the bark beetles Ips typographus and Pityog enes chalcographus (Coleoptera: Scolytidae) at traps baited with their synthetic pheromones was monitored with a portable fraction collector . Histograms of the natural arrival patterns of both species were nonr andom and clumped at shorter time scales (1-, 2-, 4-, 5-, or 6-min cel ls) but appeared random at larger time scares (10, 20 or 30 min). Mont e Carlo generation of similar histograms showed them to be random at a ll of these time scales. A stochastic computer model could graphically simulate insect orientation to odor sources based on present theories of odor-modulated anemotaxis and casting. Although this model was use d throughout, since it assumes only that insects cast perpendicular to the current wind direction, a second model could slightly improve ori entation success. However, the second model requires that the insect r emember its ground path (upwind) prior to losing the plume (after an a brupt wind direction change). The effects of casting and flight parame ters on orientation success and randomness of arrival sequence within various plumes were determined by simulation. Similarly, the effects o f random walks in plume direction, plume width, and wind speed were ex plored. The results showed that dynamic random variations in plume dir ection and especially wind speed could cause an otherwise random arriv al sequence (e.g., under constant wind) to become clumped and nonrando m. Therefore, the clumped arrival patterns of bark beetles and other i nsects, including Spodoptera litura, at pheromone sources could result from random-walk fluctuations in wind speed and wind direction.