Behaviour of pink bollworm males near high-dose, point sources of pheromone in field wind tunnels: insights into mechanisms of mating disruption

The mechanisms by which the application of formulated pheromone interferes with mating in the pink bollworm moth (PBW), Pectinophora gossypiella were examined in 0.4 ha cotton fields using high-dose (78 mg A.I.) sealed polyet hylene dispensers. Walk-in, field wind tunnels 6.2 m long were placed over two rows of cotton. Treatments consisted of a control, a tunnel in a field free of disruptant formulation; a 3-rope treatment, in which the field was free of pheromone but one of the cotton rows in the wind tunnel was treated with 3 PEW ropes; and a rope-grid treatment, in which the field was treate d with PEW ropes at the standard density of 1000 ha(-1) and one of the cott on rows inside the wind tunnel was treated with 3 PEW ropes. We released ma rked males into the tunnels near sunset or held them in field cages for 24 h prior to assay. Two pheromone traps at the tunnel's upwind end monitored the ability of males to locate point sources of pheromone. In the 3-rope tu nnel, traps placed upwind of the cotton row treated with disruptant pheromo ne captured far fewer males than those placed upwind of the untreated cotto n row. In the tunnel situated in the centre of the rope-gridded field, very few males were caught in traps in both rows, indicating a camouflage of th e pheromone plumes from the traps by the background of airborne disruptant drawn into the tunnel from the field. Activity of moths near the synthetic pheromone sources was video-recorded. Males oriented to, landed on or near, and walked on or near, PEW ropes, indicating competition between pheromone sources as a mechanism of mating disruption. Most males Visiting PEW ropes became quiescent or disappeared from the field of view after a few minutes , suggesting a habituation/adaptation of response. The rhythm of attraction of males held in the field for 24 h before release was comprised of a smal l peak of activity near 2000 h, with the majority of attraction between 230 0 and 0300 h. Much of the attraction before 0100 appears to be an advanceme nt of the male's normal diel rhythm, caused by the presence of disruptant. Together these findings indicate that mating disruption of pink bollworm us ing the PEW ropes is achieved by a combination of mechanisms: a camouflage of natural plumes, competition between pheromone sources, habituation, and some advancement of the male's rhythm of response.