Plume interaction and odour source spacing ofpheromone and non-host volatiles : : Behaviouraleffects on bark beetles and moths
Abstract: Insects have a remarkable ability to sense whether odours are released from one point source or from two closely separated ones. Therefore it is of interest to study the interactions of pheromone component odour plumes to affect the insect behaviour when the release points ofdifferent components are physically separated. It helps us to understand the mechanism that insects use to find their host or any attractive source. We studied the behavioural response of Spodoptera littoralis (Lepidoptera, Noctuidae) males, in a walking bioassay, to two components of the female sex pheromone blend, (9Z,11E-tetradecadienyl acetate) and(9Z,12E-tetradecadienyl acetate), released from two separate dispensers at various distances (0-5 cm) from each other. The main pheromone component, 9Z-11E-14:OAc was tested at 1 and 10 ng in combination with the minor component 9Z-12E-14:OAc, at 1 % of the maincomponent. The low dose always showed a lower behavioural response than the high dose at the same separation distances (0-5 cm). When low doses were used, the response was higher at 0 and 3 cm separation than other separations whereas when high doses were used there was not much difference in response at 0, 2, and 3 cm separations. A Photo Ionization Detector(PID) was used to investigate plume width and overlap. The PID showed that 5 cm separationof odour sources was enough to totally separate the odour plumes from each other, whereas at 3 cm separation, plumes partially overlapped. By comparing the behavioural results with PID data it is concluded that insects are much more sensitive than the PID, since they responded atdistances where the PID showed zero ppb. Male insects may respond well to doses that are 20 times lower than one female equivalent. A field experiment for S. litura was done in Pakistan on cotton crop by spacing sex pheromone components. Pheromone components were spacedhorizontally at 0, 3, 5 and 15 cm on plastic moth pheromone traps. The high dose attracted more males than the medium and low dose. Males were highly attracted at 0 cm separationbut spacing decreased the catch and at 15 cm spacing there was no catch. The behavioural effect of separating ph. components was further studied on the spruce bark beetle, Ips typographus L., in the field. The attraction of I. typographus to traps baited with the twoaggregation pheromone components (cis-verbenol and 2-metyl-3-buten-2-ol) separated, was investigated in a Norway spruce clear-cut. The pheromone components were separated both vertically (0-112 cm) on extended Lindgren (19 funnel) traps and horizontally (0-80 cm) on modified windvane traps. I. typographus was strongly attracted when the two components were released from the same point but spacing between components decreased the trap catch. However, at 16 cm distance, in both the vertical and horizontal test, the trap catch was not much different from the positive control (0 cm separation). The effect of odour source spacing in the field on I. typographus was also studied with regards to repellent non-host volatiles (NHV). In this test, the aggregation pheromone were separated from a blend ofrepellents (trans-conopthorin, 1,8-cineol, 3-octanol, 1-octen-3-ol, 1-hexanol and verbenone) using the same experimental design. NHV showed strong inhibitory effect up to 48 cm spacing but there was not much effect at 80 and 112 cm separations.
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