Strain specific pheromone preference of the European corn borer

The European corn borer (Ostrinia nubilalis, ECB) is a major pest of corn in the northern hemisphere. Two pheromone strains exist, which produce and prefer an opposite ratio of the two pheromone component blend. The Z-strain produces and prefers Z11-tetradecenyl-acetate (Z11) and E11-tetradecenyl-acetate (E11) in a ratio of 97:3, whereas the E-strain produces and prefers a ratio of 1:99 (Anglade et al., 1984; Klun and Robinson, 1971). Hybrids of the two strains produce and prefer an intermediate pheromone blend (35:65 Z11:E11) (Glover et al., 1991; Roelofs et al., 1987). In wind tunnel studies with single pheromone sources males preferred the ratio of their own strain with low proportions of flight to intermediate blends. Estrain males will occasionally fly to the other strain’s blend (Roelofs and Glover, 1991). The behavioral response to pheromone is controlled by a single sex-linked gene (Roelofs et al., 1987). Modeling gene-frequencies using wind-tunnel based pheromone preference (and subsequent asymmetric levels of assortative mating) predicts that the strains do not co-exist in the field, with, depending on the initial frequency, one of the two strains prevailing. Yet, the situation in the field is rather diverse. In the US races rarely interbreed, but co-exist. In Europe population genetic studies show that strains range from strict allopatry, to sympatry, and from 100% assortative mating to apparent Hardy-Weinberg equilibrium (Bourguet, unpublished). Additional factors may be required to explain the variation of interbreeding and sympatry observed in the field. One such a factor could be geographic variation in pheromone preference. In previous studies the American Z-strain (derived originally from Italian populations) was found to be the most stringent with respect to pheromone preference, with no cross attraction to the E-strain. We therefore studied the pheromone preference of a Hungarian Z-strain. In our wind tunnel experiments the Hungarian Z-strain were highly specific, similar to what has been reported for the American Z-strain. However, in sympatric populations in the field males may be exposed to a mixture of plumes from Z, E and hybrid females. We therefore examined how specificity would be affected when three pheromone sources (97:3, 50:50 and 1:99 of Z11:E11) were presented simultaneously in the wind tunnel (size: 70 by 70 by 200cm, wind speed: 0.4m/s, light: 10 lux, temperature: 19°C, RH: 55-70%; Witzgall et al., 2001). Pheromone sources were placed at 10cm distance from each other, which resulted in intermixing of the three plumes from 2/3 downwind the wind tunnel. Levels of wing-fanning, upwind flight and source landing were similar to those observed with single sources. However, males were much less specific with respect to their choice, with a substantial proportion of males flying to the hybrid blend. Z males regularly flew to the E blend (5%). Plume analysis with TiCl4 indicates that strands of the different pheromone blends may hit the antennae simultaneously, but are, judging from the behavioral response (wing-fanning, upwind flight), recognized individually. The results also indicate that the sequential behavioral steps that culminate into mate finding are not equally restrictive with respect to the ’correct’ blend. Finally, our results indicate that under field circumstances strain specificity of males may be a function of population density. Interbreeding would then be expected to increase when population densities are sufficiently high to cause pheromone plumes from females to intermix. The results are important in the light of the use of pheromones in pest monitoring and mating disruption.