Potential for the evolution of resistance to pheromone‐based mating disruption tested using two pheromone strains of the cabbage looper, Trichoplusia ni

Abstract

The reliance of most moth species on female-produced sex pheromones for mate attraction makes their communication system an ideal target for pest management. Mating disruption, by release of synthetic sex pheromone into the atmosphere, has been developed to control many lepidopteran pests and is available commercially for several species (Carde & Minks, 1995). Selection pressure provided by continuous application of mating disruptants could result in the evolution of resistance to this tactic, and this possibility should be evaluated before it occurs. The potential for resistance to mating disruption will depend on genetically-based variation in production of and response to pheromone within and amongst populations (Lofstedt, 1990, 1993; Phelan, 1992). The diverse phylogenetic pattern of lepidopteran pheromones suggests that natural selection has shaped the evolution of species-specific pheromones. Long-term use of mating disruptants that effectively control pest populations, could provide the necessary selection pressure for an alteration in the communication system and the evolution of resistance to mating disruption (Carde & Minks, 1995). In the cabbage looper, Trichoplusia ni (Hubner), the sex pheromone consists of a main component, Z7-dodecenyl acetate (Z7-12:Ac), and five minor components (Bjostad et al., 1984). Haynes & Hunt (1990) discovered an abnormal pheromone phenotype in a laboratory colony of T. ni that was the result of a single, recessive autosomal gene mutation. Mutant females release a pheromone that contains a twenty-fold increase of one minor component, Z9-tetradecenyl acetate (Z9-14:Ac) and a thirty-fold decrease of another minor component, Z5-dodecenyl acetate (Z512:Ac). Initially, male T. ni that carried the mutant gene responded like normal males, demonstrating a preference for the normal pheromone. However, after 49 generations within a pure mutant colony, males responded equally well to both mutant and normal pheromones (Liu & Haynes, 1994). Thus, there are genetically-based differences in both female pheromone and male response (Liu & Haynes, 1994). The potential for mating disruption to result in evolutionary changes in the pheromone was tested on laboratory-derived populations of T. ni using a replicated selection experiment in large field cages. We manipulated the environment (pheromone-treated vs. clean-air control) that moths were exposed to and followed the evolution of the pheromone phenotype in five closed (no gene flow) populations across four offspring generations. The normal pheromone blend applied as a mating disruptant provided selection pressure that influenced the frequency of the mutant allele in the population.