Communication of codling moth larvae: Identification and functional role of their aggregation pheromone

Pupation site-seeking larvae of the codling moth, Cydia pomonella (L.), (Lepidoptera: Tortricidae), aggregate in response to pheromone emanating from freshly spun cocoons of conspecific larvae. My main objectives were to identify the pheromone and to study its functional roles. In unmanaged apple orchards I surveyed tree trunks for cocooning larvae and found larval aggregations significantly more often than solitary larvae. In 31 two-choice olfactometer experiments, a pheromone blend of (E)-2-octenal, (E)-2-nonenal, sulcatone, and geranylacetone, in combination with either 3-carene and/or three saturated aldehydes (octanal, nonanal, decanal), elicited behavioural responses from fifth-instar larvae. Both male and female larvae produced, and responded to, aggregation pheromone. Larvae were attracted to, rather than merely arrested by, larval aggregation pheromone. They moved faster and farther upwind toward cocooning conspecifics compared to blank controls, and selected more often as first- and final choices of pupation sites those with cocooning conspecifics than those without. Finally, in Y-tube olfactometers, they anemotactically responded to, and preferred side arms with, cocooning conspecifics to those without. Aggregation behaviour by larvae does not appear to increase the rate of parasitism. In a wind tunnel, 10 larvae in aggregations were more readily located by female parasitoids Mastrus ridibundus than 10 larvae well separated from each other. Larval cocooning in aggregation or isolation had no effect on the mean rate of parasitism and the mean number of eggs deposited per parasitized host. The increased risk of aggregated larvae to be detected by egg-limited M. ridibundus is likely offset by diluted parasitism risk. A genetic algorithm model predicted that larval aggregation behaviour is selected for when the probability of mate encounter away from emergence sites is low, the time to locate mates is short, and M. ridibundus are at high population levels. In field experiments, corrugated cardboard bands treated with synthetic pheromone were more effective in capturing C. pomonella larvae than untreated bands, providing proof of concept that synthetic pheromone can be used to enhance captures of fifth-instars in trapping devices. Mature larvae of Oriental fruit moth, Grapholita molesta, and Indianmeal moth, Plodia interpunctella, were not attracted to, or arrested by, cocoon-spinning conspecifics.