Effects of Pheromone Dose and Conspecific Density on the Use of Aggregation-Sex Pheromones by the Longhorn Beetle Phymatodes grandis and Sympatric Species (Coleoptera: Cerambycidae)

Many species of longhorn beetles (Coleoptera: Cerambycidae) utilize male-produced aggregation-sex pheromones that attract both sexes. However, the reasons why and the details of how this type of pheromone is used by cerambycids and other coleopteran species that utilize analogous male-produced pheromones remain unclear. Thus, our goals were to test the hypotheses that 1) cerambycids respond to pheromones in a dose-dependent (= release rate-dependent) manner and 2) pheromone emission is density-dependent. If true, these characteristics of pheromone use could suggest that cerambycids utilize an optimal density strategy to limit competition for scarce and ephemeral hosts, i.e., the stressed or dying trees that typically constitute their larval hosts. Attraction of beetles to a range of release rates of two common pheromone components – 2-methylbutanol and 3-hydroxyhexan-2-one – was tested in field trials. Responses, as measured by the number of beetles caught in pheromone-baited traps, increased with release rates for five endemic species, even at the highest rates tested (~1450 μg/h for 2-methylbutanol and ~720 μg/h for 3-hydroxyhexan-2-one). The effect of density of conspecific males on per capita pheromone production was tested by collecting the volatiles produced by individuals, pairs, or groups of three or four male Phymatodes grandis Casey. Frequency of pheromone production was significantly different among the treatment densities, and emission rates of the pheromone (R)-2-methylbutanol decreased with increasing density. These results are discussed in the context of a possible optimal density strategy used by cerambycids, and more broadly, in relation to the use of male-produced aggregation-sex pheromones by other coleopterans. In addition, we report the identification of the pheromones of four of our five test species, specifically, Phymatodes obliquus Casey, Brothylus conspersus LeConte, Brothylus gemmulatus LeConte, and Xylotrechus albonotatus Casey.