Aggregation pheromones of two geographical isolates of the new guinea sugarcane weevil, Rhabdoscelus obscurus

Abstract

The aggregation pheromones were studied from two geographical isolates (Hakalau, Hawaii, and Silkwood, Queensland, Australia) of the New Guinea sugarcane weevil, Rhabdoscelus obscurus. Coupled gas chromatographic–electroantennographic detection (GC-EAD) and GC–mass spectrometric (MS) analyses of Porapak Q volatile extract from male and from female Hawaiian R. obscurus revealed a single EAD-active, male-specific candidate pheromone, which was identified as 2-methyl-4-octanol (1). Corresponding volatile analyses from male and from female Australian R. obscurus consistently revealed three EAD-active, male-specific candidate pheromone components that were identified as 1, (E2)-6-methyl-2-hepten-4-ol (rhynchophorol) (2), and 2-methyl-4-heptanol (3). In field experiment 1 in Hakalau, Hawaii, traps baited with a stereoisomeric mixture of synthetic 1 (3 mg/day) plus sugarcane captured more weevils than did traps baited with 1 or sugarcane alone or no bait, indicating that 1 is the pheromone of the Hawaiian R. obscurus population. In field experiment 2, conducted in Silkwood, Australia, traps baited with stereoisomeric mixtures of synthetic 1, 2, and 3 (3 mg/day each) plus sugarcane caught more weevils than did unbaited traps or traps baited with 1, 2, and 3 or sugarcane. Testing candidate pheromone components 1, 2, and 3 in experiments 2–5 in all possible binary, ternary, and quaternary combinations with sugarcane, indicated that 1 and 2 in combination, but not singly, are pheromone components of the Australian R. obscurus population. Weevils from several locations in Australia and Hawaii could not be differentiated using traditional morphological characters or ultrastructural comparisons with scanning electron microscopy (SEM). However, comparisons of mtDNA sequences (cytochrome oxidase I; regions I1 to M4; 201 base pairs) revealed 5.5% variation between the Hawaiian (N = 2) and the Australian (N = 4) samples. There was no intrapopulation variation in sequence data from the weevils from Hawaii versus Australia, suggesting that they are sibling species.