Courtship behavior and identification of a sex pheromone in Ibalia leucospoides (Hymenoptera: Ibaliidae), a larval parasitoid of Sirex noctilio (Hymenoptera: Siricidae)

Abstract

Background: Ibalia leucospoides (Hymenoptera: Ibaliidae) is a larval parasitoid that has been widely introduced as a biological control agent for the invasive woodwasp,Sirex noctilio (Hymenoptera: Siricidae) in the Southern Hemisphere. In this study, the courtship behavior and identificaion of sex pheromones are described for I. leucospoides under laboratory conditions.

Methods: For courtship behavior, both sexes were observed in a wire mesh observation cylinder (75 cm length ×10 cm diameter) for 15 minutes. The female body washes were analyzed using Gas Chromatography- Electroantennographic Detection (GC-EAD). Then the EAD-active compounds were tentatively identified using GC-Mass Spectrometry (GC-MS) and examined in olfactometer assays.

Results: The courtship behavior included rhythmic lateral movements, mounting, head-nodding cycles in males, and wing-fanning in females. GC-EAD analysis of female body washes with male antennae revealed seven compounds which elicited antennal responses, four of which are straight-chain alkanes (C₂₃, C₂₅, C₂₆, and C₂₇). The identities of these alkanes were confirmed by matching the retention times, mass spectra, and male antennal activity to those of commercially obtained chemicals. In olfactometer assays, a blend of the four straight-chain alkanes was attractive to I. leucospoides males, and there was no response to blends that lacked any of these four compounds. Female body wash was no more attractive than the four-component blend. The ratios of EAD-active components differ between hydrocarbon profiles from males and females.

Conclusion: This study is the first investigation of cuticular hydrocarbons in the family Ibaliidae. It provides evidence that the ubiquitous alkanes (C₂₃, C₂₅, C₂₆, and C₂₇) in sex-specific ratios attract I. leucospoides males.

Keywords: Courtship behavior; Cuticular hydrocarbon; Ibaliidae; Parasitoid; Sex pheromone.

Figure 1. Representative coupled gas chromatography-electroantennogram detection analysis of body wash extracts of Ibalia leucospoides female (A) and male (B), using antennae from I. leucospoides males.

Figure 2. Representative total ion chromatograms from GC-MS analysis of body wash extracts of Ibalia leucospoides females (A) and males (B).

Figure 3. Percentage of Ibalia leucospoides male responses to different stimuli in olfactometer assays.

The authentic blends were prepared at the same concentrations and ratios found in body wash extracts of individual I. leucospoides females. Table 1 provides the composition of each blend. The number of insects that responded to the stimuli are noted next to the corresponding bar. The P-value, percent responding (Resp.%), and total number of wasps for each trial is presented adjacent to the corresponding bar. The significant differences at α =0.05 are indicated by asterisks.