The Importance of Methyl-Branched Cuticular Hydrocarbons for Successful Host Recognition by the Larval Ectoparasitoid Holepyris sylvanidis

Abstract

Cuticular hydrocarbons (CHCs) of host insects are used by many parasitic wasps as contact kairomones for host location and recognition. As the chemical composition of CHCs varies from species to species, the CHC pattern represents a reliable indicator for parasitoids to discriminate host from non-host species. Holepyris sylvanidis is an ectoparasitoid of beetle larvae infesting stored products. Previous studies demonstrated that the larval CHC profile of the confused flour beetle, Tribolium confusum, comprises long chain linear and methyl-branched alkanes (methyl alkanes), which elicit trail following and host recognition in H. sylvanidis. Here we addressed the question, whether different behavioral responses of this parasitoid species to larvae of other beetle species are due to differences in the larval CHC pattern. Our study revealed that H. sylvanidis recognizes and accepts larvae of T. confusum, T. castaneum and T. destructor as hosts, whereas larvae of Oryzaephilus surinamensis were rejected. However, the latter species became attractive after applying a sample of T. confusum larval CHCs to solvent extracted larvae. Chemical analyses of the larval extracts revealed that CHC profiles of the Tribolium species were similar in their composition, while that of O. surinamensis differed qualitatively and quantitatively, i.e. methyl alkanes were present as minor components on the cuticle of all Tribolium larvae, but were absent in the O. surinamensis CHC profile. Furthermore, the parasitoid successfully recognized solvent extracted T. confusum larvae as hosts after they had been treated with a fraction of methyl alkanes. Our results show that methyl alkanes are needed for host recognition by H. sylvanidis.

Cuticular Hydrocarbon Trails Released by Host Larvae Lose their Kairomonal Activity for Parasitoids by Solidification

“…The liquid aggregate state of methyl-branched alkanes might be relevant for eliciting trail-following behavior in H. sylvanidis. This suggestion is supported by our previous study, which showed that methyl-branched CHCs on the cuticle of T. confusum larvae are exploited by H. sylvanidis females for host recognition when directly contacting the host larvae (Awater-Salendo et al 2020).…”

“…Holepyris sylvanidis discriminates between a host and a non-host species by a specific pattern of methyl-branched alkanes. The host-specific CHC pattern is common to the cuticle of different host beetle species (Awater-Salendo et al 2020 ). Hexane extracts of host larvae deposited as trails elicit trail-following behavior of H. sylvanidis .…”

Immune StimulationviaWounding Alters Chemical Profiles of AdultTribolium castaneum

“…These consisted of seven nalkanes (C25-C31), six 3-and 4-methyl-branched alkanes and seven internally methylbranched alkanes of which one was a dimethyl alkane (Table S2). The RIs of all identified CHCs did not differ by more than ± 4 from the RI values proposed by the literature (Lockey 1978;Alnajim et al 2019;Awater-Salendo et al 2020). The dominant CHCs in both naïve females and males were n-C29, 3-MeC27, 13-MeC27 and n-C27, all with a higher proportion than 10% (Table S2).…”

“…FASs are involved in the production of methyl alkanes (Blomquist & Ginzel, 2021; Holze et al, 2021). In comparison to n -alkanes (linear alkanes), methyl alkanes were shown to be more important in cue recognition (reviewed in van Zweden & d’Ettorre, 2010) and to mediate detection of T. castaneum larvae by parasitoids (Awater-Salendo et al, 2020). The regulation of methyl alkanes upon immune stimulation has been reported in other host-parasite systems as well (Baracchi et al, 2012; Richard et al, 2008).…”

“…From the stink gland analysis, benzoquinones (BQs) as well as other major compounds previously described in T. castaneum glandular secretions were identified by comparing their RIs to previous studies (Lehmann, 2015; Li et al, 2013), and by comparisons to the commercial standards analysed. Putative identification of CHCs was done by comparing the RIs with those published in studies by Alnajim et al (2019), Awater-Salendo et al (2020) and Lockey (1978).…”