Chemical tactic of facultative myrmecophilous lycaenid pupa to suppress ant aggression

Abstract

Myrmecophilous lycaenid caterpillars have close relationships with their ant hosts by means of various myrmecophilous organs, most of which are usually lost after pupation. However, some lycaenid species, including Lycaeides argyrognomon, maintain such relationships at the pupal stage and go so far as to pupate in ant nests. This invokes the hypothesis that these myrmecophilous lycaenid pupae might have alternative tactics to retain myrmecophilous interactions without ant attacks. Camponotus japonicus, Formica japonica, and Lasius japonicus exhibited distinctive aggressive behaviors against ant cuticular hydrocarbons (CHCs) from different colonies of the same species but few attacks against the crude extract of L. argyrognomon pupae. GC–MS analysis revealed that the pupal cuticular lipids contain not only CHCs but also several long-chained aliphatic aldehydes, including 1-octacosanal and 1-triacontanal, which are absent from larval cuticular lipids. With the addition of synthesized 1-octacosanal and 1-triacontanal to ant CHCs from different colonies of the same species, the aggressive behavior decreased in C. japonicus, and the duration of physical contact shortened in C. japonicus and F. japonica. However, the behavior of L. japonicus remained unaffected after the addition of those aldehydes. These results suggest that the pupae-specific cuticular aldehydes of L. argyrognomon suppress ant aggression even after the loss of certain myrmecophilous organs, though the effects varied depending on the attending ant species. Since L. argyrognomon occasionally pupate in the nests of C. japonicus in the field, the lycaenids might be better adapted to associations with C. japonicus than with the other two ant species studied.