Role of trisaccharides in larval secretion of Lycaeides argyrognomon butterfly on ant attendance

Several myrmecophilous insects participate in symbiotic relationships with ants that receive sugar-rich food rewards. For instance, certain aphid species secrete honeydew containing high concentration of melezitose, which acts as a potent feeding-stimulant and attractant for ants. Lycaenid butterfly larvae possess dorsal nectary glands that secrete sugar-rich droplets for tending ants. However, the roles of sugar components in ant foraging and larva-tending activities are unknown. Lycaeides (Plebejus) argyrognomon are larvae that are frequently and facultatively attended by various ant species, including Formica japonica, on the host plant Indigofera pseudotinctoria. The larval secretions of this insect contained small amounts of trisaccharides, melezitose and maltotriose, which were not detected in the host plant’s flower nectar, and larval secretions of two sympatric and myrmecophilous lycaenids, Zizeeria maha and Everes argiades. Melezitose and maltotriose, along with sucrose, were preferred by the worker ants. Of the four sugar mixture samples that mimicked I. pseudotinctoria floral nectar and the larval secretions of three lycaenids, respectively, the L. argyrognomon mimic was the most preferred by F. japonica ants. Moreover, the removal of trisaccharides from this mimic significantly reduced its stimulatory activity to ant feedings. These results indicated that the sugar composition of L. argyrognomon larval secretions is suited to the feeding preference of F. japonica ants, and that the trisaccharide components play a key role in increasing their preference. However, only half of the ants responded to the L. argyrognomon mimic even at the concentration corresponding to the maximum total sugar concentration in the collected larval secretions. The fact that the secretions of all L. argyrognomon larvae did not have sufficient sugar levels to stimulate ant feedings suggests that the production of sugar-rich secretions and trisaccharide components is metabolically costly for the larvae and that components other than sugars may be involved in ant attendance.