A mutualism without honeydew: what benefits forMelissotarsus emeryiants and armored scale insects (Diaspididae)?

Abstract

Mutualisms between ants and sap-sucking insects generally involve clear benefits for both partners: the ants provide protection in exchange for honeydew. However, a single ant genus associates with armoured scale insects (Diaspididae) that do not excrete honeydew. We studied three colonies ofMelissotarsus emeryiants from two localities in Mozambique. Vast numbers of the diaspididMorganella conspicuaoccupied galleries dug by the ants under the bark of living trees. Unlike free-livingM. conspicuaand other diaspidids,M. conspicualiving with ants are known to lack shields, likely because they gain protection against enemies and desiccation. Nevertheless, we documented the occurrence of rare individuals with shields inside ant galleries, indicating that their glands continue to secrete wax and proteins as building material. This is likely to constitute a significant portion of the ants’ diet, in addition to diaspidid exuviae and excretions from the Malpighian tubules. Indeed,Melissotarsusworkers cannot walk outside the galleries due to modified middle legs, forcing them to obtain all nourishment within the tree.Melissotarsusfounding queens, however, must locate a suitable host tree while flying, and acquire diaspidid crawlers. This mutualism involves ants that are highly specialised to chew through living wood, and diaspidids that can also live freely outside the bark. It is extremely widespread in Africa and Madagascar, recorded from 20 tree families, and harmful effects on plant hosts require rapid study.