Abstract
In mutualisms, each interacting species obtains resources from its partner that it would obtain less efficiently if alone, and so derives a net fitness benefit. In exchange for shelter (domatia) and food, mutualistic plant-ants protect their host myrmecophytes from herbivores, encroaching vines and fungal pathogens. Although selective filters enable myrmecophytes to host those ant species most favorable to their fitness, some insects can by-pass these filters, exploiting the rewards supplied whilst providing nothing in return. This is the case in French Guiana for Cecropia obtusa (Cecropiaceae) as Pseudocabima guianalis caterpillars (Lepidoptera, Pyralidae) can colonize saplings before the installation of their mutualistic Azteca ants. The caterpillars shelter in the domatia and feed on food bodies (FBs) whose production increases as a result. They delay colonization by ants by weaving a silk shield above the youngest trichilium, where the FBs are produced, blocking access to them. This probable temporal priority effect also allows female moths to lay new eggs on trees that already shelter caterpillars, and so to occupy the niche longer and exploit Cecropia resources before colonization by ants. However, once incipient ant colonies are able to develop, they prevent further colonization by the caterpillars. Although no higher herbivory rates were noted, these caterpillars are ineffective in protecting their host trees from a pathogenic fungus, Fusarium moniliforme (Deuteromycetes), that develops on the trichilium in the absence of mutualistic ants. Therefore, the Cecropia treelets can be parasitized by two often overlooked species: the caterpillars that shelter in the domatia and feed on FBs, delaying colonization by mutualistic ants, and the fungal pathogen that develops on old trichilia. The cost of greater FB production plus the presence of the pathogenic fungus likely affect tree growth.
Highlights
Mutualisms are interspecific interactions involving two or more species where each partner obtains resources that it would obtain less efficiently if alone, and so derives a net fitness benefit [1,2,3]
Like Coelomera chrysomelid beetles [37] and mutualistic ants, P. guianalis caterpillars at their third larval stage recognize the prostomata and gnaw an entrance hole to shelter in the host tree domatia
The difference with the damage caused by Coelomera is that it is direct as they feed on young leaves [37], whereas P. guianalis caterpillars, like mutualistic ants, feed on the food bodies (FBs) produced by the plant
Summary
Mutualisms are interspecific interactions involving two or more species where each partner obtains resources that it would obtain less efficiently if alone, and so derives a net fitness benefit [1,2,3]. These mutualistic partnerships are transmitted from one generation to the in one of two ways. Biotic pollination, seed dispersal by animals, ant-plant associations and interactions between rhizobia or mycorrhiza and plant roots are transmitted horizontally and can be mutualistic [4],[6,7,8,9]. Hostplant selection by founding ant queens, for example, seems to be driven by chemical compounds [13,14,15,16]; to enter into the domatia of certain myrmecophyte species, founding ant queens must be the right size or be able to recognize and to gnaw an entrance hole into the prostomata or thinner area, generally devoid of vessels [10],[17,18,19]
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