Division of labour and its regulation in a primitively eusocial wasp

Abstract

Social insect species that exhibit overlap of generations, cooperative brood care and reproductive caste differentiation have been termed eusocial. Among these, those that also exhibit morphological caste differentiation are termed highly eusocial while those that have morphologically identical queen and worker castes are termed primitively eusocial. Because queens and workers are not morphologically differentiated in primitively eusocial species, caste differentiation is often post-imaginal. The processes by which some individuals succeed in becoming reproductive queens while others end up as sterile workers in primitively eusocial species is of obvious interest. Such post-imaginal reproductive caste differentiation is absent in highly eusocial species because reproductive caste differentiation in these species is usually pre-imaginal. On the other hand, further differentiation of the workers into different kinds of task specialists is a phenomenon that has been extensively studied in highly eusocial species (Wilson, 1971). There has seldom been an opportunity to study both post-imaginal reproductive caste differentiation as well as post-imaginal division of non-reproductive labour in the same species because these two phenomena usually do not occur in the same species. Ropalidia marginata is an old world tropical primitively eusocial wasp widely distributed in southern India. Although classified as primitively eusocial by the traditional criteria of overlap of generations, cooperative brood care, reproductive caste differentiation and absence of morphological differentiation between queens and workers (Gadagkar, 2001a,b), R. marginata appears to have acquired some features resembling highly eusocial species. One such feature concerns the mode of division of non-reproductive labour among the adult wasps. R. marginata exhibits a remarkably well-developed honeybee like age polyethism. Workers show a strong tendency to feed larvae, build the nest, forage for pulp and forage for food, in that order, as they age. The frequency and probability of performance of different tasks is strongly influenced by the age of the individual (Naug and Gadagkar, 1998a). Like in the honeybees, age polyethism in R. marginata is very flexible and this flexibility comes from the fact that workers are allocated tasks based more on their relative age rather than their absolute age. This can be demonstrated experimentally by creating artificial colonies containing only young individuals (young cohort colonies) or containing only old individuals (old cohort colonies). In young cohort colonies, precocious foragers, i.e., some individuals who begin to forage at an early age when they would not do so in normal colonies, compensate for the absence of older individuals (Naug and Gadagkar, 1998b). In old cohort colonies, hard working nurses, i.e., some individuals who feed larvae at rates higher than they would in normal colonies, compensate for the absence of younger individuals (Agrahari and Gadagkar, submitted). Juvenile hormone modulates post-imaginal reproductive division of labor in primitively eusocial species and promotes the production of queens (e.g., Polistes) while it modulates age polyethism and promotes the production of foragers in highly eusocial species (e.g., the honeybee). Since R. marginata shows both post-imaginal regulation of reproductive division onabor as well as age polyethism, it is a particularly interesting model system to study the effect of juvenile hormone. A single, topical application of 100 ~g. of juvenile hormone-III per female wasp accelerates ovarian development of wasps held in isolation. Similar application to wasps released back on to their natal nests has no effect on their rate of behavioral development as witnessed from the age of first performance of feed larva, build, bring pulp and bring food. It appears, therefore, that in R. marginata, juvenile hormone has retained its function of modulating reproductive division of labor and has not acquired the function of modulating age polyethism (Agrahari and Gadagkar, 2003).