Social Organisation in Laboratory Colonies of Ropalidia Marginata

Ropalidia marginata (Lep.) (Hymenoptera: Vespidae) is a primitively eusocial wasp which lacks morphological caste differentiation. By constructing time activity budgets for individually identified wasps and subjecting these to multivariate statistical analysis, we have shown that adult female wasps can be classified into one of the three behavioural castes namely Sitters, Fighters and Foragers. In recent times we have often found it necessary to use laboratory colonies for a variety of experiments. It is important therefore to see if social organisation of the laboratory colonies is similar to that in natural colonies. This report describes the results of 306 hrs of observations of 2 laboratory colonies that were established by marking and releasing all adults present on two natural colonies

Ovarian developmental variation in the primitively eusocial wasp Ropalidia marginata suggests a gateway to worker ontogeny and the evolution of sociality

Social insects are characterized by reproductive caste differentiation of colony members into one or a small number of fertile queens and a large number of sterile workers. The evolutionary origin and maintenance of such sterile workers remains an enduring puzzle in insect sociobiology. Here, we studied ovarian development in over 600 freshly eclosed, isolated, virgin female Ropalidia marginata wasps, maintained in the laboratory. The wasps differed greatly both in the time taken to develop their ovaries and in the magnitude of ovarian development despite having similar access to resources. All females started with no ovarian development at day zero, and the percentage of individuals with at least one oocyte at any stage of development increased gradually across age, reached 100% at 100. days and decreased slightly thereafter. Approximately 40% of the females failed to develop ovaries within the average ecological lifespan of the species. Age, body size and adult feeding rate, when considered together, were the most important factors governing ovarian development. We suggest that such flexibility and variation in the potential and timing of reproductive development may physiologically predispose females to accept worker roles and thus provide a gateway to worker ontogeny and the evolution of sociality

Constructing Dominance Hierarchies in a Primitively Eusocial Wasp

It is well known that dominance-subordinate interactions form an important part of social organisation in primitively eusocial wasps (1). In many studies of social insects, the rank of an animal is based merely on the frequency with which it shows dominance behaviour. This is misleading because a high frequency of dominance behaviour may be accompanied by an equally high frequency of subordinate behaviour. Moreover showing dominance over a high ranking individual should be weighed differently from showing dominance over a low ranking individual. For these reasons, we have used a modified version of the index of fighting success that was developed for red deer to solve similar problems(2)

Serial polygyny in the primitively eusocial wasp Ropalidia marginata: implications for the evolution of sociality

Social insects usually live in colonies comprising one or a small number of reproductive individuals and a few or large number of sterile individuals. In termites only, both sexes are represented among the reproductives as well as among the sterile workers. In other social insects, namely ants, bees, and wasps, males do not participate significantly in the social life of colonies, which involves primarily the fertile queens and sterile female workers (Wilson 1971). The haplodiploid genetic system found universally in the Hymenoptera creates an asymmetry in genetic relatedness such that a female is more closely related to her full sister (coefficient of genetic relatedness, r = 0.75) than to her offspring (r = 0.5). This makes inclusive fitness theory (Hamilton, 19640, b) particularly applicable to the evolution of sterile worker castes in the social hymenoptera (Wilson 1971; Hamilton 1972)

Perennial Indeterminate Colony Cycle in a Primitively Eusocial Wasp

The colony cycle of primitively eusocial wasps consists of three phases: the pre-emergence phase, the post-emergence phase and the declining phase. Most species of wasps in temperate regions follow a seasonai colony cycle. Being initiated synchronously in Spring by overwintered females, colonies grow through summer and are abandoned in Fall, after producing reproductives. Females produced in Fall mate and overwinter while the remaining individuals, including the males, die. In the tropics, however, colonies are aseasonal and may be initiated throughout the year. Colony cycles in most species, either temperate or tropical, may thus be termed determinate since they are abandoned after a fixed time after initiation. Ropalidia marginata follows a colony cycle which often encompasses multiple repeats of a typical determinate colony cycle. We therefore call it an indeterminate colony cycle. A major portion of the brood and cells are destroyed and a large fraction of adults leave the nest during the declining phase of each unit of the colony cycle but a small number of females may stay back on the nest and begin a new unit of the cycle. This leads to two interesting questions: why is there a decline if the colony is to continue? and why is the decline not complete as in the determinate colony cycle. We propose two alternate hypotheses.One is that the colony cycle is a response to predation by Vespa tropica. It may be adaptive to issue 'swarms' of dispersing wasps periodically to found new nests, before all is lost to the predator. This hypothesis predicts that queen replacements need not necessarily coincide with the beginning of every unit of the colony cycle. The second hypothesis is that the queen produces reproductives and dies at the end of each unit of the colony cycles. One of her daughters may however stay on and use her natal nest to produce her brood. This hypothesis predicts that queen replacements should necessarily coincide with the beginning of every unit of the colony cycle. Our present results are incapable of distinguishing between these hypotheses. In some colonies queen replacements always coincide with new units of the colony cycle but in others, this is not so. Further studies of such perennial indeterminate colony cycles are bound to be rewarding