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In battles between rival groups, aggression levels, and ultimately the outcome, may be influenced by the number of individuals and the resource-holding potential (RHP) of the individuals in each group. Since fighting in social animals can involve intragroup cooperation, the distribution of individual RHPs within the group could influence overall group ability. Thus, the total group fighting prowess might not simply equate to the sum of individual RHPs. We investigated the effects of within-group variance in individual RHP during battles in wood ants, Formica rufa. Rival groups were matched in number of individuals and mean body size of individuals but differed in the variance among body sizes. Larger individuals were most aggressive and small individuals were least aggressive. Overall, small individuals had the highest attrition rate and large individuals the lowest, with medium-sized individuals experiencing moderate attrition rates. Consequently, aggression and attrition rates in low-variance groups did not differ significantly from rates in high-variance groups, indicating that when group size and mean body mass are equal, the distribution of individual RHPs does not influence the overall fighting ability of the group. Rather, it appears that large individuals are able to compensate for poor performance by weaker group members.

The fighting ability components of ‘resource-holding potential’ (RHP) may vary with body size, weapon size and energetic status. Theory predicts that agonistic signals should vary with RHP and any decision to ‘give up’ should be made by the opponent with lower RHP. In some species, an encounter is only resolved when one opponent is killed or critically injured. We investigated fatal fighting between groups of red wood ants, Formica rufa, and determined whether the probability that a focal group inflicts the first fatality in an encounter is influenced by the same correlates of RHP demonstrated to affect noninjurious fight outcome. In addition, we manipulated group size, an additional factor which is likely to be linked to RHP during intergroup conflicts, to determine whether members of larger groups concentrate attacks on their less numerous opponents. We found that, during most encounters, focal groups that inflicted the first fatality contained individuals of larger mean body mass than groups that received the first fatality. There was no evidence that energetic status directly influenced outcome, although groups that received the first fatality experienced a significant reduction in total body sugar compared to nonfighting controls. Finally, we found no evidence to suggest F. rufa concentrate their attacks in a way that leads to enhanced group RHP.

The structures of volatile chemicals released by parasitic wasps in the family Bethylidae are shown to correspond to the subfamily to which the species belong. Species in the Epyrinae release skatole (3-methylindole) and species in the Bethylinae release a spiroacetal (2-methyl-1,7-dioxaspiro [5.5]undecane): these compounds are chemically very different. The enantiomeric composition of the spiroacetal differs between congeneric species. Chemical release is a discrete event under the active control of both male and female wasps. Structural differences between the mandibular glands and intramandibular glands suggest the mandibular glands to be the source of released volatiles. Real-time mass spectrometry shows that the spiroacetal is released by Goniozus nephantidis females during dyadic resource contests, with release more common during more aggressive interactions. Chemical tagging with deuterium further shows that the volatile is released by the loser of an agonistic interaction and not the winner. The function of spiroacetal and skatole release by bethylids is discussed.

When two animals compete directly for an indivisible resource, the outcome may be influenced by differences in the contestants' intrinsic contest abilities and prior owner status, collectively termed ‘resource-holding potential’, and also by any difference in the values that contestants place on the resource. Using owner–intruder contests and owner–owner contests between females of the parasitoid wasp Goniozus nephantidis, we investigated the influence of resource value asymmetries generated by differences in contestant age and by the size of the host resource, while taking resource-holding potentials into account. When owners and intruders competed, intruder success increased with both intruder age and relative body size, suggesting that larger females had greater contest ability and that older intruders valued more greatly the opportunity to obtain a host. The size of the host did not influence the outcome of owner–intruder contests but in owner–owner contests, in which asymmetries in ownership status were absent, both larger females and owners of the larger, more valued, hosts were advantaged. Overall, studies of G. nephantidis show that asymmetries in both resource-holding potentials and resource values interact to influence contest outcomes. Despite a large literature on animal contests, there have been few studies that have evaluated these influences both simultaneously and separately.

The question of whether biological control is most likely achieved by deploying single or multiple species of biological control agents is much debated. While utilizing several natural enemies may enhance control, there is also the potential for disruptive inter-specific interactions. Such interactions may be studied in the laboratory by focusing on the details of the interactions themselves and attempting to infer population level consequences from their sum, or by focusing more directly on the overall effects on natural enemy populations: we term these approaches ‘reductionist’ and ‘holistic.’ Here we conduct a holistic laboratory study on interactions between three species of parasitoid wasps that are parasitoids of the coffee berry borer, Hypothenemus hampei (Ferrari) (Coleoptera: Scolytidae): Cephalonomia stephanoderis Betrem, C. hyalinipennis Ashmead and Prorops nasuta Waterston (all Hymenoptera: Bethylidae). We find evidence for both intra- and inter-specific resource competition. Interactions between C. stephanoderis and P. nasuta , both indigenous to Africa, appear to be approximately symmetrical, while C. hyalinipennis , naturally found in the coffee plantations of Chiapas, Mexico, may exert a disruptive influence. C. hyalinipennis also has a low population growth rate. We now consider it to be a detrimental invader of the Mexican coffee agro-ecosystem that should not be encouraged by augmentative release or introduced into other regions. Overall, the most successful species, in terms of both emergence and female production, was P. nasuta . We compare these results with those from prior reductionist and holistic studies, and with observations on patterns of establishment of these bethylid species in the field. Given that it is increasingly clear that disruptive inter-specific interactions are generally common when multiple species are deployed in biological control, screening of potential agents should consider such interactions alongside the more ‘traditional’ focus on host specificity.

The coffee berry borer, Hypothenemus hampei (Ferrari) (Coleoptera: Scolytidae) causes substantial reductions in coffee production. It originates from Africa but is now present in almost all of the major coffee producing countries. Classical biological control attempts around the world, including South and Central America, the Caribbean, Indian sub-continent, Indonesia, and Polynesia, including introductions of the African bethylid wasps Prorops nasuta Waterston and Cephalonomia stephanoderis Betrem, have not yet proved sufficiently successful. In Mexico, a bethylid wasp indigenous to North America and Europe, Cephalonomia hyalinipennis Ashmead, has naturally started to attack the borer. The presence of multiple species of natural enemies has the potential to disrupt biological control via negative interspecific interactions. We evaluate both inter- and intraspecific competition among these three bethylids in the laboratory, focussing on pairs of adult females competing directly for hosts. In interspecific contests, the loser is frequently killed. C. stephanoderis is the most successful species, while P. nasuta females never killed their opponents. Intraspecific interactions often involved fighting behavior but were non-fatal. We discuss the implications of the differing strengths of such inter- and intraspecific interactions for the coexistence of these natural enemies of the coffee berry borer and for biological control.

When social animals engage in inter-group contests, the outcome is determined by group sizes and individual masses, which together determine group resource-holding potential (‘group RHP’). Individuals that perceive themselves as being in a group with high RHP may receive a motivational increase and increase their aggression levels. Alternatively, individuals in lower RHP groups may increase their aggression levels in an attempt to overcome the RHP deficit. We investigate how ‘group RHP’ influences agonistic tactics in red wood ants Formica rufa . Larger groups had higher total agonistic indices, but per capita agonistic indices were highest in the smallest groups, indicating that individuals in smaller groups fought harder. Agonistic indices were influenced by relative mean mass, focal group size, opponent group size and opponent group agonistic index. Focal group attrition rates decreased as focal group relative agonistic indices increased and there was a strong negative influence of relative mean mass. The highest focal attrition rates were received when opponent groups were numerically large and composed of large individuals. Thus, fight tactics in F. rufa seem to vary with both aspects of group RHP, group size and the individual attributes of group members, indicating that information on these are available to fighting ants.

Animal interactions often involve chemical exchange but simultaneous evaluation of chemistry and behaviour has been problematical. Here we report findings from a novel method, atmospheric pressure chemical ionization-mass spectrometry (APCI-MS) coupled with manipulation of molecular-mass achieved by rearing organisms on deuterium-enhanced nutrients. This allows real-time monitoring of the occurrence and quantity of volatile chemicals released by each of two interacting individuals, in tandem with behavioural observations. We apply these methods to female–female contests in the parasitoid waspGoniozus legneri. We show that this species emits the spiroacetal 2-methyl-1,7-dioxaspiro[5.5]undecane. Chemical release is most common in more behaviourally aggressive contests, which occur when prior resource owners successfully resist take-over by similar-sized intruder females. Volatiles released during contests are always emitted by the loser. Aggression in contests is reduced after spiroacetal release. We suggest that the spiroacetal functions as a weapon of rearguard action. We anticipate that APCI-MS, which is rapid, non-intrusive and relatively inexpensive to operate, will be widely applied in studies linking chemistry and behaviour.