Inter- and intraspecific interactions in Melittobia digitata and M. femorata (Hymenoptera: Eulophidae)
Deyrup, Leif D.
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This research examined aspects of the nature, roles, and interrelationships between the fundamental wasp behaviors, stinging, chewing/feeding, and aggression using Melittobia as a model. In the field, these tiny gregariously developing wasps attack solitary bees, wasps and associates; in the laboratory, they accept a wide range of alternative hosts. The genus includes 8 North American species; the work focused on M. digitata, a species widely used under the name WOWBug in science curricular activities. When individual mated and unmated female wasps were simultaneously offered two flesh fly (Sarcophaga bullata) pupae as potential hosts, virgin females laid eggs on only one of the paired pupae more often than mated females did, but the “unused” second pupa developed to adulthood significantly less often than did controls, suggesting that the female nonetheless had stung it. Because direct visualization of stinging and its effects were not possible with fly pupae due to their enclosing puparia, female Melittobia were placed with mealworm pupae (Tenebrio molitor; Coleoptera) for two days, then removed before they could lay eggs. Exposed mealworms responded to gin-trap-reflex stimulation significantly less than controls, and those that had melanized sting marks on them were even less likely to move; these findings suggested a paralytic component to the venom. Highly marked mealworms were significantly more likely to remain wholly or partially in the pupal stage, suggesting a developmental component to the venom. To confirm this, venom-milking techniques were developed. Milked venom from M. digitata was injected into mealworms and pupae of an economically important natural host, the leafcutter bee (Megachile rotundata). All responded to injected venom with paralysis and in some cases with developmental delay, indicating that these reactions are a generalized phenomenon and that venom components might be useful for physiological studies or have potential application in pest control. After mating, Melittobia chew out of their natal host cocoon. Observations showed that a female wasp stings a spot, then other females bite/chew cooperatively at the site. Chewing was experimentally elicited using milked venom and artificial pits. Using venom and dissected glands as stimulants, similar results were obtained in two other species, M. femorata and M. australica. This apparently cooperative behavior promises insights into possible evolutionary origins of components of eusocial behavior. The stinging-chewing sequence suggests an evolutionary derivation of the escape chewing behavior of dispersal-ready females from host-feeding. Experiments using mud dauber wasp (Trypoxylon politum) host hemolymph as the critical cue showed it was possible to switch Melittobia females back and forth between the two behaviors, stimulating chewing in a feeding context and feeding in a chewing context. Whereas chewing and feeding are commonly accepted for female Melittobia, the widely held assumption has been that males do not feed. Experiments showed that males can and do feed on the hemolymph of another male killed in combat. In addition, males fed hemolymph lived significantly longer than males that were not fed. Finally, male aggression is widely accepted for male Melittobia, yet M. femorata females suffer body damage when multiple foundresses are placed on a host. Through marking and observation it was determined that this species is territorial and violent. For comparison, tests were done on both M. digitata, and M. australica. Melittobia digitata did not share any territorial or aggressive tendencies, whereas M. australica was territorial but without mutilation and violence.