The position of the emerging point has rarely been investigated as a factor possibly affecting the future nest settlement behaviour in Hymenoptera, in particular within nest aggregations. We studied the emergence and dispersion patterns of the digger wasp Stizus continuus. Individuals emerged daily in clumped patterns, possibly revealing a certain synchrony of emergence from the same nests, and protandry appeared both at seasonal and daily level. Differences between the number of females that nested relatively close or far from their emergence holes (EH) were either significant or not, depending on the year, and observed dispersal distances from the natal nests did not differ from those obtained by random simulations. By contrast, females nested close to the nearest conspecific nest. Size did not affect the dispersion patterns. EH are thus not important cues for nest establishment, and conspecific nests are probably the key cue for nest-founding females. In addition, males did not prefer to establish territories close to their natal nest.
Studies which quantitatively analyse how aculeate parasitoids exploit their window of opportunity to find and attack a host are scarce, despite the recognized importance of parasitic pressure as a driving force that promotes aggregate nesting in their hosts. We have studied the activity and behaviour of the velvet ant Nemka viduata, an ectoparasitoid of immature stages of the digger wasp Stizus continuus. Due to the resource exploited by the parasitoid (mature larvae and prepupae), and in general agreement with basic optimal foraging theory, we expected a major activity at late stages of the host seasonal provisioning period, an independence from the host daily provisioning patterns and a spatial positive association with host nest density. In accordance with these predictions, during the season, the parasitoid resulted was more active at the end of the host provisioning period, and across the day, it showed an inverse quadratic pattern of activity, in contrast to the positive one shown by the host. Thus, at both temporal scales, N. viduata activity was highly asynchronous with that of the host. At a spatial scale, however, the activity of the velvet ants was correlated with host nest density, although there is weak evidence suggesting that areas of high host density suffered from a higher rate of parasitism. Multivariate analyses confirmed a number of relevant factors associated with velvet ants' activity, including nest density (positive), air temperature and the hour of the day (both negative). In addition, the activity of both male S. continuus and male N. viduata entered in the models in association with female parasitoid's activity, probably because of their mating strategy.
The behavioural ecology of ichneumonid wasps that attack aculeate Hymenoptera is still largely unknown. Field observations and morphological analyses were devoted to investigate host detection and rate of parasitism by Acroricnus seductor (Scopoli), a natural enemy of the black and yellow mud dauber wasp Sceliphron caementarium (Drury). At the study site, about half of the host nests suffered parasitism by A. seductor. No significant difference was found between the rate of parasitism in sheltered (inside human building) and unsheltered (outside building) nests. Larger nests did not suffer a higher rate of parasitism, and larger brood cells were not more likely to be parasitized. As revealed by contents of parasitized cells, A. seductor appeared to act as a kleptoparasitoid, devouring spider prey and young host larvae. Analysis of video recordings obtained in the field revealed the basic behavioural sequence of host detection. Acroricnus seductor female taps with the antennae the host nest surface and, once a suitable host brood cell is found, inserts the ovipositor through the mud wall, possibly facilitated by the secretion of a mud-softening substance. Behavioural data, together with the presence of modified tips on the apex of female antennae and the relative thicknesses of female fore tibiae, strongly suggest that A. seductor uses echolocation to detect the host.
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