H. W. Nell

ADAPTIVE SUPERPARASITISM AND PATCH TIME ALLOCATION IN SOLITARY PARASITOIDS: THE INFLUENCE OF THE NUMBER OF PARASITOIDS DEPLETING A PATCH

An ESS model that predicts more superparasitism and longer patch times with an increasing number of searching parasitoids in a patch, was tested in experiments with Leptopilina heterotoma, a solitary larval parasitoid of Drosophila. The observed egg distributions and patch times were in quantitative agreement with the predictions of the model; oviposition and patch time decisions are clearly influenced by the number of conspecifics in the patch. Both in the model and in the experiment patch quality was kept constant (the number of hosts and the patch area per parasitoid were kept constant). The model predicted and the experiments showed that parasitoids gain less offspring per unit of time when searching a patch together: superparasitism leads to mutual interference. No self-superparasitism should have occurred when parasitoids searched alone. This prediction was only met with females that had been kept in isolation in the days before the experiment; when stored in groups of four, self-superparasitism did occur. This indicates an ability of the parasitoids to assess the probability of future superparasitism by conspecifics.

The function of host discrimination and superparasitization in parasitoids

SummaryHost discrimination, i.e. the ability to distinguish unparasitized hosts from parasitized ones, and to reject the latter for egg laying is present in many parasitic wasp species. This property is classically considered as an example of contest competition, and is supposed to have a number of functions. However, different species do not react to each others marks and lay eggs in hosts parasitized by the other species. Apparently the marks used for recognition are specific.Multiparasitization is the best strategy when hosts are scarce and the egg supplies of the parasitoids are not limited. Interspecific host discrimination is not an ESS.Superparasitization within one species would have selective advantage if the number of unparasitized hosts is small and the wasp has a reasonable chance to lay her egg in a host that is not parasitized by herself, and if the chance for her offspring to survive the competitive battle with the first parasitoid larva is not too small. This is shown to be the case.However, marks are not individual and wasps cannot distinguish hosts parasitized by themselves from those parasitized by others. The hypothesis is tested that the egg laying strategy (i.e. the decision to superparasitize) of wasps is dependent on the number of conspecifics that is searching simultaneously for hosts, since this determines the chance that a parasitized host encountered by a wasp is parasitized by herself.It is shown that host discrimination cannot be regarded as a case of contest competition. Other aspects of superparasitization, related to interference and population regulation, sex allocation and encapsulation are briefly discussed.

Superparasitism and Host Discrimination By Asobara Tabida Nees )Braconidae: Alysiinae), a Larval Parasitoid of Drosophilidae

This paper presents the results of a study of superparasitism and host discrimination by Asobara tabida Nees. We found that: (1) A. tabida females are able to distinguish unparasitized hosts from those previously parasitized by themselves or by a conspecific; (2) There is no evidence that A. tabida females are able to distinguish hosts in which they laid an egg themselves from hosts parasitized by conspecifics; (3) A. tabida females, unlike those of Leptopilina heterotoma cannot discriminate between hosts with different numbers of eggs; (4) Superparasitism may occur because: (a) inexperienced females of A. tabida may initially lay two eggs during one oviposition. (b) a female A. tabida may re-attack a host after oviposition within the period needed for building up the factor which causes avoidance of superparasitism. (c) the restraint to oviposit in parasitized hosts breaks down when a female A. tabida only meets parasitized hosts and does not lay eggs during a period of at least 8 hours. (d) females that have never oviposited in unparasitized hosts do not refrain from oviposition in parasitized hosts. We discuss whether superparasitism by insect parasitoids can be adaptive under particular circumstances.

Adaptive superparasitism in solitary parasitoids: marking of parasitized hosts in relation to the pay-off from superparasitism

Abstract. 1 The pay‐off from an egg laid in a parasitized host is an important parameter in models on adaptive superparasitism in solitary insect parasitoids. 2 For Leptopilina heterotoma, a parasitoid of larval Drosophila, the pay‐off from a second egg laid in a host is 0.43 offspring when the interval between the two ovipositions is less than 3h. For longer intervals, this pay‐off decreases to almost zero for an interval of 24 h. 3 When a female encountering a parasitized host is able to estimate the interval since the first oviposition, it is expected that she will take this into account in her host selection decisions. This is, however, not in the direct interest of the female that lays the first egg, and marks the host. 4 We studied whether superparasitism in hosts containing a young egg is more common than in hosts containing an older egg, when searching in a patch containing once‐parasitized and unparasitized hosts. 5 The acceptance/encounter ratio of parasitized hosts increased for intervals longer than 6h, as predicted when the interests of the marking female and the longevity of the mark are taken into account. 6 Superparasitism occurred more often when parasitoids had previously searched a host patch 7 days before the experiment compared to when parasitoids had searched a patch 1 day before, a phenomenon predicted by dynamic optimal diet models.

Adaptive superparasitism and patch time allocation in solitary parasitoids:searching in groups vs.sequential patch visits.

A deterministic ESS model was made to predict optimal degrees of superparasitism, when solitary parasitoids exploit a patch simultaneously or sequentially. Some predictions of this model are: (1) increasing levels of superparasitism and (2) longer patch times (measured as the sum of search times of all females visiting a patch) with increasing numbers of females visiting the patch. (3) More superparasitism and (4) longer patch times were predicted when females visited a patch simultaneously than when the same number of females visited a patch sequentially

Positive and negative effects of herbivory on the population dynamics of Senecio jacobaea L. and Cynoglossum officinale L.

SummaryHerbivore effects were studied on populations of the biennial plant species Senecio jacobaea and Cynoglossum officinale. During a three year period (1985–1988) population characteristics (herbivory, number of seedlings, rosettes and flowering plants) were compared in-and outside exclosures, as well as parameters reflecting vegetation cover. In S. jacobaea, a strong negative effect of Tyria jacobaeae was found on seedling establishment, rosette growth and flowering. On the other hand, vertebrate herbivores (mainly rabbits) had an indirect positive effect by limiting the development of the surrounding vegetation (esp. grasses). The increasing vegetation cover in protected populations caused a reduction in germination, seedling- and rosette-growth. Herbivory on C. officinale was low (<10%), no direct effects of herbivores on plant populations were shown. Indirect effects of herbivory through an increasing vegetation were even more pronounced as in S. jacobaea. Therefore, although both plant species may first benefit from herbivore-exclusion, their populations are dependent on rabbits eating other plants (esp. grasses) and reducing competition.