No abstract

Host 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 other's 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.

a b s t r a c tWe calculated a Living Planet Index (LPI) for the Netherlands, based on 361 animal species from seven taxonomic groups occurring in terrestrial and freshwater habitats. Our assessment is basically similar to the global LPI, but the latter includes vertebrate species and trends in population abundance only. To achieve inferences on trends in biodiversity more generally, we added two insect groups (butterflies and dragonflies) and added occupancy trends for species for which we had no abundance trends available. According to the LPI, the state of biodiversity has slightly increased from 1990 to 2014. However, large differences exist between habitat types. We found a considerable increase in freshwater animal populations, probably because of improvement of chemical water quality and rehabilitation of marshland habitats. We found no trend in the LPI for woodland populations. In contrast, populations in farmland and open semi-natural habitats (coastal dunes, heathland and semi-natural grassland) declined, which we attribute to intensive agricultural practices and nitrogen deposition, respectively. The LPI shows that, even in a densely populated western European country, ongoing loss of animal biodiversity is not inevitable and may even be reversed if adequate measures are taken. Our approach enabled us to produce summary statistics beyond the level of species groups to monitor the state of biodiversity in a clear and consistent way.

The course of competition between the solitary endoparasitoids Asobara tabida Nees von Esenbeck, 1834 and Leptopilina heterotoma (Thomson, 1862) in multiparasitized hosts (Drosophila melanogaster Meigen, 1830 and D. subobscura Collin, 1936) was followed in time. The outcome of competition was determined, as well as the mechanisms used by the parasitoids to eliminate a competitor and the moment at which this elimination occurred. The effect of various intervals between oviposition periods, of various temperatures, host stages and host species was studied. The distinguish mortality due to competition from mortality due to other factors, the survival of the parasitoids in multiparasitized hosts was compared with that in singly parasitized hosts.

(1) The parasitic wasps Leptopilina heterotoma (Thomson) and Asobara tabida (Nees) usually refrain from laying eggs in hosts that have already been parasitized by themselves or by a conspecific (=intraspecific host discrimination). (2) In contrast to intraspecific host discrimination, interspecific host discrimination, in which females refrain from laying eggs in hosts that have already been parasitized by the other species, is not found in these sympatric parasitid species. (3) Because we expected that development of interspecific host discrimination would increase the fitness of the wasps, we wondered why this strategy had not evolved in L. heterotoma and A. tabida. (4) We therefore developed a simulation model of the parasitization process, in which interspecific host discrimination can be included. (5) By varying the time needed for host location, survival chances and the proportions of hosts parasitized, we obtained estimates for the number of offspring in situations with and without interspecific host discrimination. (6) The results imply that, assuming that female wasps carry an ample supply of eggs, the development of interspecific host discrimination by L. heterotoma or A. tabida will not lead to increased fitness, even under extreme circumstances. Hence, interspecific host discrimination will not evolve.