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The parasitic Hymenoptera in a beech forest on limestone. 3, the role of leaf litter for the parasitoid species

Ulrich W.

Polish Journal of Ecology

1998

Vol. 46, no 3

311-326

In a mixed beech forest growing on limestone, the effect of an enhanced and a reduced layer of leaf litter on the emergence of parasitic Hymenoptera (as well as their hosts) was studied. A long term increase (4 years with an initially 5-fold value) of the leaf layer and a short term experiment (1 year, factor 5) resulted in arise of the total density of soil living parasitoids. The abundance of parasitoids which attack hosts in the herb layer or canopy declined. However, in both groups diversity and evenness was lowered. A reduction of the layer of leaf litter resulted in lower densities of the parasitoids of soil living hosts, but higher abundances of the parasitoids of hosts in other strata. The lacking leaf layer caused a reduction in the number of species and lowered both diversity and evenness. The lacking leaf layer also caused changes in the species composition. Experiments with dead snails to attract necrophagous insects and their parasitoids covered by different amounts of leaf litter revealed a species specific reaction to the leaf cover. Total parasitism rates were not reduced under a thick layer of leaf litter. No protective function of the leaf cover could be detected.

The parasitic Hymenoptera in a beech forest on limestone. 2, study of the sex ratios and their dependence on ecological factors

Ulrich W.

Polish Journal of Ecology

1998

Vol. 46, no 3

291-310

The sex ratio of the parastic Hymenoptera of a beech forest on limestone near Gottingen (FRG) was studied using ground-photo-eclectors. Judged from the mean sex ratios the parasitoids could be separated into 4 groups. Parasitoids of miners and gall-makers had the highest sex ratios (30 to 40% males) and less than 5% of the species were thelytokous. The species which attack soil-living sapro- or mycetophagous Diptera as well as the parasitoids of ectophytophages had (on average) 20 to 30% males and only 1.5% of them were thelytokous. Parasitoids of sap-suckers and egg-parasitoids usually had sex ratios below 20% males and 29% of them were thelytokous. The lowes sex ratios (below 10% males) and the highest number of thelytokous species (30%) were found among hyperparasitoids and parasitoids of soil-living Staphylinidae. No clear trend was detectable between the sex ratio and the density of the species. The density fluctuations and the degree of aggregation of the species influenced the sex ratio. The data are consistent with the hypothesis that K-selected parasitoid species have higher sex ratios on average. Koinobiontic species (mostly parasitoids of gall-makers and miners) had (on average) significantly higher sex ratios if their host lives in the herb or crown layer (32% versus 21%). They also produced more males if they are univoltine (32% versus 22%) and hibernate as larvae or pupae (30% versus 17%). After a MANOVA-analysis the voltinism turned out to be the main infuencing factor. There was a trend towards higher percentages of males in the summer generation.