Soil microorganisms control plant ectoparasitic nematodes in natural coastal foredunes.

Belowground herbivores can exert important controls on the composition of natural plant communities. Until now, relatively few studies have investigated which factors may control the abundance of belowground herbivores. In Dutch coastal foredunes, the root-feeding nematode Tylenchorhynchus ventralis is capable of reducing the performance of the dominant grass Ammophila arenaria (Marram grass). However, field surveys show that populations of this nematode usually are controlled to nondamaging densities, but the control mechanism is unknown. In the present study, we first established that T. ventralis populations are top-down controlled by soil biota. Then, selective removal of soil fauna suggested that soil microorganisms play an important role in controlling T. ventralis. This result was confirmed by an experiment where selective inoculation of microarthropods, nematodes and microbes together with T. ventralis into sterilized dune soil resulted in nematode control when microbes were present. Adding nematodes had some effect, whereas microarthropods did not have a significant effect on T. ventralis. Our results have important implications for the appreciation of herbivore controls in natural soils. Soil food web models assume that herbivorous nematodes are controlled by predaceous invertebrates, whereas many biological control studies focus on managing nematode abundance by soil microorganisms. We propose that soil microorganisms play a more important role than do carnivorous soil invertebrates in the top-down control of herbivorous ectoparasitic nematodes in natural ecosystems. This is opposite to many studies on factors controlling root-feeding insects, which are supposed to be controlled by carnivorous invertebrates, parasitoids, or entomopathogenic nematodes. Our conclusion is that the ectoparasitic nematode T. ventralis is potentially able to limit productivity of the dune grass A. arenaria but that soil organisms, mostly microorganisms, usually prevent the development of growth-reducing population densities.

Toxicity of binary mixtures of cadmium-copper and carbendazim-copper to the nematode Caenorhabditis elegans.

For ecological risk assessment, the additive model may be used to empirically predict toxic mixture effects. Detailed toxicity tests were performed to determine whether effects of mixtures of copper-cadmium and copper-carbendazim on Caenorhabditis elegans were similar to the effects of the individual compounds. Effects on the course of reproduction, the length of the juvenile period, the length of the reproductive period, and body length were analyzed. Dose-response data were compared to the additive model and tested for four deviation patterns from additivity: No deviation, synergistic/antagonistic deviation, dose ratio-dependent deviation, dose level-dependent deviation. During the exposure, the cadmium-copper effect on reproduction changed from a synergistic, to a dose ratio-dependent deviation from additivity. More cadmium in the mixture decreased the toxicity and more copper increased the toxicity. The effect of copper-carbendazim on reproduction was synergistic at low dose levels and antagonistic at high dose levels and independent of time. Mixture effects on the juvenile and reproductive period were similar to single component effects. It was concluded that the observed time-dependence of toxic interactions was small and that interactions on the timing of reproduction were not found. The additive model underestimated mixture effects on reproduction and body length.