Life history changes in the benthic cladoceran Chydorus piger induced by low concentrations of sediment-bound cadmium.

The effect of sediment-bound cadmium on several life history parameters of the benthic cladoceran Chydorus piger, was tested in the laboratory. It was investigated whether a test with C. piger is an ecologically realistic alternative for the Daphnia test applied to sediments. Therefore, a culture of C. piger was exposed to a control and five cadmium concentrations, equilibrated with natural detritus (0.036, 0.063, 0.26, 0.55 and 1.0 mmol/kg). Continuous records of growth and reproduction were made which took as long as 20 weeks. Longevity of C. piger declined markedly from 112 days to 20 days as cadmium concentrations increased. Intrinsic rate of increase and age at first reproduction (AFR) tended to be stimulated at very low doses but this effect was not significant. However, they were negatively influenced by moderate cadmium levels. While a significant cadmium effect was found for all traits investigated, the effective doses of cadmium varied strongly. C. piger was found to be even more sensitive to cadmium than for example Daphnia and Hyalella, naturally occurring cadmium levels in detritus being effective to C. piger. Experiments with a benthic cladoceran can therefore give important information about the ecological effect of cadmium and possibly other toxicants deposited in sediments.

Sublethal effects of esfenvalerate residues on pyrethroid resistant Typhlodromus pyri (Acari: Phytoseiidae) and its prey Panonychus ulmi and Tetranychus urticae (Acari: Tetranychidae).

The effect of residues of esfenvalerate on oviposition of the resistant strain of the predatory mite Typhlodromus pyri and its main prey, European red mite Panonychus ulmi and two-spotted spider mite Tetranychus urticae, were investigated. T. pyri showed a significant linear reduction in oviposition after 24 h in the presence of increasing levels of esfenvalerate residue applied at the field rate. Furthermore, when given a choice, T. pyri preferred to lay eggs on residue-free surfaces. Of the two prey species, only P. ulmi showed significant avoidance of increasing levels of residues of the field rate concentration of esfenvalerate, as measured by runoff mortality, however both P. ulmi and T urticae, when given a choice, showed a preference for esfenvalerate-free surfaces. As with the predatory mite T. pyri, both prey species showed a significant linear reduction of oviposition with increasing esfenvalerate residues and a preference to lay eggs on esfenvalerate-free surfaces. Esfenvalerate residues as high as 15X field rate were not repellent to pyrethroid-resistant T. pyri. The possible effects of these sublethal effects on predator-prey dynamics and implications for integrated mite control programmes in apple orchards are discussed.

Comparison of cultivars of ornamental crop Gerbera jamesonii on production of spider mite-induced volatiles, and their attractiveness to the predator Phytoseiulus persimilis.

We investigated whether volatiles produced by spider mite-damaged plants of four gerbera cultivars differ in attractiveness to Phytoseiulus persimilis, a specialist predator of spider mites, and how the mite-induced odor blends differ in chemical composition. The gerbera cultivars differed in resistance, as expressed in terms of spider mite intrinsic rate of population increase (rm). In order of increasing resistance these were Sirtaki, Rondena, Fame, and Bianca. To correct for differences in damage inflicted on the cultivars, we developed a method to compare the attractiveness of the blends, based on the assumption that a larger amount of spider mite damage leads to higher attraction of P persimilis. Spider mite-induced volatiles of cultivars Rondena and Bianca were preferred over those of cultivar Sirtaki. Spider mite-induced volatiles of cultivars Sirtaki and Fame did not differ in attractiveness to P. persimilis. Sirtaki plants had a lower relative production of terpenes than the other three cultivars. This was attributed to a low production of cis-alpha-bergamotene, trans-alpha-bergamotene, trans-beta-bergamotene, and (E)-beta-farnesene. The emission of (E)-beta-ocimene and linalool was lower in Sirtaki and Fame leaves than in Bianca and Rondena. The importance of these chemical differences in the differential attraction of predatory mites is discussed.