Are low humidity levels a limiting factor for spider mite control by phytoseiid predators under fluctuating climatic conditions?

Low humidity levels are assumed to be an important limiting factor for biological control of spider mites by phytoseiid predatory mites, mainly because of the vulnerability of the egg stage for drought. In this study, we evaluated the efficacy of several species of phythoseiid predatory mites for control of the spider mite Tetranychus urticae Koch at low(55%) and high (80%) humidity levels on greenhouse cucumber in two greenhouse trials where predators were released either before or after spider mite introductions. In the preventive trial,the best control was achieved by Neoseiulus californicus McGregor at both humidity levels.Spider mites were completely eradicated. None of the other species of predatory mites was able to reduce spider mite densities, except Transeius montdorensis (Schicha) at the low humidity level. In the curative trial, N. californicus was again the best performing predatory mite,together with Phytoseiulus persimilis Athias-Henriot. Spider mites were completely controlled at both humidity levels by these species. This suggests that low humidity levels during daytime do not necessarily disrupt the control of spider mites by these predators, possibly because of the humidity fluctuations between day and night. Humidity did also directly affect spider mites;densities were up to 3 times higher on plants in the greenhouse with low humidity levels than on plants with a high humidity level. This increased growth rate of spider mites at low humidity levels may be another explanation for the failure of spider mite control in practice. Yet, in order to enhance the biological control of spider mites at low humidity levels, it might be better to focus on methods that support the establishment of effective spider mite predators, rather than selecting drought adapted strains of predatory mites.