Entomopathogenic nematodes as natural enemies for control of Rhizoglyphus robini (Acari: Acaridae)?

Bulb mites of the genus Rhizoglyphus are cosmopolitan pests of onion, garlic and ornamental plants. Despite the growing awareness against the use of pesticides, growers continue to use insecticides as the main control method. Thus, the search for environmentally safe alternative control methods, such as effective biocontrol agents, is of paramount importance. In the present study, the bulb mites were exposed to the infected juveniles of 20 strains of Steinernema and Heterorhabditis species applied at a single dose of 300 IJs per mite, and the invasion rate and mite mortality were assessed. Furthermore, the effect of the culture supernatants of the selected symbiotic bacteria of the genus Xenorhabdus on bulb mite mortality was tested. Our data show that both Steinernema and Heterorhabditis nematodes are able to invade and kill bulb mites, but in general, the invasion and resulting mortality were relatively low. The highest invasion rate of ca 30 IJs per mite was observed in Heterorhabditis taysearae. The mortality reached the maximum of 30% in the most efficient species, namely Steinernema huense, H. bacteriophora and H. amazonensis. The effect of the culture supernatants was generally low, though there were considerable differences in the efficacy among Xenorhabdus species and strains. To conclude we demonstrate that some EPN species are able to invade and kill bulb mites but their effect is in general quite low. Based on present data, EPNs and the metabolites of their symbiotic bacteria do not seem to represent a viable option for bulb mite biocontrol as a standalone approach.