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The life cycle of the Western Flower Thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), one of the most important glasshouse pests, includes a soil passage composed of three instars that deserve more attention in terms of biocontrol strategies. It has been repeatedly reported that two polyphagous predatory mites, Stratiolaelaps miles (Berlese) and Hypoaspis (Geolaelaps) aculeifer (Canestrini) (Acari: Laelapidae), also prey on these thrips stages, in addition to several other soil inhabiting prey species. However, the potential thrips consumption rates have never been quantified for these predatory mites. Therefore, an arena experiment was carried out to investigate the potential predation rates of the two mites on second instar larvae, prepupae, and pupae of F. occidentalis. In addition, the fecundity on the thrips diet was assessed and compared to oviposition rate on a nematode prey. All thrips instars were accepted as prey by each mite species. Females of H. aculeifer preyed on 3.5 (± 0.5) thrips instars and laid 2.5 (± 0.87) eggs per day, whereas females of S. miles preyed on 1.64 (± 0.3) thrips and laid 0.8 (± 0.53) eggs. Males of both species killed 0.6 (± 0.3) thrips per day. The fitness of the two predatory mites on F. occidentalis as prey and their suitability as biocontrol agents are elucidated. Reasons for reduced thrips control in the soil environment, in contrast to the results obtained in arena assays are discussed.

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In biological control programmes, the two predatory soil mites Hypoaspis aculeifer and H. miles are often applied against soil-borne pests like mushroom flies, springtails and mites. Although the mites show high consumption rates on varying prey types in Petri dish experiments and in greenhouses, their overall efficiency is sometimes limited. We hypothesized that intraspecific interactions, like cannibalism, could contribute to this decreased competence. Therefore, experiments were conducted to show the propensity of H. aculeifer and H. miles to cannibalise. Adult mites and nymphs were introduced as predators with conspecific eggs, larvae, nymphs, adult females or males as prey and the number of killed individuals was recorded. Additionally, the oviposition rate on conspecific prey was quantified and the correlation with the number of prey consumed was calculated to assess the influence of cannibalism on egg production. The results illustrate that cannibalism occurs infrequently in both Hypoaspis spp., the only exception being H. aculeifer nymphs, which cannibalised one conspecific egg per day. Moreover, cannibalism never occurred in the presence of alternative prey. Oviposition rate decreased during the experiment in both species but it was positively correlated with the cannibalism rate only for H. aculeifer. The benefit of cannibalism for populations of H. aculeifer and H. miles is discussed.

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