Intraspecific variation in humidity susceptibility of the predatory mite Neoseiulus californicus: Survival, development and reproduction

Biological control of spider mites on crops grown in arid environments is a serious problem, which is difficult to solve with the currently commercially available predatory mites (Acari: Phytoseiidae). In phytoseiids the degree of adaptation to low humidity underlies considerable intraspecific variability allowing the screening for a dry resistant strain within a given species. We considered the generalist predator Neoseiulus californicus an ideal species for such a screening due to its well-known plasticity in life history, and behavioural and ecological traits. Within the frame of a multi-institutional project on the identification and mass rearing of a dry adapted N. californicus strain, we evaluated life history parameters such as adult female survival, oviposition and offspring sex ratio, egg hatchability and development, and juvenile survival and development of eight strains at three to four humidity levels (64.0–75.6% at 25 °C). Strains came from Hérault/France (FR), Tuscany/Italy (FL), Valencia/Spain (SP), La Cruz/Chile (CH), Sicily/Italy (SI) and California/USA (BOKU, ISZA, ARO). The influence of low humidity on life history traits decreased during ontogeny, with the egg being the most sensitive to dry ambient conditions. The high variability of the N. californicus strains in their performance at the lowest humidity tested (64.0%) allowed to classify the strains into three distinct groups. (1) Egg hatch probabilities were nearly zero in the FL, SP and ISZA strains (0.07–0.01). (2) The second group with the FR, CH and ARO strains was characterized by low egg survival (0.14–0.18) but high juvenile survival probabilities (0.69–0.82). (3) Among the eight evaluated strains the BOKU and SI strains had the highest egg (0.37 and 0.38) and juvenile survival probabilities (0.96 and 0.83). We conclude that BOKU and to a somewhat lesser degree SI are the most promising N. californicus strains for arid environments and should henceforth be evaluated on different crops in semi-field and field trials.