Spatial distribution and coincidence of Neoseiulus californicus and Tetranychus urticae (Acari : Phytoseiidae, Tetranychidae) on strawberry

In strawberry greenhouses in La Plata (Buenos Aires, Argentina), Tetranychu s urticae is a major pest and Neoseiulus californicus is its most important established phytoseiid predator. The purpose of this study was to determin e the spatial distribution and coincidence of N. californicus and T. urtica e on strawberry. T. urticae populations exhibited density variations that w ere followed, with some temporal delay, by those of the predator. In genera l terms, N. californicus exhibited lower aggregation than T. urticae. The i ndex of dispersion (I) of T. urticae had values significantly greater than 1 in 100% of the sampling dates. The percentage of infested leaflets increa sed with prey density in a curvilinear way, ranging from 80 to 100% when de nsity was higher than 100 individuals/leaflet. N. californicus had values o f I higher than unity in 86.7% (G1) and 53.8% (G2) of the cases. Taylor's b and Iwao's beta values were greater than 1 for both populations, this indi cating aggregation. Iwaos results showed larger aggregation units of T. urt icae (alpha=53.24) than of N. californicus (alpha=3.61), probably due to it s higher fecundity and oviposition behaviour of laying eggs in clumps. Both populations were overdispersed, although in general terms, N. californicus exhibited lower aggregation than T. urticae. This would create refuges for the prey, thus increasing the persistence of the system. Index of coincide nce of predator (Ic) was high most of the time even at low densities of bot h populations, decreased abruptly at the end of an interaction cycle and wa s similar at greenhouse and plant spatial scales. The high spatial coincide nce of N. californicus with T. urticae suggests an important dispersal capa city of the predator and a high ability to detect leaflets with prey. The r efuge index for the prey decreased with predator density and was higher at greenhouse scale than at plant scale, both at similar predator densities. A t greenhouse scale and despite high predator density, the prey had a 10%-re fuge, which would lead to the system persistence. Results of the present st udy suggest that N. californicus is a promising established natural enemy f or controlling T. urticae on strawberry.