Heat Units, Release Rate, Prey Density, and Plant Age Effects on Dispersal by Neoseiulus fallacis (Acari: Phytoseiidae) After Inoculation into Strawberry

From April to October, 100 adult female Neoseiulus fallacis (Garman) (plus immatures) were released at point locations in strawberry to thereafter disperse and control twospotted spider mite, Tetranychus urticae Koch. Between 166 and 250 degree-days (DD10, 3-8 wk), predators dispersed 2.5-13.3 m(2)/100 DD. At release of 10, 50, or 100 adult females per point but with the same total mites, the fewest mites released at the most points gave the most coverage. Multivariate models showed that the more dense the spider mites or older the planting, the slower the spread of mites, initially. but overall these 2 factors were less important than degree-days (or time for spread) or release rate in explaining spread of predators. Releases before 31 May or after 15 July gave less spread per degree-day than in midsummer; later releases gave coverage in the next season. A model of release options predicts that under conditions of prey density and plant age that require large releases, 10,000/ha (100 per site) or 7,000/ha (10 per site) would give coverage in 500 DD. Under small release conditions, only 200-500/ha (10 per site) released by 1 April would give coverage by 1 September (1,000+ DD). Options for intermediate conditions are discussed.