The influence of relative humidity on egg hatch in Euseius finlandicus, Typhlodromus pyri and Kampimodromus aberrans (Acari, Phytoseiidae)

The effects of different relative humidities (50%, 55%, 65%, 75%, 85% and 1 00% RH) on egg hatch were tested at 25 +/- 1 degrees C for three strains of Euseius finlandicus Oudemans, two strains of Typhlodromus pyri Scheuten an d two strains of Kampimodromus aberrans Oudemans. At decreasing humidity, e gg mortality increased earlier in K. aberrans than in E, finlandicus and T. pyri. E. finlandicus and T. pyri had similar humidity responses. Differenc es between strains within each species were small. RH50 values (relative hu midity at which 50% of eggs hatch) were 50.4%, 52.4% and 53.1% for strains of E. finlandicus, 51.8% and 53.3% for strains of T. pyri, and 55.3% and 57 .5% for strains of K. aberrans, respectively. Eggs of the Finnish E. finlan dicus strain were most drought resistant (50.4% RH50), eggs of the French K . aberrans strain were most sensitive (57.5% RH,,). The results are discuss ed in relation to the abundance of these species on certain plants and to t he humidity responses of other phytoseiids.