The codling moth (Cydia pomonella L.) is a serious pest of pome fruit. Diapausing cocooned larvae overwinter in cryptic habitats in the soil or in the bark of infested trees. The entomopathogenic nematode Steinernema feltiae (Filipjev) (Rhabditida: Steinernematidae) is used to control diapausing codling moth larvae. The objective of this study was to define environmental conditions favouring the performance of the nematodes. Cocooned larvae were more susceptible than non-cocooned larvae. Susceptibility of pupae was low. To determine the influence of decreasing water activity (a w -value) on the activity of the nematodes, mortality of codling moth larvae and Galleria mellonella L. were tested in sand-sodiumpolyacrylate mixtures of variable water activity. S. feltiae was able to infect both insects at a w -values [0.9. Cocooned larvae of both insects died at lower a w -values than non-cocooned larvae. Mortality of cocooned larvae did not further increase after half an hour of exposure to nematodes, whereas the mortality of non-cocooned larvae increased with increasing exposure time. LC 50 and LC 90 considerably decrease with increasing RH. The negative influence of the relative humidity (macro environment) was less important than the effect of the water activity in the bark substrate (micro environment). The micro environment can be manipulated by applying S. feltiae with higher volumes of water. A surfactant-polymerformulation significantly increased nematode efficacy and can buffer detrimental environmental effects.