The influence of humidity on the effect of Steinernema feltiae against diapausing codling moth larvae (Cydia pomonella L.) (Lepidoptera: Tortricidae)

Codling moth (CM) is a serious and global pest of pome fruit. It overwinters in cryptic habitats as cocooned diapausing larvae. Field trials with the entomopathogenic nematode Steinernema feltiae (Rhabditida: Steinernematidae) report control of diapausing CM of up to 70%, but results are variable. 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. S. feltiae was unable to infect CM at a water activity (aw-values) < or = 0.9. Mortality of cocooned larvae was reported at lower aw-values than of non-cocooned larvae. Exposure time and impact of external relative humidity (RH) was studied. Mortality of cocooned larvae did not further increase after half an hour of exposure, whereas the mortality increased with increasing exposure time in non-cocooned larvae. LC50 and LC90 considerably decreased with increasing RH. The influence of the relative humidity was less pronounced when surpassing 80% than the effect of the volume of applied water. When S. feltiae was formulated in a surfactant-polymer-formulation (SPF), mortality significantly increased when compared to application in water.