Parasite-induced modification of host behavior increasing transmission to a next host is a common phenomenon. However, field-based studies are rare and the role of environmental factors in eliciting host behavioral modification is often not considered. We examined the effects of temperature, relative humidity (RH), time of day, date, and an irradiation-proxy on behavioral modification of the ant Formica polyctena (Förster, 1850) by the brain-encysting lancet liver fluke Dicrocoelium dendriticum (Rudolphi, 1819). This fluke induces ants to climb, and bite to vegetation by the mandibles in a state of temporary tetany. A total of 1,264 individual ants expressing the modified behavior were observed over 13 non-consecutive days during one year in the Bidstrup Forests, Denmark. A sub-set of those ants (N=172) was individually marked to track attachment and release of infected ants in relation to variation in temperature.