ENHANCED COLD TOLERANCE OF THE ENTOMOPATHOGENIC NEMATODE STEINERNEMA-FELTIAE THROUGH GENETIC SELECTION

Cold sensitivity of entomopathogenic nematodes severely restricts thei r biological control potential in some environments. We selected the S N strain of Steinernema feltiae together with its bacterial symbiont, Xenorhabdus bovenii, for improved cold tolerance by repeated passage t hrough the wax moth Galleria mellonella larvae at 15 degrees C. Nemato de virulence (total insect mortality and speed of kill) and establishm ent (initiation of nematode development following penetration) were ev aluated after six (=12-24 generations) and 12 passages (=24-36 generat ions). Cold selection enhanced nematode virulence at the cooler temper atures. Virulence measured as total insect-mortality at 8 degrees C im proved by 5.3- and 6.6-fold after six and 12 passages, respectively. O nly small improvements (1.2-1.5-fold) were observed in speed of kill. Nematode establishment improved at all temperatures after 12 passages; the highest increase of 9-fold was observed at 8 degrees C. Our resul ts lend support to the hypotheses that functional traits along a conti nuously distributed environmental variable are genetically correlated and that the area under the fitness function is not always constant. T rade-offs in percentage mortality and speed of kill by the selected ne matodes were observed at the warmer extreme after six passages of sele ction only. The implications of rapid changes in thermal sensitivity f or economic mass-production of nematodes are discussed.