Quality and environmental factors affecting Trichogramma brassicae efficiency under field conditions

Field parasitism of the egg parasitoid Trichogramma brassicae Bezdenko (Hymenoptera, Trichogrammatidae) (synonymous to T. maidis Pint. et Voegele) on Ostrinia nubilalis Hübner was compared to four single quality parameters (walking speed, fecundity on the factitious host Ephestia keuhniella Zeller, fecundity on the natural host O. nubilalis , and life span) previously measured in the laboratory and, a quality index calculated from three of the measured parameters. A single quality parameter (fecundity on E. keuhniella ) and the calculated quality index showed a correlation to T. brassicae field parasitism, for different T. brassicae populations. The number of female Trichogramma released clearly influenced parasitism but not in the same proportion for all populations tested, reflecting that numbers released can only to some extent compensate for low quality in Trichogramma. Variation indegree hours above 18 °C also influenced field parasitism. When incorporating degree hours to the quality index a significant correlation R 2 = 0.56 (P = 0.01) is obtained. In this work fecundity of T. brassicae on E. keuhniella can be as good an indicator for the potential field performance of T. brassicae , as the quality index which takes into account three quality parameters. Since environmental factors can obscure the potential performance of a population, i.e., one which under optimal laboratory conditions performs well, the quality index gives only a partial indication of how the released strains will perform in the field.