Beschreibung:
<jats:title>Summary</jats:title><jats:p>1 Laboratory studies demonstrated that the susceptibility of larvae of the lepidopteran crucifer pest <jats:italic>Plutella xylostella</jats:italic> to the insect pathogen <jats:italic>Bacillus thuringiensis</jats:italic> (<jats:italic>Bt</jats:italic>) was influenced by the host plant.</jats:p><jats:p>2 Larvae reared on the resistant cabbage cultivars Minicole F<jats:sub>1</jats:sub> and Red Drumhead were significantly more susceptible to <jats:italic>Bt</jats:italic> (the LC<jats:sub>50</jats:sub> fell to one half) than larvae fed leaves of susceptible cultivars.</jats:p><jats:p>3 However, a third resistant cultivar, Aquarius F<jats:sub>1</jats:sub>, had no synergistic effect on <jats:italic>Bt</jats:italic>‐related mortality.</jats:p><jats:p>4 Actual uptake of <jats:italic>Bt</jats:italic> was monitored in the bioassays, as a preliminary experiment showed that the plant resistance reduced consumption of <jats:italic>Bt</jats:italic>‐treated leaf discs. However, differences in feeding rate did not explain the observed differences in mortality.</jats:p>