The effect of stress on the susceptibility of Plutella xylostella to Bacillus thuringiensis

The following stress factors were tested for their effect on the susceptibility of Plutella xylostella (L.) to Bacillus thuringiensis Berliner: (a) exposure to low doses of the synthetic pyrethroid cypermethrin, (b) treatment with low doses of the secondary plant compound, azadirachtin and (c) development on partially resistant cabbage cultivars. B. thuringiensis-related mortality of cypermethrinand azadirachtin-stressed larvae did not differ significantly from that of non-stressed larvae. However, larvae reared on different cabbage cultivars differed in their response to B. thuringiensis. LC50 values of larvae reared on some resistant cultivars were significantly lower than LC50 values of larvae reared on susceptible cultivars. However, not all resistant cultivars caused this effect. Although stress treatment with resistant cultivars affected consumption of B. thuringiensis-treated leaf discs, these differences in feeding rate did not explain the observed differences in mortality.