The development of resistance to Bacillus thuringiensis toxic proteins is a growing concern because it could threaten both conventional and gene transfer use of this environmentally safe biological insecticide. The most common mechanism of resistance involves changes in binding affinity of toxin receptors in the insect midgut membrane. This has not been the case in Heliothis virescens.W e have investigated changes in midgut proteolytic activity as a possibility to explain the resistance observed in this insect species.We have developed an improvement of known methods to demonstrate proteolytic activity i n crude extracts. Using this method we have found differences in the proteolytic activity profile of midgut extracts of a susceptible and a resistant H . virescens strain. We also have studied the in vitro processing of CrylA(b) toxin and protoxin b y midgut contents of both strains. SDS-PAGE of the in vitro degradation products showed differences between the strains. The resistant strain degrades protoxin more slowly and processes the active toxin more quickly than the susceptible strain. o 1996 WiIry-1 i s , Inr.

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The western flower thrips Frankliniella occidentalis (Pergande) is a very significant pest of a number of different agricultural crops in the south-east of Spain. The importance of thrips as a pest is not due mainly to the direct damage inflicted on the plant, but to the loss in commercial value which occurs as a consequence of the development of dark spots caused by the tomato spotted wilt virus (TSWV) which they transmit. The economic threshold is therefore almost zero, which enhances the problems of resistance management. The present work is part of a global project that attempts to evaluate the status of insecticide resistance in field populations of thrips obtained from several agricultural crops. We have studied, in either individual or pooled insects, some enzyme systems classically related to detoxification of insecticides: esterase and glutathione-S-transferase (GST). The activity of these enzymes from laboratory populations selected with various classes of insecticides has also been measured using several appropriate substrates. An increase in GST mean activity was found in two field-collected strains. Differences in frequency distributions of esterase and GST activities were found for both field-collected strains and for a laboratory strain selected with acrinathrin. These activities were compared with those of a wild-type reference strain.

The western flower thrips, Frankliniella occidentalis (Pergande), is a serious pest in the south-east of Spain owing to its direct feeding on crops, transmission of the tomato spotted wilt virus and its very high level of resistance to insecticides. Mechanisms of resistance were examined using field populations of F. occidentalis with different susceptibilities to acrinathrin, methiocarb (selective insecticides), endosulfan, metamidophos and deltamethrin (broad-spectrum insecticides). Esterase activity towards alpha-naphthyl acetate and p-nitrophenyl acetate in resistant strains was significantly higher than in the reference strain (MLFOM) for both model substrates. This higher activity was significantly correlated with acrinathrin and methiocarb resistance.