The sulphur status of a plant is a strong influencing factor for GSH concentration in its tissue (Schnug et al. 1995). Consequently higher GSH levels in plants devoured by parasites increase the pests vitality as observed in Epi-lachna species fed on soybean leaves with different S02 fumigations. From these results it has been concluded that air pollution improves the success of pests on crop plants (Hughes and Voland 1988). High amounts of S containing compounds devoured may also improve the vitality of the parasite. According to Capua et al. (1991) mites foraging on Allium sativum, which is well known for its high content of reduced S compounds (Haneklaus et al. 1997), had a higher GST activity. As a consequence of this also the rate of epoxidation (a detoxifying reaction involving GSH) of aldrin was also increased in the parasites.
Hughes et al. (1987) found also a better performance of Trichoplusia spec. on Brassica grown on sludges. As sludges are usually rich in sulphur, the sludge grown crops had higher GSH contents. This strengthens the hypothesis that pests benefit from higher GSH contents in their forage.
Host plants induce GST activity in the parasite (due to isothiocyanates and other allelochemicals) (Yu 1992). Yu (1982) showed that plants with typically higher sulphur concentrations increase GST activity in Spodoptera spec. Consequently, highest in GST was Brassicajuncea (Brassica species are among the plants with the highest sulphur concentrations) and lowest was Cynodon dactylum. Yu (1996) concluded from this that GST plays an important role in allelochemical resistance in phytophagous Lepidoptera. The same conclusions were drawn by Wadleigh and Yu (1987) who reported an important role of GST in isothiocyanate detoxification in phytophagous insects and proposed the involvement of GST in the food-plant adaptation of phytophagous insects.
Some authors could not find diet specific differences (e.g. Egaas et al. 1992), but this does not prove the null hypothesis, because the diet compared may be inadequate, for instance, when only diets of supposed same sulphur content are compared like in Egaas et al. (1992).
Finally it should be mentioned that obviously also varieties with higher resistance against an insect, induce higher GST activity in their predators, compared to a non-resistant variety (Lesyczynski et al. 1994).
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