Binding of an insecticidal transgene product to soil: Biological activity of soil-bound avidin and the effects of time and microbial activity

The insecticidal effects of avidin fed in artificial diet either unbound or bound to one of two New Zealand soils (a sandy loam and a clay soil), on a lepidopteran pest insect, Epiphyas postvittana were investigated. Immediately after binding of avidin to soil, 70-75% of the avidin retained the ability to bind biotin. Both soil-type and duration of incubation after binding of avidin to the soil markedly affected its insecticidal activity. After 6 months incubation, residual activity of avidin in the sandy loam sample had been almost completely lost, whereas some insecticidal activity was still present in the clay sample. Loss of activity of avidin did not correlate with the numbers of culturable bacteria in the two soil types nor did the presence of avidin affect the number or type of culturable bacteria present in soil. The concentration of avidin at which insecticidal effects were observed was conservatively estimated at approximately 100-fold higher than that expected in the field, based on known expression levels in insecticidal plants.