TOXICITY AND PROTECTANT POTENTIAL OF THE ESSENTIAL OIL OF TETRADENIA-RIPARIA (LAMIALES, LAMIACEAE) AGAINST ZABROTES-SUBFASCIATUS (COL, BRUCHIDAE) INFESTING DRIED PINTO BEANS (FABALES, LEGUMINOSAE)

The essential oil of Tetradenia riparia had limited acute toxicity to adult Zabrotes subfasciatus, with females, which are larger, being less susceptible than males. However, freshly-applied essential oil at a predicted concentration of 250 mug/cm2 of pinto bean surface completely suppressed the production of F1 adult progeny, with an EC50 of 72 mug/cm2. Eggs were sensitive to this preparation with an EC50 of 50 mug/cm2 of bean surface for F1 adult progeny per parental female. This concentration-dependent effect upon the eggs was greatly enhanced by physical damage to the eggs during the treatment process, resulting in a population size 21% of that achieved in controls. Larvae within the beans were well protected from the oil treatment with an EC50 of approximately 3980 mug/cm2 for F1 adult progeny per parental female. However, the EC50 for numbers of subsequent adult F2 progeny for these emergent F1 adults was 121 mug/cm2 of pinto bean surface. This value was similar to that for the freshly-applied oil against adults, but the data followed a distinctly different distribution that was characterized by limited adult emergence during days 55-85. Emergence occurred even at extremely high concentrations, whereas no adults could emerge (because either no eggs were laid or none survived) for these concentrations when the treatment was directed at either the parental adults or eggs. The activity of the oil increased at 210 days post-application with an EC50 of 23 mug/cm2 of pinto bean surface for hatched eggs per parental female at 25 days post-inoculation. The essential oil is quite repellant to ovipositing females after this period of incubation with an EC50 of only 10 mug/cm2 of pinto bean surface. Volatiles emitted after this duration of incubation had a weak but significant effect on fecundity of females selecting untreated beans in a choice test. The essential oil had no effect on bean germination at 180 days. The logistic dose-response transition equation, y = a + {b/[1 + (x/c)d]}, may better fit resulting population size estimates for experiments involving a 'dose-response' than probit analysis, because midpoints can be determined for transitions in which partial suppression data are limited. We conclude that the essential oil of T. riparia may make an effective seed treatment against Z. subfasciatus at 1.7 l/to; but to be used as a treatment for beans for consumption, additional research must evaluate mammalian toxicity and palatability of these treated beans.