LETHAL AND SUBLETHAL EFFECTS OF THE NEEM INSECTICIDE FORMULATION, MARGOSAN-O, ON THE PEA APHID

The effects of 'Margosan-O' (MO) on the pea aphid, Acyrthosiphon pisum (Harris), were determined. MO significantly reduced population increa se of A. pisum in a concentration-dependent manner. At a concentration equivalent to 100 mg litre-1 of azadirachtin, population increase was c. 3.5 times lower than the control. In more detailed studies, MO sig nificantly reduced the number of molts, longevity, and fecundity of A, pisum that had been reared on treated broad bean, Vicia faba L., plan ts. MO also reduced the longevity and fecundity of young adult A. pisu m exposed to MO-treated broad bean. MO was slow-acting against A. pisu m. Mortality caused by MO stabilised seven days after newborn A. pisum were exposed to treated broad bean and 10 days for adults. The seven day LC50 for individuals exposed from birth was 27.50 mg azadirachtin liter-1 while the 10 day LC50 for adults was 53.32 mg liter-1. Contrar y to previous studies suggesting that neem insecticides are not contac t toxicants, we found that MO applied topically to adult A. pisum caus ed effects similar to those found in individuals that fed upon treated plants. However, MO was slower-acting when applied topically. Mortali ty in adult A. pisum caused by topically applied MO stabilised 17 days after treatment with a resultant LD50 of 2.91 mug azadirachtin g-1.