AbstractThe effects of ‘Margosan‐O’ (MO) on the pea aphid,Acyrthosiphon pisum(Harris), were determined. MO significantly reduced population increase ofA. pisumin a concentration‐dependent manner. At a concentration equivalent to 100 mg litre−1of azadirachtin, population increase wasc. 3.5 times lower than the control. In more detailed studies, MO significantly reduced the number of molts, longevity, and fecundity ofA. pisumthat had been reared on treated broad bean.ViciafabaL., plants. MO also reduced the longevity and fecundity of young adultA. pisumexposed to MO‐treated broad bean. MO was slow‐acting againstA. pisum. Mortality caused by MO stabilised seven days after newbornA. pisumwere exposed to treated broad bean and 10 days for adults. The seven day LC50for individuals exposed from birth was 27.50 mg azadirachtin liter−1while the 10 day LC50for adults was 53.32 mg liter−1. Contrary to previous studies suggesting that neem insecticides are not contact toxicants, we found that MO applied topically to adultA. pisumcaused effects similar to those found in individuals that fed upon treated plants. However, MO was slower‐acting when applied topically. Mortality in adultA. pisumcaused by topically applied MO stabilised 17 days after treatment with a resultant LD50of 2.91
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