The neem tree (Azadirachta indica), whose seed and leaf extracts are known to kill more than 60 types of insects, has recently been the subject of several interesting studies. Researchers at Virginia Polytechnic Institute and State University effectively control the Colorado Potato Beetle (CPB) in eastern Virginia using extracts from seed kernels of the tree. Meanwhile, in Louisiana, USDA researchers have found that leaves from the India native may block fungi from producing aflatoxins in stored grains. Another study in Israel conducted on pest and predacious mites indicates that neem extracts are considerably more toxic to the pest than to the predator.

Neem, Predacious and Phytophagous Mites

Although neem is claimed to have low toxicity to beneficial insects, there are few details to support this belief. A study was conducted in Israel to determine the effects of neem extracts on the predacious mite, Phytoseiulus persimilis, and one of the pest spider mites upon which it preys, Tetranychus cinnabarinus. P. persimilis is used as a biological control agent in greenhouses for the control of spider mites in several countries and in the field in Israel and the U.S. Since it is susceptible to many chemical insecticides, scientists were interested in finding how vulnerable it is to neem extract, which is of interest as an alternative to chemical insecticides in Integrated Pest Management systems.

Neem extracts in four different solvents (methanol, ethanol acetone and pentane) were compared. Discs made from young bean leafs were dipped in different concentrations of extract in each solvent. Control leaf discs were dipped in each untreated solvent. After air drying, each leaf was situated on a wet filter paper, and 30-40 adult female T. cinnabarinus were placed on the disc to act as food for the 10 adult female P. persimilis which were then introduced. After 48 hours, mortality and fecundity (number of eggs per female per day per females surviving) were determined.

The solvents alone caused negligible mortality among the mites. All extracts were considerably more toxic to the pest than to the predator. The pentane extract was the most effective, with the highest level of toxicity to the pest (T. cinnabarinus) and the lowest level of toxicity to the predator (P. persimilis). The reduction of fecundity of P. persimilis, however, was nearly as great as for the pest, indicating that toxicity index alone may not be representative of the susceptibility of a predator to an insecticide. Effects on fecundity should be considered before a product is claimed to be harmless to natural enemies. It should be noted that the pentane extract (at 0.1 percent) caused both a high mortality of the pest (70 percent) and a relatively low reduction of fecundity of the predator (15 percent).

Mansour, F. et al. 1987. Effects of neem (Azadarachta indica) seed kernel extracts from different solvents on the predacious mite Phytoseiulus persimilis and the phytophagous mite Tetranychus cinnabarinus. Phytoparasitica, 15(2) pp125-130.