Cocaine each day keeps the bugs away.

Although it might seem otherwise, coca plants do not make cocaine just for people to use and abuse. One of many alkaloids manufactured by plants, cocaine works to keep insects away, says James A. Nathanson, a neurobiologist at the Massachusetts General Hospital in Boston.

Previously, researchers had noticed that few insects nibble coca plants, a surprising observation given that their leaves are perpetually young and tender because of people harvesting them for the drug trade, says Nathanson. Also, since cocaine can cause mammals to become anorexic, he also wondered what effect cocaine might have on insect feeding behavior.

Nathanson and his colleagues put groups of five three-day-old month caterpillars on tomato leaves sprayed with different concentrations of cocaine. Just being neaer the leaves made the insects rear up, shake, and walk away, behaviors that worsened when the caterpillars tasted the leaves. The concentrations were about equivalent to those that exsit naturally in coca leaves, says Nathanson. Cocaine also killed mosquito larvae.

The Boston group then studied the effects of several other compounds that function in the nervous system the same way as cocaine. They also investigated cocaine's chemical cousins.

Cocaine bothers insects by preventing their nerve cells from taking up key chemical messengers, ipn particular one called octopamine. Octopamine functions like norepinephrine, the messenger that conveys the "fight or flight" response in mammals. When cells fail to take up octopamine, it accumulates, making the cells overly excited, the group concludes in the Oct. 15 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES.

Because octopamine plays little role in mammalian nervous systems, these findings could lead to new pesticides that deter insects the same way that cocaine does, but without affecting people, adds Nathanson.