Effect of Long-Chained Esters on the Insecticidal Properties of L-Canavanine

The ability of the several esters of L-canavanine, a potentially insecticidal natural product of leguminous plants, to enhance the potency of the parent compound was evaluated with the propyl, butyl, isobutyl, and octyl esters of L-canavanine. These compounds, administered either by a single parenteral injection or by dietary consumption, were tested for their intrinsic toxicity in studies conducted with terminal instar larvae of the tobacco hornworm Manduca sexta [Sphingidae]. Parenterally injected propyl or butyl esters were somewhat more toxic than canavanine, but the isobutyl and octyl esters were far more deleterious to larval growth and development. A single injection of the isobutyl or octyl ester (1.25 mg/g larva, molar equivalent of L-canavanine dose) was lethal to all the test animals before the end of the larval instar. Much of the toxicity of the octyl ester arguably was due to release of octyl alcohol, since the free alcohol is highly pernicious. In contrast, the isobutyl ester was far more toxic than isobutanol. A different picture emerged from chronic exposure to the tested esters via dietary consumption. By this administrative route, although all of the esters were marginally more toxic than canavanine, the tested compounds exhibited much less toxicity than occurred by parenteral injection. Little difference was noted in the relative growth-inhibiting activity between the esters, and all of the free alcohols exhibited little deleterious effect on larval development.