Insecticidal activity of monoterpenes against Rhyzopertha dominica (F.) and Tribolium castaneum (Herbst)

Abstract

Secondary metabolites play an important role in plant resistance to insects. The monocyclic monoterpenes 1,8-cineole (eucalyptol) and R-(+)-limonene have been considered economically important in this context. Cineole is a component of different species of Eucalyptus spp. and limonene is a constituent of Citrus spp. essential oils. These substances were bioassayed to determine possible fumigant, contact, and ingestion activity against Rhyzopertha dominica (F.), lesser grain borer, and Tribolium castaneum (Herbst), red flour beetle, which are important pests of stored grain. It was possible to show that both compounds are insecticidal against the two insects, mainly in the contact and/or ingestion action.

Introduction

Rhyzopertha dominica (F.) and Tribolium castaneum (Herbst) cause large economic losses of stored wheat grain. The conventional way to control these insect pests has been with the use of insecticides, either directly applied to grains or by gas fumigation. However, concerns have arisen about the persistence of insecticide residues in grains which can be harmful to mammals.

Some secondary metabolites of plants play an important role in plant–insect interaction, and are commonly responsible for plant resistance to insects (Mann, 1987). Some essential oils have acute toxicity, repellent action, feeding inhibition, or harmful effects on the reproductive system of insects. Additionally, secondary metabolites from higher plants have recently been used as pesticides or models for new synthetic pesticides as, for instance, toxaphene (insecticide and herbicide) and cinmethylin (herbicide) (Duke et al., 1988). These chemicals were developed from plant-derived products such as terpenoids that can be found in the essential oil secreted by the glandular trichomes of Artemisia (Compositae) or closely related genera. Pine oil, a by-product of the sulphate wood pulping industry, has the monoterpene α-terpineol among its major constituents. This substance protects living trees for up to 10 m by repelling three bark beetle species (Nijholt et al., 1981). Alfaro et al. (1984) reported that pine oil is a feeding deterrent to Pissodes strobi (Peck) (Coleoptera: Curculionidae). Monoterpenes found in essential oil from different plants have insecticidal activity against termites (Baker and Walmsley, 1982). Coats et al. (1991) described fragrant volatile oils containing monoterpenes or their related compounds: alcohols, ketones, aldehydes, carboxylic acids, and oxides, such as d-limonene, β-myrcene, α-terpineol, linalool, and pulegone. These substances have toxicity to house fly (Musca domestica L.), German cockroach (Blatella germanica L.), rice weevil (Sitophilus oryzae L.), red flour beetle (Tribolium castaneum Herbst) and Southern corn rootworm (Diabrotica unidecimpunctata howardi Barber) (Rice and Coats, 1994a, Rice and Coats, 1994b). A large reduction in the population of ticks (Boophilus microplus Canestrini) caused by the essential oil of the grass Melinis minutiflora (de Beauvois) was reported by Ayacardi et al. (1984). More recently, the GC/MS analysis of the M. minutiflora essential oil revealed a complex mixture of substances which included the monoterpene 1,8-cineole as a major active component against ticks (Prates, 1992; Prates et al., 1993).

R-(+)-limonene (p-mentha-1,8-diene) and 1,8-cineole (1,3,3-trimethyl-2-oxabicycle[2.2.2.]octane) are natural products from plants with low toxicity to mammals. Limonene is a major component of Citrus spp. essential oils, and is also used as an ingredient of soaps, perfumes, and food additives (Karr and Coats, 1988). Cineole (or eucalyptol) is a constituent occurring in variable amounts in the essential oil of Eucalyptus spp. leaves (Gibson et al., 1991).

The main purpose of this work was to test cineole and limonene as ecologically safe alternative insecticides, which could also be economically produced and used against R. dominica and T. castaneum.