Toxic properties of d-limonene in insects and the earthworm Eisenia fetida

Abstract

<p>The monocyclic hydrocarbon monoterpenoid, d-limonene, is the principal component of the essential oils of citrus. In recent years, it and similar compounds have been touted as possible alternatives to conventional insecticides. Studies were completed to determine the spectrum of insecticidal properties of d-limonene in several common insects via several routes of entry and to evaluate the suitability and efficacy of the compound as a general-use insecticide. Additionally, the effects of d-limonene on parameters of growth and reproduction in the German cockroach, Blattella germanica, were investigated. The toxic properties of d-limonene were also investigated in a non-target invertebrate, the earthworm Eisenia fetida. A non-invasive electrophysiological monitoring technique was used to assay for neurotoxicity effects of d-limonene in the earthworm;d-Limonene was not found to be a uniformly potent toxin against the insects tested. It exhibited no residual or oral toxicity, low topical toxicity, moderate fumigant and ovicidal activity, and significant repellency and larvicidal activity. The compound was synergized with piperonyl butoxide. The material was found to have interesting, subtle effects on insect growth and reproduction. It did not function as a feeding stimulant or arrestant, but significantly accelerated the growth rate of German cockroach nymphs when administered in the diet. Topical application of d-limonene to the oothecae of gravid German cockroaches caused embryotoxicity, and topical application of high levels to the female prior to mating was found to decrease reproductive potential and shorten the female lifespan. Neither pre-mating vapor exposure nor feeding of treated diet was found to influence reproduction;In the earthworm Eisenia fetida, d-limonene's LC[subscript]50 was 6.0 ppm, and application of the synergists piperonyl butoxide and sesame oil did not affect toxicity significantly. A variety of behavioral, morphological, and neurobiological effects ensued rapidly following exposure, and the neurotoxic effects were regionally specific. Nervous system effects such as giant fiber conduction velocity decreases, medial giant fiber impulse rebounding, and spontaneous spiking of lateral giant fibers were noted following d-limonene treatment, but the effects were reversed once exposure ceased.</p>