Toxicological, biological and physiological effects of diatomaceous earth on the bean weevil Acanthoscelides obtectus (Say) and the cowpea weevil Callosobruchus maculatus (F.) (Caleoptera: Bruchidae)

The cowpea weevil Callosobruchus maculatus (FABRICIUS) and the bean weevil Acanthoscelides obtectus (SAY) are considerably destructive pests of pulses both in storage and in the field, and are widely distributed throughout the tropics and subtropics. Recently developed naturally occurring diatomaceous earth (DE) formulations can be used as alternative substances to chemical insecticides. It is well known that DE kills the insects by absorbing the epicuticular lipid layers leading to high rate of water loss through the cuticle. The objective of this study was to determine the efficacy of two DEs, Fossil-Shield and Silico-Sec, on the basis of toxicity, reproductive biology, kinetics of uptake and physiological effects on the two bruchids. Both DEs were highly toxic to the bruchids. The F1 progeny production declined rapidly with increasing DEs dosage form 100mg/kg to 2800mg/kg. Mortality of bruchids increased as temperature increased from 20°C to 35°C but it decreased at high relative humidity of 84%. The cowpea weevils C. maculatus were more susceptible to DE than the bean weevils A. obtectus. The reproductive strength of both bruchids was significantly affected by DEs. It was determined that the fecundity, egg hatchability, host seed acceptance and first instar larval survival decreased with increasing dosage of DEs from 400mg/kg to 2400mg/kg. Therefore, it can be concluded that DEs lead to progeny reduction not only by direct killing but also hindrance of reproductive behaviour of bruchids. These studies showed that Fossil-Shield was more effective than Silico-Sec. The kinetics of Fossil-Shield® uptake by both weevils can be described precisely by a first order two-compartment model. According to the model, DE uptake occurs as a two site reaction. Firstly, pick-up and loss of DE from the epicuticle of insects happens quite rapidly. Secondly, picked up DE particles are incorporated into the epicuticle lipid layer. This model indicates that the cowpea weevils take up more DE than bean weevils. Gas chromatographic analysis revealed that DE could effectively absorb the epicuticular lipids from treated insects within a short period (i. e. six hours) after treatment. The highest body water loss rate occurred at 40% relative humidity (r. h.). Even at 99.9% r. h., DE-treated insects continuously lost body water. They died after loosing about 50% of their body water. Microcalorimetric studies revealed that metabolic heat production rates in DE treated bruchids were significantly higher than in untreated insects. The loss of hemolymph volume and tissue water content of more than 50% was lethal to the cowpea weevils C. maculatus. However, hemolymph could buffer the tissue water content up to a certain extent. Predominantly at 15°C and 25°C ambient temperature, the DE treated cowpea weevils C. maculatus showed different respiration patterns compared to the untreated insects. The carbon dioxide release rate in untreated insects was discontinuous, whereas in treated it occurred at a fairly continuous rate. It can therefore be argued that the death of DE treated C. maculatus and A. obtectus occurs due to high water loss together with stress related high physiological metabolic activity.