Gravimetric Method for Determining Stage of an Obligate Internally Feeding Stored-Product Insect Pest, the Cowpea Weevil

Abstract

We conducted two experiments to develop a gravimetric method for determining the stadia of Callosobruchus maculatus developing in cowpeas. An important step was to develop regres- sion equations for predicting weight changes in uninfested cowpeas to correct for changes that occur in weight as a result of changing environmental conditions, such as temperature, relative humidity, and barometric pressure. This was vital because we needed to have a way of determining weight changes in infested peas that were caused by the insects developing within them; even the weight of uninfested peas changed over the course of the experiments. The observed weight of each infested pea was corrected based on the proportion weight change in uninfested peas each time weighing was done to get an estimate of the cumulative change in weight caused by the presence of the insect. For each infested pea, cumulative change in weight was plotted against time, and plots were analyzed to determine whether a pattern existed that could be used to consistently distinguish stadia. In the second experiment we used an ultrasonic detector to validate results from the gravimetric method. We found that the gravimetric method provided accurate information on duration of the egg, larval, and pupal stages of C. maculatus; however, the gravimetric method could not reliably distinguish the four larval stages of C. maculatus. Our paper also discusses practical applications of the gravimetric method for determining the stadia of internally feeding insects in the context of the current and previous research.