Use of wet, air-dried, or oven-dried bulk mass to quantify insect numbers: an assessment using Chilothorax distinctus (Müller) (Coleoptera: Scarabaeidae)

Abstract

Abstract We examined the use of bulk mass to predict the number of individuals in samples of the dung beetle Chilothorax distinctus (Müller) (Coleoptera: Scarabaeidae: Aphodiinae: Aphodiini). We first developed linear regression equations to characterise the relationship between the number of beetles in a sample and sample wet mass, air-dried mass, or oven-dried mass. We then applied these equations to samples containing unknown numbers of beetles to obtain a predicted number. The predicted number was subsequently compared to the number obtained by counting each beetle by hand. Wet mass was as suitable as air-dried or oven-dried mass to estimate beetle numbers and was quicker to obtain. The predicted number of beetles in individual samples based on wet mass deviated from the actual number by 0.6–19.9%. For results combined across samples, the discrepancy was 2.2%. We conclude that quantifying C. distinctus by bulk wet mass rather than by hand count provides a reasonable alternative that accelerates the pace of sample processing while providing substantial cost savings. These results add to the small body of literature assessing the accuracy of bulk insect mass as a predictor for the actual number of individuals in large samples of conspecifics.