Determining the instar of a weevil larva (Coleoptera: Curculionidae) using a parsimonious method

Tracking the larval development of an insect is important in both applied and basic ecology. Yet, it is difficult to discriminate between the different larval instars of holometabolous insect species, particularly in the field. The methods currently available are of limited use as they rely on an a priori determined size distribution of every immature instar and are irrelevant whenever the distributions of two instars actually overlap. We developed a model that computes, for a given species, the most probable instar of a larva based on its individual head capsule width and the size distribution of the last larval instar. The model presented here is specifically for species of curculionid that develop through four immature instars, but can be adapted for species of weevil with a different number of instars or insects of other taxonomic units. Our method computes the risk error associated with assigning a larva to any of the possible instars and might not assign a larva if its size falls within the overlapping zone of the size distributions of two successive instars. Thus, this parsimonious method might be widely used, notably for wild-caught larvae, and can be readily used thanks to the R package CINID that we developed for that purpose.