Two non-invasive molecular techniques to identify live scarab larvae

Abstract

Very little attention has been paid to the taxonomy of scarab larvae, and so the ability to identify them morphologically to species level can sometimes be challenging. Furthermore, this is greatly compounded by the larvae of closely related species looking extremely similar in size, colour and shape, and taxonomic keys to distinguish scarab larvae are often underdeveloped and generally rely on the use of minute morphological characteristics often only accessible by dissection. Whatever the status of the scarab under investigation, pest or endangered, the accurate identification of the species is not the only crucial point to prioritize. Indeed, modern molecular identification tools, are usually based on the use of invasive techniques clearly not compatible with subsequent behavioural, developmental or fitness studies that require physiologically unaffected live specimens. Here, we propose two noninvasive DNA sampling methods suitable for the identification of closely related scarabs. These methods were tested using two New Zealand endemic scarabs, Costelytra zealandica (White) and Costelytra brunneum (Broun). Our results show that the use of the frass and larval exuviae as sources of DNA were both efficient methods to identify the two species studied. This study also showed that small quantities of frass can be used reliably even 7 days after excretion, which is of major importance in ecological studies where timeframes rarely allow daily monitoring. We believe our approach is readily transferable to other holometabolous insects.