Larval development, metabolism and diet are possible key factors explaining the decline of the threatened Dytiscus latissimus

Abstract

Abstract In Western Europe, the diving beetle Dytiscus latissimus (Coleoptera: Dytiscidae) has become rare and went extinct in several countries during the last century. This study investigated whether larval development rate, metabolism and feeding ecology differ between D. latissimus and the congeneric D. lapponicus to explore factors explaining its decline. During instar I and II, D. latissimus larvae developed faster and gained more weight than D. lapponicus larvae. In accordance, D. latissimus larvae had higher oxygen consumption rates than D. lapponicus larvae, which signifies a greater energy expenditure. Food preference tests showed that D. latissimus larvae strongly prefer caddisfly larvae (Trichoptera: Limnephilidae) with early instars being obligatory dependent on caddisfly larvae for their development. Only instar III larvae readily fed on alternative prey items. In contrast, D. lapponicus larvae had a broader diet and even rejected caddisfly larvae. Based on field observations, availability of caddisfly larvae strongly declined before the end of the larval development of D. latissimus, suggesting that time constraints on food availability limit completion of larval development. Our results suggest that food limitation during (early) larval stages is a possible bottle‐neck for this species, potentially explaining its disappearance from former localities. Promoting caddisfly larvae in the vicinity of D. latissimus oviposition sites, may possibly safeguard the present distribution of D. latissimus and support the species recovery. Although more research is needed, promoting leaf litter in shores may be beneficial to the shredding caddisfly larvae and in turn for their predator D. latissimus.