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

Abstract

AbstractIn Western Europe, the diving beetleDytiscus 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 betweenD. latissimusand the congenericD. lapponicusto explore factors explaining its decline.During instar I andII,D. latissimuslarvae developed faster and gained more weight thanD. lapponicuslarvae. In accordance,D. latissimuslarvae had higher oxygen consumption rates thanD. lapponicuslarvae, which signifies a greater energy expenditure.Food preference tests showed thatD. latissimuslarvae strongly prefer caddisfly larvae (Trichoptera: Limnephilidae) with early instars being obligatory dependent on caddisfly larvae for their development. Only instarIIIlarvae readily fed on alternative prey items. In contrast,D. lapponicuslarvae 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 ofD. 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 ofD. latissimusoviposition sites, may possibly safeguard the present distribution ofD. latissimusand 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 predatorD. latissimus.