Host Acceptance and Larval Competition in the Banded and European Elm Bark Beetles, Scolytus schevyrewi and S. multistriatus (Coleoptera: Scolytidae): Potential Mechanisms for Competitive Displacement between Invasive Species

Abstract

A recent survey revealed that the newly invasive banded elm bark beetle, Scolytus schevyrewi, was much more abundant than the long-established European elm bark beetle, S. multistriatus, in areas of Colorado and Wyoming, USA. This study was initiated to determine whether competitive displacement of S. multistriatus by S. schevyrewi might be mediated by a sequence of behavioral interactions between the species on and below the bark surface. At the first step in the sequence, host acceptance behavior (i.e., time to tunnel into the host) was monitored among female S. schevyrewi and S. multistriatus. There was no substantial difference in host acceptance behavior when females were offered small cut logs (bolts) of Siberian elm, Ulmus pumila, infested with con- or heterospecific females. At the second step, mating and oviposition usually follow after a female has excavated a gallery. Mean oviposition per parental female in 1 wk was not significantly different among treatments: S. schevyrewi at medium density of 2.86 parental females per dm², at high density of 5.71 per dm²; S. multistriatus at medium density, at high density; and mixed species (i.e., S. schevyrewi and S. multistriatus combined, with each at medium density). At the third step, progeny production and progeny size were monitored among the same density treatments, but the number of parental beetles and size of bolts were doubled. Differences in progeny production would reflect larval competition, since the number of eggs initially laid was not different. Both S. schevyrewi and S. multistriatus were sensitive to intraspecific competition because size of progeny was smaller in high than medium density treatments, but total progeny production was similar at both densities. Scolytus schevyrewi was a stronger interspecific competitor than S. multistriatus. Smaller S. multistriatus progeny, and four-fold more S. schevyrewi progeny were produced when both species developed on the same host. The contributions of these proximal behavioral events toward the mechanism for competitive displacement are described in the context of the host colonization behavior of Scolytus spp. The competitive advantage of S. schevyrewi larvae established through this study, and the more rapid response of S. schevyrewi to elm hosts established through other investigations, may be the key mechanistic components that facilitate the displacement of S. multistriatus by S. schevyrewi.