Morphology of invasion: body size patterns associated with establishment of Coccinella septempunctata in western North America

Edward W Evans

doi:10.14411/eje.2000.072

Abstract

The Old World ladybird Coccinella septempunctata has rapidly established itself as an abundant, widespread species throughout North America. Overwintering individuals of this species, and of native ladybirds, were collected from early season alfalfa in northern Utah during the period of initial establishment of the invader (1989 to 1999), and were measured for body size. Adult body size can vary widely within insect species, often reflecting differential success of individuals as immatures in obtaining food. Here I examine patterns of ladybird body size to address two questions associated with the establishment of C. septempunctata: (1) is there evidence for adverse impact on native species?, and (2) why has the invader has been so successful in establishment? As an indirect test of adverse competitive effect of the invader on native species, I determined whether mean body size of adults of the five most common native species (Coccinella tranversoguttata, Hippodamia convergens, H. quinquesignata, H. sinuata, and H. tredecimpunctata) declined over the period 1991-1997 as the invader increased rapidly in abundance. No such decline was observed for any of these species, thus providing no evidence that the invader's establishment has significantly increased scramble competition for food among immature ladybirds. I also compared body size distribution of the invading species with that of native species. The invader was distinctive in having particularly large variation in body size among individuals (i.e., in having relatively high proportions of both unusually large and small individuals). Such results are consistent with the hypothesis that the invader's success derives from being a generalist with much ecological flexibility in regard to the conditions under which it engages and succeeds in reproduction.

Highlights

Since its discovery in the northeastern U.S in 1973 (Angalet et al, 1979), the Old World ladybird Coccinella septempunctata L. (Coleoptera: Coccinellidae), a predator principally of aphids (e.g., Hodek, 1973; Gordon, 1985; Hodek & Honek, 1996), has spread rapidly to establish itself as a dominant species throughout North America (e.g., Schaefer et al, 1987; Elliot et al, 1996)
In light of its rapid rise in abundance, two obvious issues arise con cerning the biology of this invader and the native ladybird species with which it shares North American habitats: (1) has the arrival of C. septempunctata had adverse impact on native ladybird species?, and (2) why has the invader been so successful in establishing in a new continent?
The simplest hypothesis to account for such declines is that native species have suffered from increased scramble competition for food (e.g., Evans, 1991; Elliott et al, 1996; see Niemela & Mattson, 1996), an alternative possibility is that these spe cies have been victimized by C. septempunctata through intraguild predation (Hironori & Katsuhiro, 1997; Obrycki et al, 1998; see Agarwala & Dixon, 1992)

Summary

Introduction

Since its discovery in the northeastern U.S in 1973 (Angalet et al, 1979), the Old World ladybird Coccinella septempunctata L. (Coleoptera: Coccinellidae), a predator principally of aphids (e.g., Hodek, 1973; Gordon, 1985; Hodek & Honek, 1996), has spread rapidly to establish itself as a dominant species throughout North America (e.g., Schaefer et al, 1987; Elliot et al, 1996).

Results

Conclusion