Harmonia axyridis (Coleoptera: Coccinellidae): Smelling the rat in native ladybird declines

John J. SLOGGETT

doi:10.14411/eje.2017.058

Abstract

In the last two decades a huge amount of research has focused on the invasive harlequin ladybird, Harmonia axyridis, particularly on potential or actual deleterious effects that have arisen after it has colonised new regions. A focus of this work has been real or anticipated declines in native ladybird abundance since the introduction of H. axyridis, for which it is deemed responsible. Scientists have generally painted a very bleak picture of the effects of H. axyridis on native species: in this paper I argue that the picture painted is often too bleak. I use the case of the 2-spot ladybird, Adalia bipunctata, the species most often invoked as threatened by H. axyridis, to illustrate my point. While there is little question that H. axyridis has led to a decline in A. bipunctata populations in Europe, it seems likely that prior to the invasive ladybird's arrival A. bipunctata occurred in artificially high numbers in the urban environments in which it was typically studied. Pollution in towns and cities led to enhanced numbers of prey aphids on plants there which initially favoured A. bipunctata, and later H. axyridis. Thus one species, A. bipunctata, that has benefitted from an association with humans has been replaced by another, H. axyridis, just as brown rats replaced black rats in Europe and North America. Viewed with a longer perspective, A. bipunctata has more likely declined back to pre-industrial levels: the artificially high level from which it has declined recently was not a 'natural' one, and thus its decline from this level does not imply that it is now threatened or endangered. More broadly, we need a wider perspective, encompassing other ladybirds, longer timeframes and better comparisons with other (non-ladybird) invasive species to more clearly assess whether H. axyridis really poses as much of a threat as is often proposed.

Highlights

In the last two decades, a huge amount of research has focused on just one species of ladybird, the invasive Asian species Harmonia axyridis (Pallas) (Sloggett, 2005, 2012; Roy et al, 2016)
The focus of this work has been actual or anticipated declines in native species abundance since the introduction of H. axyridis (e.g., Michaud, 2002; Martins et al, 2009; Adriaens et al, 2010; Roy et al, 2012, 2016; Grez et al, 2016), the detailed picture is complex (Harmon et al, 2007; Bahlai et al, 2015). This focus has been coupled to numerous experiments demonstrating strong intraguild predation of native species by H. axyridis in the laboratory and field (e.g., Cottrell & Yeargan 1998a, b; Yasuda et al, 2004; Ware & Majerus, 2008; Thomas et al, 2013; Rondoni et al, 2015), competitive displacement could be more significant in explaining native species’ declines (Harmon et al, 2007; Snyder, 2009; Bahlai et al, 2015)
Adalia bipunctata was for a long time a widely used research model and its apparent disappearance was likely for many researchers a personal manifestation of the effects of H. axyridis

Summary

Introduction

In the last two decades, a huge amount of research has focused on just one species of ladybird, the invasive Asian species Harmonia axyridis (Pallas) (Sloggett, 2005, 2012; Roy et al, 2016). Personal observations and data from before and after the arrival of H. axyridis in its European range to argue that our perception of how dangerous H. axyridis is to A. bipunctata, and by implication other native ladybirds, might be greater than the threat it poses.

Results

Conclusion