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Pros and Cons of Biological Control 411
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Fig. 23.1 Map showing the invasion of Harmonia axyridis in Europe from the first sightings
in the year 2000. This ladybeetle has been used for the biocontrol of aphids in greenhouses
and the field since 1982 (map extracted from Crop Protection Compendium and
based on information provided by A.J.M. Loomans, Plant Protection Service NL, photograph
A.J.M. Loomans)
native to Europe. The situation in the US therefore warrants being concerned
about the invasion of H. axyridis but can not be used to predict the potential
environmental impact in Europe with high certainty.
23.5 Why Has H. axyridis Become Invasive?
Since it is well known that, once established, only a small fraction of all exotics
subsequently become invasive, the question posed here more generally relates
to our ability to predict whether a species will become invasive or not. Despite
the many studies conducted on H. axyridis, the reasons for the displacement
of native coccinellids by H. axyridis and its general success in North America
are still not well understood. Intraguild predation has been examined as a
mechanism leading to displacement of native species by H. axyridis. Several
studies showed that H. axyridis is intrinsically superior to many other coccinellids
(e.g. Michaud 2002b), and results of Yasuda et al. (2004) suggest that
intraguild predation by H. axyridis strongly influence aphidophagous guild
structure. Thus, H. axyridis acts as a top predator in the guild of aphidophagous
insects, which may be one factor for its success as an invader.