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Regulatory animal and plant protection strives to safeguard agricultural species from pests, diseases, and competition from foreign non-beneficial species. The traditional concern for protecting only economic species (crops, livestock, grazing lands) has recently been broadened to include endangered native species and ecosystems in general, upon the acceptance that ecosystems other than agricultural also provide tangible economic benefits to humanity (Perrings et al. 2000), not to mention what are considered the more intangible benefits arising from our innate affinity with nature (Wilson 1984).

The problem with invasive species is as critical now as it has ever been. It could be argued that regulatory animal and plant protection has not done a good job at protecting economic species, let alone performing its more recent task of protecting ecosystems. Still, that apparent failure in the prevention of invasive species must be measured in light of the magnitude of the ever increasing problem, the modest amount of resources directed toward it, and educated guesses about what shape the world would be in without regulatory protections.

Eradication is the complete and permanent removal of all wild populations of an alien plant or animal species from a defined area, by means of a time-limited campaign.This measure is therefore different from control, i.e. the reduction of population density and abundance in order to keep damage at an acceptable level, and containment, aimed at limiting the spread of a species by containing its presence within defined geographical boundaries (Bomford and O’Brien 1995). Following this definition, also the removal of very few individuals is an eradication, if these have the potentialities of reproducing and establishing in the wild (i.e. this does not include the removal of single animal individuals but includes removal of seeds or plant propagules in the wild, or of a few pairs of animals).

Eradication of unwanted alien species is an increasingly important tool for conservation of biological diversity. In fact, although the most effective way for mitigating the impacts caused by biological invasions is the prevention of new unwanted introductions, once prevention has failed and an alien species has invaded a new area, eradication is the best alternative, considering the costs and undesired effects related to permanent control or to a “do-nothing” policy.

This chapter will briefly review the positive aspects of biological control and will highlight a few examples. It will further review negative aspects of biological control introductions. One of the examples where biological control led to detrimental environmental effects was the introduction of the ladybeetle , and this case will be outlined in more detail. This example will also be used to explore some of the population biology mechanisms which can contribute to the net effects of introduced natural enemies. Finally, some information on recent developments and improvements in risk assessment of biological control agents is provided.

Though alien species and biological invasions have become very topical in recent years, it is astonishing (or, should I say, embarrassing) how limited our knowledge still is.We lack information on the specific characteristics (traits) of those species which become invasive, and we do not really know how successful invaders differ from closely related non-invaders. This makes plausible prognoses extremely difficult (if these will ever be possible) but it has also to be stressed that explanation and prediction are two different things.We know that it is unrealistic to expect to detect single characteristics, or even syndromes, which fully explain invasiveness. Biological invasions are to a large extent idiosyncratic, triggered by a huge variety of events and driven by specific factors. A far more realistic approach, therefore, focuses on a complex interaction of many factors, including species traits, environmental aspects and human influence, which need to be more fully unravelled. Today, it is widely accepted that it is futile to manage particular invasions without attending to ill-managed landscapes.