Environmental risks of biological control
Adding a new species to an ecosystem is bound to impact in some way. The complex nature of communities precludes prediction of the ramifications that might occur through the trophic levels within the system. The potential risks from biological control introductions come from either direct effects on non-target species or indirect effects on the community into which the new species arrives. Novel hosts might include indigenous species that are taxonomically related to the intended host including beneficial or valued species, and species which occur in or near the habitats where the target host is found, or to where it disperses.
Until recently, there has been little research undertaken to demonstrate the impact of biological control agents on populations of non-target species. It has been shown that the weevil Rhinocyllus conicus (Froehlich), introduced into the USA to control exotic thistles, has also adopted native thistle species as hosts and impacts at the population level have been demonstrated (Louda 1999, Louda et al. 2005). This is one of few examples where the impact of a biological control agent on non-target species has been quantified. In New Zealand, the braconid parasitoid Microctonus aethiopoides Loan, introduced to control the weevil lucerne pest Sitona discoideus Gyllenhal, has been found to parasitise a number of non-target weevil species in the field, with parasitism levels of up to 70% (Barratt et al. 1997). Ironically, one of the species attacked by M. aethiopoides in New Zealand is the weed biological control agent, R. conicus (Ferguson et al. 1998), although the impact of this on thistle control has not been assessed.
The most well documented examples of adverse impacts from biological control programmes have involved the introduction of vertebrates. The Indian mongoose (Herpestes auropunctatus) was introduced to the West Indies and Hawaii to control rats in sugarcane, but being unable to climb trees, it was only able to control the Norway rat and not the less numerous tree rat. The latter, having been suppressed by the former, then became more abundant. The mongoose also attacked domestic and native birds and lizards, and the reduction in numbers of lizards resulted in an increase in numbers of the sugarcane beetle (Pimentel 1980). The other classical example is the introduction of the cane toad (Bufo marinus) from Central and South America in to Queensland to control two pests of sugar cane, the grey backed cane beetle and French's beetle. While biological control of the sugar cane pests failed, the toad has spread spectacularly within Australia and reached very high population densities and posed a threat to native species in some areas.
There are examples also of generalist predators which have been very damaging to non-target organisms, for example the predatory snail, Euglandina rosea. This was released in many countries, often contrary to the advice of scientists, to control the giant African snail, Achatina fulica, a pest of crops and natural ecosystems. However, E. rosea has had a devastating effect on native snails, in some instances causing extinctions (Civeyrel and Simberloff 1996).
Given the regulatory safeguards that we have in place today, such poorly conceived deliberate introductions are unlikely. However, impacts of invertebrates are less obvious and hence more difficult to investigate. Consequently there are few well documented cases of adverse impacts of biological control agents. There is circumstantial evidence to suggest that insect biological control agents have the capacity to cause species extinctions, particularly in island communities. In Fiji, the introduction of the tachinid Bessa remota (Aldrich) to control the coconut moth, Levuana iridescens Bethune-Baker is reported to have caused the extinction of its host (Howarth 1991). Similarly, the citrus psylla, Trioza erytreae (Del Guericio) was apparently exterminated from Reunion Island by the eulophid parasitoid Tetrastichus dryi Waterston (Aubert and Quilici 1983). In both cases, the parasitoids maintained their populations on alternative species, despite the decline of the target hosts. Whether or not L. iridescens is actually extinct is still hotly debated.
Aubert B. and Quilici S. (1983). Nouvel équilibre biologique observe à la Réunion sur les populations de psyllides après l’introduction et l’éstablissement d’hymenopteres chalcidiens. Fruits 38: 771-780.
Barratt B.I.P., Evans A.A., Ferguson C.M., Barker G.M., McNeill M.R. and Phillips C.B. (1997). Laboratory nontarget host range of the introduced parasitoids Microctonus aethiopoides and Microctonus hyperodae (Hymenoptera: Braconidae) compared with field parasitism in New Zealand. Environmental Entomology 26: 694-702.
Civeyrel L. and Simberloff D. (1996). A tale of two snails: is the cure worse than the disease? Biodiversity and Conservation 5: 1231-1252.
Ferguson C.M., Cresswell A.S., Barratt B.I.P. and Evans A.A. (1998). Non-target parasitism of the weed biological control agent, Rhinocyllus conicus Froelich (Coleoptera: Curculionidae) by Microctonus aethiopoides Loan (Hymenoptera: Braconidae). Pp. 517-524 In: Pest Management - Future Challenges: Proceedings of the 6th Australasian Applied Entomological Research Conference, M. Zalucki, R. Drew and G. White (Ed.) The Cooperative Research Centre for Tropical Pest Management, Australia.
Howarth F.G. (1991). Environmental impacts of classical biological control. Annual Review of Entomology 36: 489-509.
Louda S.M. (1999). Negative ecological effects of the musk thistle biological control agent, Rhinocyllus conicus. Pp. 213-243 In: Nontarget effects of biological control introductions, P.A. Follett and J.J. Duan (Ed.) Kluwer Academic Publishers, Norwell, Massachusetts, USA.
Louda S.M., Rand T.A., Arnett A.E., McClay A.S., Shea K. and McEacherne A.K. (2005). Evaluation of ecological risk to populations of a threatened plant from an invasive biocontrol insect. Ecological Applications 15: 234-249.
Pimentel D. (1980). Environmental risks associated with biological control. Ecological Bulletin 31: 11-24.
Environmental risks of biological control