Selecting biological control agents
Factors influencing host selection in the target range
Geographic range and biotypes
As mentioned earlier, host range and other biological characteristics of organisms may vary from one population to another across their native range. Where these differences can be defined the population can be described as a strain or biotype: a group of organisms of the same species that have clear-cut physiological but not necessarily morphological distinctions from others within the species. Biotypes can normally interbreed, although biotypes can have sexual and parthenogenetic forms (Gerard et al. 2006). The existence of biotypes can often be inferred from the diversity of host records that appear in the literature. The Thompson and Simmonds catalogue (Thompson and Simmonds 1964-1965) records a wider host range for many oligophagous parasitoids than can be measured in any one population (Hill et al. 1985).
The existence of previously unrecognised biotypes is gradually becoming much better understood. The target or the natural enemy may actually consist of several distinct populations that vary biologically but are lumped under a single taxonomic name. The existence of variation between populations of natural enemies creates both risks and opportunities in the development of a biological control programme. For example, unless biotypes are distinguished, a complex of narrowly specific parasitoids might be incorrectly viewed as one widespread, more polyphagous species (Sands and Van Driesche 2004), risking the rejection of potentially useful agents. Selecting the wrong source population can compromise the probability of establishment or successful control of the target pest.
Biotrophic plant pathogens such as rust fungi can be particularly specific to different forms of target weeds. Puccinia chondrillina has successfully controlled the narrow-leafed form of skeleton weed, Chondrilla juncea, but after over 30 years has not adapted to attack the other forms. Similarly, the biological control programme for blackberry in Australia has involved the selection of a range of pathotypes of Phragmidium violaceum that are specific to different groups of the Rubus fruticosus aggregate (Morin et al. 2006).
On the other hand, the inclusion of genotypes from a population that has not been subjected to host range tests can lead to unpredicted host use following release (Quentin Paynter, pers. comm.). This was the case when the gorse pod moth, Cydia succedana, was introduced to New Zealand from the UK in 1992 as a biological control agent for gorse. This species now makes occasional use of non-target plants at levels that were not predicted by host-range testing (Paynter et al. 2004). This has been attributed to the release of untested genotypes from Portugal that have a subtly different pattern of host use (Quentin Paynter, pers. comm.). It is now standard practice in New Zealand to release only those populations of weed control agents for which host range tests results exist. Sands and Van Driesche (2004) recommended that host range tests for parasitoids should be also be conducted using the population that is to be released to avoid the possibility of using different biotypes which might have different host ranges.
Variation between populations has provided a potentially powerful tool for biological control of Sitona lepidus in New Zealand. Microctonus aethiopoides Loan was introduced to New Zealand in 1982, and provides adequate control of Sitona discoideus Gyllenhal. When clover root weevil (S. lepidus Gyllenhal) became established, the New Zealand resident M. aethiopoides failed to parasitise the new pest, even though this was a common parasitoid of S. lepidus in its native range (Barratt et al. 1997). Before a new strain of the parasitoid could be introduced from Europe, rearing experiments indicated that interbreeding between the new S. lepidus strain and the resident S. discoideus strain led to poor performance in hybrids, and the prospect of failure in the control of S. discoideus should the new biotype be introduced (Goldson et al. 2003). Instead, a parthenogenetic strain, incapable of interbreeding with the biotype already in New Zealand, has been introduced (Gerard et al. 2006). These forms of the parasitoid can now be distinguished using molecular techniques (Vink et al. 2003) and there may be cryptic species involved. Goldson et al. (1997) also discuss the potential of differing parasitoid strains in biological control.
For many years the best-adapted control agents were thought to occur at the 'centre of origin' of a pest. The competing theory that the most dynamic control agents would be 'new associations' between natural enemies and unusual hosts, often from the edge of the native range of a pest (Hokkanen and Pimentel 1989) was considered particularly high-risk by Simberloff and Stiling (1996).
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.
Gerard P.J., McNeill M.R., Barratt B.I.P. and Whiteman S.A. (2006). Rationale for release of the irish strain of Microctonus aethiopoides for biocontrol of clover root weevil. New Zealand Plant Protection 59: 285-289.
Goldson S.L., McNeill M.R. and Proffitt J.R. (2003). Negative effects of strain hybridisation on the biocontrol agent Microctonus aethiopoides. New Zealand Plant Protection 57: 138-142.
Goldson S.L., Phillips C.B., McNeill M.R. and Barlow N.D. (1997). The potential of parasitoid strains in biological control: observations to date on Microctonus spp. intraspecific variation in New Zealand. Agriculture, Ecosystems and Environment 64: 115-124.
Hill R.L., Cumber R.A. and Allan D.J. (1985). Parasitoids introduced to control larvae of the Noctuidae (Lepidoptera) in New Zealand (1968-1978). DSIR Entomology Division, 24 p.
Hokkanen H.M.T. and Pimentel D. (1989). New associations in biological control: Theory and practice. Canadian Entomologist 121: 829-840
Morin L., Evans K.J. and Sheppard A.W. (2006). Selection of pathogen agents in weed biological control: critical issues and peculiarities in relation to arthropod agents. Australian Journal of Entomology 45: 349-365
Paynter Q.E., Fowler S.V., Gourlay A.H., Haines M.L., Harman H.M., Hona S.R., Peterson P.G., Smith L.A., Wilson-Davey, J.R.A., Winks, C.J. and Withers T.M. (2004). Safety in New Zealand Weed Biocontrol: A nationwide survey for impacts on non-target plants. New Zealand Plant Protection 57: 102-107
Sands D.P.A. and Van Driesche R.G. (2004). Using the scientific literature to estimate the host range of a biological control agent. Pp. 15-23 In: Assessing host ranges for parasitoids and predators used for classical biological control: a guide to best practice, R.G. Van Driesche and R. Reardon (Ed.) USDA Forest Service, Morgantown, West Virginia.
Simberloff D. and Stiling P. (1996). Risks of species introduced for biological control. Biological Conservation 78: 185-192.
Thompson W.R. and Simmonds F.J. (1964-1965). A Catalogue of the Parasites and Predators of Insect Pests. Commonwealth Agricultural Bureaux, Bucks, United Kingdom.
Vink C.J., Phillips C.B., Mitchell A.D., Winder L.M. and Cane R.P. (2003). Genetic variation in Microctonus aethiopoides (Hymenoptera: Braconidae). Biological Control 28: 251-264
How can models improve biosafety?
Fundamental vs ecological host range