A. The Concept
CBC has been in practice for over a century: the concept having been conceived, developed, and refined by entomologists for the management of invasive alien species, principally arthropod pests and weeds. However, the theory that underpins the practice has only recently been conceptualised formally through the enemy release hypothesis (Keane and Crawley2002);
so recent, in fact, that it failed to make the original chapter (Evans2002a). In essence, the theory posits that exotic plant species become invasive because of increased fitness in the absence of their coevolved natural enemies.
And, therefore, the solution to address the problem of invasive neophytes became obvi-ous: source, import, and release the natural enemies, or CBC agents, from the centre or region of origin of the target weed species into the invaded ecosystem or country to reduce its fitness and, thus, increase the competitive abil-ity of the indigenous flora. Supportive data quickly followed, based on a desk study that analysed biotrophic plant pathogens (predomi-nantly, rusts, smuts, and powdery mildews) associated with over 470 plant species from Europe naturalised in the USA (Mitchell and Power 2003). The results showed that, on average, the exotic plants had 84 % fewer fungi than in their native European range and, significantly, those with the least fungal natural enemies proved to be the most invasive and troublesome.
It could be argued, of course, that plant invasiveness cannot be explained in such sim-plistic terms, and that multiple factors are involved: release from natural-enemy pressure being one of many components, such as resource availability (Blumenthal2006). In the recently proposed endophyte-enemy release hypothesis, for example, it is posited that exotic
plant species may also lack their coevolved endophytic fungi (Evans 2008). Those endo-phytes that form beneficial associations with their coevolved hosts—such as increasing plant resistance to abiotic and biotic stresses (Rudgers et al.2004; Schulz and Boyle2005)—
and thus act as bodyguards, would not be a necessity in a new environment with little or no natural-enemy pressure. Indeed, neophyte hosts arriving without their coevolved endo-phytes would be fitter and, therefore, more invasive, as nutrients are not sequestered by or diverted to these fungal mutualists, thereby freeing-up more resources for growth and reproduction. Nevertheless, if their coevolved natural enemies should ever catch up with such vulnerable, endophyte-deficient plants, the consequences could be disastrous. This may help to explain not only the ‘silver-bullet’
phenomenon—whereby, the release of a single CBC agent results in spectacular and sustain-able control of the target weed (Van Wilgen et al. 2004; Page and Lacey2006; Barton et al.
2007)—but, also, the devastating impact of newly-arrived pathogens on their coevolved crop hosts (Large1940) more recently, referred to as pathogen pollution and tagged with the label ‘emerging infectious diseases’ (Anderson et al.2004; Evans and Waller2010). If there are any doubters about the devastating impact of coevolved fungal pathogens on exotic plant species (and, therefore, the potential efficacy of CBC as a management strategy for invasive alien weeds), examples in the agricultural sec-tor are all too common (Large 1940; Quimby 1982; Evans2002b; Agrios2005).
CBC practitioners had, in fact, been adher-ing to this simple concept encapsulated in the enemy release hypothesis since the late nine-teenth century, with the aim of reducing the competitiveness of invasive alien weeds through the release of their coevolved natural enemies and thus restoring the balance of nature. However, this remained the domain of entomologists until the 1970s (McFadyen 1998), when plant pathologists first entered the field to immediate and spectacular effect (Cullen et al.1973; Burdon et al.1981). Never-theless, to most conservation policy-makers the CBC strategy is either unknown or viewed
with scepticism, bordering on hostility. As touched upon in the General Introduction on the management of invasive alien plants in the Gala´pagos Islands, the debate has centred pri-marily on an eradication policy versus the other extreme of ‘embracing’ them within the island ecosystems (Vince 2011). A third option, the use of CBC, has received little or no attention.
Ironically—in the light of this recent Gala´pagos con-troversy—it was Charles Darwin who first hinted at the controlling impact of natural enemies after observing at first-hand the invasiveness of alien plant species in island ecosystems during the voyage of the Beagle: “cases could be given of introduced plants which have become common throughout whole islands in a period of less than 10 years . . . . . the geometrical tendency to increase must be checked by destruction at some period of life . . . lighten any check, mitigate the destruction ever so little, and the number of the species will almost instantaneously increase to any amount” (Darwin1859, pp 118–119).
B. The Practice
Best practices must always be followed if this still-evolving field of weed management is to be accepted by both conservationists and farmers alike, but also, more crucially, by the decision-makers holding the purse strings. Because of its very nature—no saleable product, for example—
CBC is funded invariably by central or local government agencies. Thus, both political and public concerns need to be addressed. Fortu-nately, scientific and safety standards—until recently set and regulated by the CBC practi-tioners themselves—have been high; negative impacts have been minimal, and all were entirely predictable (Evans2000; Culliney2005). Unfor-tunately, in sharp contrast, the spurious and ill-considered attempts at biological control by non-specialists—typically, involving the impor-tation of generalist predators (snails, snakes and toads, for example, in a parody of the song ‘The woman who swallowed a fly’) and, therefore, doomed to failure and potential ecological dis-aster—are the ones that have most attracted press coverage and hence caught the public imagination. Thus, CBC—or, “the intelli-gent introduction of counter-pests” (¼ natural
enemies) (Elton 1958)—needs to be distanced from this form of unregulated, unintelligent, unscientific biological control and from the per-ception of the general public that it is a hazard-ous option for the management of invasive alien species, and of exotic weeds, in particular.
1. Selecting the Agents
Because only coevolved natural enemies should ever be considered for CBC, fungal pathogens must be sourced from the centre of origin or diversity of the exotic weed target. Invariably, these potential agents are obligate biotrophs—
most typically, rust fungi—although hemibio-trophs can also be, and, indeed, have been, taken into consideration (Evans2002a). Thus, the first step is a literature and herbarium survey—both botanical and mycological—in order to determine the natural distribution of the plant and thereby to delimit the target area for the field survey and agent collection, as well as to identify and collate any associated myco-biota. This is not always straightforward, since the plant host may be rare or of no economic interest in its native range, and so poorly-studied and collected. Typically, the plant spe-cies may have significantly more site and fungal records from its invasive than from its native range. This is exemplified by lantana weed (Lantana camara): as an exotic ubiquitous plant in India, 30 fungal pathogens have been recorded—some with genus or species-specific epithets, erroneously suggesting coevolutionary traits—whilst in its neotropical native range, only nine pathogens have been recorded follow-ing comprehensive surveys in Brazil (Barreto et al. 1995). These Indian fungi constitute an assemblage of non-specific opportunistic pathogens with no impact on the fitness of L. camara—hence, its invasive status—compared to the damaging biotrophic pathogens found in its natural range (see Fig.6.2d). Conversely, for some exotic plant species there is a complete absence or scarcity of published records of fun-gal pathogens, in both their invasive and natural ranges. As a bonus, this preliminary ‘desk-bound’ survey can often uncover rare and even new, previously unpublished records of fungal
pathogens before the field surveys even begin (Evans1987a).
An assessment of the data collated from the literature, herbarium, and field surveys can also provide useful indications as to the specificity of the associated fungal pathogens and the damage that they inflict on their hosts, from which early decisions can be reached concerning their potential as CBC agents. This infor-mation is also helpful to prioritise those agents war-ranting further study, or, in certain cases, to discontinue the programme if suitable agents are not identified.
2. Screening the Agents
Potential agents can be screened either in the country of origin—with no quarantine measures—or, in an intermediate country where the target weed is absent—with low-level quarantine—or, in the weed-affected country—
with high-level quarantine. Host-specificity test-ing is the most time-consumtest-ing and, thus, the most expensive phase of a CBC programme.
In parallel with this screening, the taxonomic position, life cycle, and infection parameters of the selected agent(s) need to be established.
Often, this is not a straightforward process because the pathogen may be taxonomically difficult or new to science (Evans and Ellison 2005), and, especially in rust fungi, the life cycle may be unique and the infection parameters challenging (Evans 1987b; Ellison et al. 2006;
Seier et al.2009).
The protocol that underpins specificity screening—and so provides the main data for the risk assessment—follows the centrifugal phylogenetic testing sequence, initially devel-oped for arthropods (Wapshere1974a,b), and is based on genetic relatedness, in contrast to earlier, more emotive ones that concentrated on threats posed to crop plants. Indeed, the very rigorousness of the test—for example, the frequency of false positives due to optimal infection conditions and artificially high in-oculum loads—even led to the claim that potentially beneficial agents could be rejected (Wapshere1989). Additional techniques can be used to better interpret, and give added value to the test results. A clear-staining methodology
(Bruzzese and Hasan 1983) has been used to identify resistance factors to the agent both on and within the challenged test plant species, and to clarify any ‘suspect’ symptoms, such as hypersensitive reactions (Evans and Tomley 1996; Evans2000).
3. Risk Assessment
Once a potential agent has been given the ‘all clear’, in the sense that it has demonstrated a sufficiently high level of specificity to the target weed so as not to pose a threat to non-targets in the country of release, a document—usually called a pest risk assessment (PRA)—is prepared detailing all the scientific data gener-ated from the field, greenhouse, and laboratory studies. This is presented to the relevant quarantine authorities in the receiving country and, after peer review (at the government, state, or even public level), the decision whether or not to import and release the agent is reached independent of the stakeholders. Additional testing—typically, to include rare and locally-unique plant species or to ‘tweak’ infection parameters—may be required at this stage.
Conflicts of interestinevitably occur, of course, that can delay or even lead to the abandonment of CBC programmes. The Australian Biological Control Act of 1984—based on socio-economic and ecological evidence and still the only example of legislation of CBC in the world (Sheppard et al. 2003)—was invoked specifi-cally to minimise the threats from lobby groups using the Rule of Law principle, such as bee-keepers. Conflicts continue, however (Chew2009).
C. Case Studies 1. Past: Updated
Several of the programmes covered previously (Evans 2002a) are updated and analysed in greater detail to reflect the progress made in this continuously-evolving specialist field.
In particular, the increasing ecological and eco-nomic awareness and the associated benefits of
CBC are highlighted, with a summary of the lessons learned from each case study.
a) Mistflower: Ageratina riparia (Asteraceae) One of the earliest CBC initiatives targeting the use of fungal pathogens, the mistflower story has been long and eventful. From its beginnings during the early 1970s in Hawaii (Trujillo 1985)—with the unexpected and highly suc-cessful control of an invasive weed by a CBC agent pertaining to a fungal family that, hith-erto, had been a ‘minor’ player on the plant pathology stage—there have been many twists and turns reflecting advances in the field to improve the science and safety of CBC. Thus, our knowledge of the taxonomy, biology, ecol-ogy, and pathogenicity of the fungal agent, the white smut Entyloma ageratinae (Entylomata-ceae, Exobasidiomycetes)—as well as of its impacts on and benefits to invaded ecosystems in Hawaii, New Zealand, and South Africa—has changed and expanded immeasurably since these pioneering days (Barreto and Evans 1988; Morin et al. 1997; Trujillo 2005; Barton et al. 2007; Barton and Fowler 2008; Waipara et al.2009; Heystek et al.2011). To aid identifi-cation of the mistflower fungus, the use of more advanced microscopy, such as SEM, was called upon (Barreto and Evans1988) and, of course, in more recent times, molecular systematics have come into prominence, which can help to resolve the kind of taxonomic controversy that initially plagued this CBC programme. Simi-larly, improvement of the host-range screening protocol, especially the quantity and quality of the plant test list has been a priority.
In the case of the mistflower fungus, for example, 44 plant species were tested in Hawaii; whilst for South Africa, a further 18 species were included and for New Zealand, an additional 34 species were screened (Barton2012). Moreover, the success story continues with the news that the white smut has ‘arrived’
recently in Australia—transported either accidentally or deliberately—and is already having a significant impact on mistflower populations in New South Wales (McFadyen, 2012, personal communication).
Lessons learned: the over-riding message from this success story is that even the most seemingly ineffectual natural enemy in its native range can exert significant control over its host in the exotic range, and that seemingly fragile, habitat-restricted indigenous plant species can become dominant and invasive in new ecosystems. Thus, mistflower is almost an endangered species in Mexico, proving difficult to locate and apparently confined to fast-flowing mountainous rivers in Veracruz State, with white smut appearing as a minor leaf disease (Barreto and Evans1988). Indeed, in a dictionary of plant pathology, Entyloma has only a short entry, concluding with the state-ment: “no serious diseases caused” (Holliday 1989); whilst there is only a passing mention of the genus in the most modern and comprehensive text on plant pathology (Agrios 2005). Therefore, perhaps more than any other, this CBC programme highlights both the dan-gers of the indiscriminate movement of plant species and the elegant yet simple solution to their management if they should ever become invasive. Finally, it also offers an insight into
‘natural control’by natural enemies, since it is one, if not the only documented example of an exotic plant species introduced together with a coevolved fungal pathogen. When it was first collected in Jamaica in the 1970s, both the fungus identified as a Cercosporella sp. and the host plant were thought to be indigenous (Leather 1967). However, almost certainly, mistflower was imported from its Mexican home as a living specimen shortly after its discovery in the mid-nineteenth century—
probably in the then-fashionable Wardian case—together with its pathogen, and escaped the botanical garden to become naturalised.
Such a scenario has been observed in the Central Highlands of Sri Lanka, where natural habitats around a botanical garden (Hakgala) are heavily invaded by mistflower (Author 1999, personal observation). However, in sharp contrast to Sri Lanka and other recipient countries, it never became an aggressive invader
in Jamaica, especially in the favourable upland ecosystems, because of the presence of the white smut: a perfect example, perhaps, of accidental CBC.
b) Rubber-vine: Cryptostegia grandiflora (Asclepiadaceae)
After nearly a decade of research (Evans2000), the coevolved rust Maravalia cryptostegiae from Madagascar was released into the north-ern region of Queensland in 1994–1995 against its woody climbing host Cryptostegia grandi-flora, described as the greatest single threat to biodiversity in tropical Australia (McFadyen and Harvey 1990). Preliminary impact assess-ments were promising (Fig.6.1c, d), leading to the confident prediction “that rubber-vine weed will be brought under substantial if not complete control within the next decade”
(Evans 2002a). Data from independent, long-term monitoring studies were released shortly
afterwards, which showed over 40 % reduction in weed populations with almost zero seedling recruitment (Vogler and Lindsay2002; Tomley and Evans 2004). This was followed up by an economic impact assessment—made possible because of the weed’s agricultural as well as ecological significance—which put the net benefit of the programme up to 2005 at over AU$ 230 million, with a cost:benefit ratio of 1:108 (Page and Lacey 2006). This makes it one of the most successful CBC weed pro-grammes in the long history of Australian CBC initiatives (Palmer et al. 2010), and, therefore, in the world. The same weed is also becoming problematic in Mexico (Rodriguez-Estrella et al. 2010) and Curac¸ao in the Netherlands Antilles (Author 2003, personal observation), and a sister species, C. madagas-cariensis, is having a major impact on biodiver-sity in north-east Brazil (Herrera and Major 2006; Alves et al. 2008; Fig. 6.1a, b). Since a tried, tested, and successful ‘technology’ for Fig. 6.1. (a) Flowers and fruit of rubber-vine weed
(Cryp-tostegia madagascariensis), Ceara´ State, NE Brazil—note the ‘clean’, pest-free foliage. (b) Rubber-vine weed form-ing dense stands under and growform-ing over the endemic
‘carnau´ba’ palm, Copernicia prunifera—an ecologically and economically important plant in the region.
(c) Impact of the biotrophic rust Maravalia cryptostegiae on rubber-vine weed (Cryptostegia grandiflora) in
northern Queensland (Australia), shortly after its release.
(d) Inset of the rust forming uredinial pustules on the lower leaf surface. (e) White asexual fruiting structures (acervuli) of the hemibiotrophic ascomycete, Sphaerulina (Phloeospora) mimosa-pigrae, on the giant sensitive plant, Mimosa pigra in its native Mexican range—the fungus has since been released in the Northern Territory of Australia
management of rubber-vine weed is already in place, it should be a relatively simple and inex-pensive process to transfer this to the affected regions; provided, of course, that stakeholders and officials in those countries share and buy into the CBC strategy.
Lessons learned: biotrophic fungi, such as rusts, have coevolved with their hosts over mil-lennia, and this is reflected in their complex life histories, intimately linked with and adapted to a particular host. Thus, for each potential CBC fungal agent of weeds, there is an immediate challenge: to sort out the taxonomy of an under-studied pathogen and to elucidate its life cycle. Indeed, the mistflower study also threw up some intriguing taxonomic and evo-lutionary dilemmas—such as the presence of pycnia in the Ustilagomycotina (Barreto and Evans 1988)—which have not been pursued since, probably because the subject (white smuts) and the context (weed pathology) are not in the mainstream of mycology. Invariably, more cutting-edge science is now required to solve these problems, providing new insights into fungal biology. Until this ‘academic’
phase of the CBC programme is completed to the satisfaction of the stakeholders, investment in the more applied phases—determining infection protocols, extended host-range screening—may not be forthcoming. For the rubber-vine rust, the challenge was especially daunting, since the taxonomy and life cycle of one of its closest relatives, coffee rust (Hemileia
phase of the CBC programme is completed to the satisfaction of the stakeholders, investment in the more applied phases—determining infection protocols, extended host-range screening—may not be forthcoming. For the rubber-vine rust, the challenge was especially daunting, since the taxonomy and life cycle of one of its closest relatives, coffee rust (Hemileia