EVOLUCIÓN HISTÓRICA DE LA PSICOMOTRICIDAD

i) cultivated plants botanically related to the target weed It is unclear why this is included as a safeguard, as it is merely an extension of the phylogenetic selection principal applied to a subset of plants. If the cultivated plant is closely related to the target weed, this is ample justification for choosing it under component A of Wapshere’s (1974) selection process. Indeed, it makes sense, from a risk assessment perspective, to select economically or ecologically important plant species from among the many phylogenetically equidistant possibilities, rather than obscure species.

ii) cultivated plants for which there is little or no entomological or mycological knowledge

This criterion is to some extent self-contradictory. If such plants are related to the target-weed they would be selected under the previous criterion. If not related, inclusion of them would imply that the phylogenetic selection principal itself may be erroneous and would

The centrifugal phylogenetic method used to select plants for host-specificity testing of weed biological contr

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cast doubt on the whole process. In practice, criteria i) and ii) have generally been lumped together, as there are extremely few examples of cultivated plants that have a poorly known entomofauna or mycoflora. iii) cultivated plants which have evolved apart, or which for geographic or climatic reasons have not been extensively exposed to the candidate agent (uncertainty of behaviour of agent faced with new potential host) Recent evidence suggests that, where related plants have evolved apart, they are less likely to serve as hosts, because of divergent selection in characteristics that provide host-cues to the insects. The example of the genus Heliotropium mentioned earlier, in which Australian “Heliotropium” that had evolved apart were not recognised as host plants by a specialist phytophage of South American Heliotropium species, illustrates this. Moreover, endemic African Solanum-feeding insects do not appear to have colonised the introduced weed, Solanum mauritianum in South Africa to any great extent (Olckers & Hulley, 1991), providing complementary evidence that the evolution of geographic separated plant species is more likely to lead to divergent evolution of their specialist natural enemies and reduced risk of subsequent colonisation by these should they encounter the novel congeneric non-target species in a new habitat. If the agent were not a specialist, then this would be discovered as a consequence of normal testing under component A of the CPM. In either case, it is not clear why such safeguard species need to be included. iv) cultivated plants known to be attacked by organisms closely related to the candidate agent

It is a requirement of the importation permit to provide documentation of the host-range of close relatives to the candidate agent. However, the usefulness of this information for selecting test plants is not very clear, since there are many large insect genera which contain species that specialise on one or a few plants, but in quite different plant families, e.g. Chrysolina (Garin et al., 1999) and Longitarsus (Dobler, 2001). In the case of Chrysolina, a well-known source of successful biological control agents for Hypericum perforatum, this criterion would have required testing candidate agents against species of Asteraceae, Apiaceae, Scrophulariaceae, Plantaginaceae and Lamiaceae, without changing the ultimate test result that the insects were specific to the genus Hypericum (Clusiaceae).

The phylogenetic data indicate that the host associations of “close relatives” are not particularly informative for the selection of test plants, as these large insect genera contain several lineages that often correlate with shifts to new plant families. Moreover, the host associations

of congeneric species within the same phylogenetic lineage are more likely to reinforce evidence of host plant restriction, as Wapshere (1974) himself realised when playing down the importance of this criterion. It is clear, though, that a phylogenetic approach, rather than reliance on taxonomic circumscription, is equally important in the case of the phytophagous insects themselves as for their host plants.

v) any plants on which the candidate agent has been previously recorded

It is essential to test the veracity of literature records, either by confirmatory field observations in the native range during the exploration phase or by experiment if these differ from current field observations. Many such records are associated with taxonomic literature and often involve adult collection records where the insect may not even have been feeding. Such information would normally be obtained prior to seeking approval for formal host-specificity testing. True polyphages would be eliminated at this stage, and stenophagous agents with unacceptably broad ranges would be detected thorough normal application of the centrifugal-phylogenetic method. Using safeguard species in practice

Most of the concerns of these safeguard criteria can be catered for by a strictly phylogenetically based test plant selection process. However, there remains the slight possibility that the species may be a disjunct oligophage, which would not necessarily be detected. Reported cases of such disjunct host-associations (e.g. Thompson, 1993; Gomez-Zurita et al., 2000) suggest that this may not be due to true host restriction, but is geographically linked and dependent on the availability of different host-plants. Such species may thus not be true host specialists and could have much broader fundamental than realised host-ranges. While this poses a concern, an ability to develop on a broader range of plants should be still detected by strict phylogenetically-based testing. However, in some insect groups, there is a closer phylogenetic association with particular plant chemistries, than with overall plant phylogeny (e.g. Becerra, 1997). If disjunction in host usage is due to such an association, one case for which a “safeguard” plant selection may still be required would involve agents from insect taxa known to be associated with particular plant chemistries. Ultimately, the value of the safeguard criteria must be judged by their effectiveness. How many potential biological control agents are there that have been rejected because of testing against a safeguard test plant, but would not have been rejected when tested solely against plant species selected on phylogenetic principles? This is a difficult question to answer as few cases of agent

The centrifugal phylogenetic method used to select plants for host-specificity testing of weed biological contr

ol agents: Can a

nd should it be moder

nised?

rejection are published. However, those published suggest that species selected uniquely on safeguard criteria have not played a role in rejection (e.g. McFadyen & Wegler- Beaton, 2000).

The case of Microthrix inconspicuella, a candidate agent for Emex australis (Polygonaceae), is instructive in this regard. This moth was originally rejected by Harley et al. (1979) because of significant larval feeding on apple (Rosaceae). However, subsequent testing by Shepherd (1990a) showed that M. inconspicuella could not complete its life-cycle on apple, though it was ultimately not introduced into Australia because the host-range within the Polygonaceae was found to be too broad (Shepherd, 1990b). In this case, testing the safeguard species, apple, proved to be a “red herring”, while selection of test plants based solely on phylogenetic relatedness would have been sufficient to show that the risk of introduction was unacceptably high.

Palmer & Tomley (1993) rejected a cerambycid beetle tested for the biological control of Baccharis halimifolia (Asteraceae) in Australia because, although oviposition and larval feeding were restricted to Baccharis, adults fed on green bark from a wide range of plants in several families including distant economic crops such as citrus (Rutaceae). However, the broad range in adult feeding was evident from plants tested within the Asteraceae, suggesting that strict phylogenetic testing would have raised the same doubts about the range of adult feeding. Adult feeding is much more restricted in the field in its native range (Palmer & Tomley, 1993), indicating that the issue here, and many ambiguous cases, is more to do with the types of tests used and interpretation of test data rather than the use or not of safeguard species. Recently, Briese & Walker (2002) carried out a statistical analysis of plant categories used for testing a candidate control agent for Heliotropium amplexicaule, and found that the key parameters measured (i.e. feeding, survival and oviposition of the chrysomelid beetle) decreased with increasing phylogenetic distance from the target weed, as expected, while the addition of safeguard species contributed no additional information about the host- range of the agent (Figure 1). The prevailing evidence therefore suggests safeguard criteria have little value in determining the risk of a candidate biological control agent to non-target species.

Figure 1. Median rankings of suitability as a host-plant for

Deuterocampta quadrijuga of test plants with increasing degrees of phylogenetic separation from the target weed, H. amplexicaule. Bars indicate the range in rankings and bracketed figures show the number of tested species in each grouping (after Briese & Walker, 2002). N.B. One of the two “Other Heliotropioidae” belongs to the genus Tournefortia, which has been found to be paraphyletic within the South American Heliotropium (Gottschling et al., 2001), and which may explain its higher suitability ranking.

Modernising the centrifugal