Plant Protection Quarterly Vol.8(4) 1993 159 disturbance. Weed taxa that are closely al- lied to economically important plants or Overview and use of biological control in pasture indigenous plants e.g., Poaceae should not be excluded from consideration due situations to the high level of host specificity dem- onstrated by some organisms. Australian D.A. McLaren, Department of Conservation and Natural Resources, Keith native plants that are weeds outside their Turnbull Research Institute, PO Box 48, Frankston, Victoria 3199, Australia. natural range may also make suitable tar- gets for biological control, particularly where substantial geographical barriers Summary It has been estimated that ragwort exist between their introduced and native A number of successful biological con- populations have been reduced by range. The protocols for conducting such trol programs against pasture weeds are 60–70% through the combined actions of programs are not developed, but require discussed and related to the state of the the cinnabar moth, Tyria jacobaeae (L.), the attention before they should be under- programs in Australia. Factors that af- ragwort flea beetle, Longitarsus jacobaeae taken. fect the establishment of biological con- (Waterhouse) and the ragwort seed fly, The ultimate criteria for the successful trol agents and how biological control Botanophila seneciella (Meade) (Brown control of environmental weeds, regard- can be integrated into pasture manage- 1990). less of the techniques used, should be ment programs are discussed. Biological control of blackberry, Rubus measured as either the level of replace- constrictus Lef & M., has been successful ment of the target infestations with other How successful have biological in Chile where the rust fungus, Phrag- vegetation, or the level of protection pro- control programs been against midium violaceum (Shulz), has signifi- vided to uninfested vegetation by a reduc- pasture weeds? cantly reduced infestations (Oehrens and tion in the rate of spread. Control pro- There have been a number of examples of Gonzalez 1977). The rust fungus builds grams that fail to reduce the fitness of successful biological control of pasture up its innoculum over summer and heavy weed infestations to levels below the weed species around the world. St. John’s rust infestations defoliates blackberry ‘critical ecological threshold’ that allows wort, Hypericum perforatum L., occupied plants. This allows light to penetrate natural regeneration, or significantly re- more than 1 424 300 hectares in North through the blackberry thicket, enabling duce dispersal rates cannot be regarded as America (Tisdale 1976). Biological control seed from competing plant species to ger- successful. Where the target weed is re- was started in 1946 with the introduction minate and start growing up through the placed by undesirable vegetation with an of two chrysomelid beetles, Chrysolina blackberry. This gradually takes nutrients equal or greater weed status, limited suc- quadrigemina (Suffrian) and Chrysolina and light away from the blackberry until cess may be claimed but the contribution hyperici (Forster). Significant reductions it is displaced by other plant species. of the control program to the protection were observed by 1954 and St. John’s Musk thistle, Carduus nutans L. has or enhancement of biological conserva- wort now occupies less than 3% of its been successfully controlled by the wee- tion must be rated as negligible. In this re- original density over much of its former vil Rhinocyllus conicus Froel. in Virginia. spect, biological control programs for distribution in California. The beetles The weevil has reduced musk thistle den- environmental weeds should be incorpo- only feed on St. John’s wort in sunny situ- sity by 95% and six years after release the rated into integrated management plans ations, so its distribution has now been weevil had spread 32 km (Kok and Surles that aim to manipulate post-control suc- largely restricted to shaded locations. Bio- 1975). cession from weedy to native vegetation. logical control programs against St. Evaluation processes that record changes John’s wort have been most successful in Biological control of pasture weeds in vegetation composition and structure areas with summer droughts, where de- in Victoria and the reproductive fitness of the target foliated plants succumb to water stress Table 1 provides a summary of the cur- weed as well as the more traditional and die. It is still a problem in localities rent status of biological control projects measures of plant density, biomass and with summer rainfall such as British Co- against pasture weeds in Victoria. area of distribution are required to deter- lumbia in Canada, as the plant will regen- St. John’s wort has been partially con- mine success of environmental weed con- erate and recover after defoliation and its trolled by biological control in Victoria, trol programs. seed will germinate more readily especially in agricultural areas experienc- (Williams 1985). ing Mediterranean conditions. The best An excellent example of biological con- results have occurred when farmers have trol in Australia is the spectacular success improved their pasture management of the moth, Cactoblastis cactorum (Berg), practices by sowing strong competing in controlling prickly pear cactus, Opuntia pasture and by keeping it well fertilized stricta Haw. Prickly pear once occupied (Parsons 1957). It appears that the combi- more than 20 million hectares of pastoral nation of stress imposed by the biological and agricultural land in south west to control agents and the competition from central Queensland and was invading the improved pasture, displaces the new country at the rate of two hectares weed. St. John’s wort is still a serious every minute (Waterhouse 1978). Fifty weed in many areas of Victoria and re- one biological control agents were intro- search is currently being carried out on duced over a 22 year period (1913–1935) the host specificity of a fungus, Colleto- of which seventeen were released and 12 trichum gloeosporioides, and introduction established. C. cactorum was introduced of a mite, Aculus hyperici, for its control. in 1925 and had controlled most of the There are three insect species estab- prickly pear infestations by 1932 (Wilson lished on ragwort in Victoria. These are 1960). the ragwort leaf and crown boring moth, Another pasture weed, ragwort, Senecio Cochylis atricapitana (Stephens), and the jacobaea L., has been biologically control- ragwort flea beetles, Longitarsus flavi- led in Western Oregon in North America. cornis Ste. and L. jacobaeae. Thus far, the 160 Plant Protection Quarterly Vol.8(4) 1993 flea beetles have only established in high What are our chances of success The ragwort flea beetle, L. flavicornis is altitude, high rainfall locations. Where and what factors affect the likeli- another insect species that appears to be the flea beetles have established, they hood of successfully controlling a limited in its distribution by climate and have reduced ragwort infestations but pasture weed? insect strain. So far, L. flavicornis has only have spread slowly and taken a long time The overall figure on worldwide success established in high altitude, high rainfall to control the weed. The ragwort leaf and in biological control programs on weeds sites. Attempts were made to bring in sev- crown boring moth is building up at es- is 24% (Julien 1989). In Australia, it has eral other strains of this flea beetle from a tablished sites and is expected to start been estimated that biological control has range of climatic localities in Europe. having an impact on ragwort infestations saved in excess of $1 billion dollars However, this species has still only estab- (McLaren 1992). (Harley and Forno 1992). Factors that will lished in higher altitude/rainfall locali- An illegal introduction of a strain of the determine the likelihood of success are: ties. Another flea beetle species L. blackberry rust fungus, P. violaceum, was jacobaeae was introduced in 1988 and has found in Victoria in 1984 (Marks et al. Climate, host range, agent strain established at some lower altitude sites. 1984). This rust only attacks four of the Biological control agents have evolved Another way climate and insect strain nine blackberry species in Victoria but has with their host in their country of origin may impact on effectiveness of biological been shown to have a significant impact for many thousands of years. As a conse- control is shown by impact of the ragwort on these species (E. Bruzzese personal quence, many insects have become spe- seed fly, Botanophila jacobaeae, on ragwort communication). A strain of P. violaceum cifically adapted to particular sets of cli- infestations in New Zealand. There is that attacks the most widespread black- matic conditions and in many cases poor synchronization of emergence of the berry species (Rubus procerus) was re- formed strains that will only survive in seed fly and the flowering period of rag- leased in 1990 and observations suggest particular environmental locations. Out- wort, as it emerges approximately six that this could have a considerable impact side this climatic range, the species strain weeks before the peak flowering period, on blackberry populations in the next may not reproduce or complete their life resulting in 80–90% of ragwort seeds es- decade. cycle. It is therefore important that caping predation (Dymock 1987). The Biological control of Paterson’s curse searches are made for biological control milder climate in New Zealand enables re-commenced in 1988 with the release of agents in areas that have similar climatic ragwort to flower over a prolonged pe- the Paterson’s curse leaf miner, Dialectica conditions to those that the insect species riod. It may be possible to overcome this scalariella (Zeller). This insect is now es- will encounter when released on plant in- problem by importing seed flies from lo- tablished in the warmer parts of the state festations in Australia. cations in Europe that have similar cli- but has yet to have an impact on Climate may impact on establishment mates to those in New Zealand.