Keynote Presenter Release strategies in weed biocontrol: how well are we doing and is there room for improvement?
S.V. Fowler,1 H.M. Harman,2 J. Memmott,3 P.G. Peterson4 and L. Smith1
Summary A large number of factors associated with the agent and habitat can be manipulated when making a biocontrol release. Yet, it is only recently that an experimental approach has been taken, particularly with the release of arthropod weed biocontrol agents, for example examining factors such as release size to test ideas originating from theoretical or retrospective studies. For release size, both retrospec- tive analyses and experimental studies with arthropod agents usually show that larger releases have a higher probability of establishing. However, a limited number of large releases at a few sites also run the risk of chance extinction from locally major environmental effects. These risks can be minimized by releasing at many widespread sites, so there is often a trade-off between a few large releases and many smaller one. Unless these risks and relationships are known, a mixed strategy with arthropods, using a range of release sizes, is likely to be optimal at least at the start of a release programme. Re- cent theoretical attention has been applied to specific mechanisms creating lower individual fitness in small populations, termed Allee effects. It appears that Allee effects, including genetic inbreeding and problems in finding mates, may have been underestimated: They can be powerful, as witnessed by deliberate extinctions using sterile male release. Genetic issue with biocontrol releases, such as strain selection, inbreeding depression, drift and creation of laboratory-selected strains, have also received considerable, mostly theoretical, analysis. However, the actual rate of establishment success per agent species in weed biocontrol programmes is now 80–100%. These overall analyses do hide problems in establishing particular agent species: Programmes in New Zealand targeting ‘recalcitrant’ envi- ronmental weeds have been hampered by partial or complete failure to get potentially critical agent species established. A case study of heather beetle in New Zealand is used to illustrate the frustrations and research challenges in a release programme.
Keywords: release size, Allee effects, genetics of biocontrol releases, establishment rates, heather beetle.
Introduction until recently relied on retrospective analyses (Hall and Ehrler, 1979; Beirne, 1985; Simberloff, 1989; Hopper Classical biological control offers just about the only op- and Roush, 1993). Many of these studies called for a portunity to make multiple experimental releases of an more rigorous experimental approach in classical bio- exotic organism into a new environment. Despite this, logical control programmes, and recently, this call has most studies of the effect of different release strategies been heeded (Memmott et al., 1998; Grevstad, 1999a; Memmott et al., 2005), although there are usually con- straints in operational programmes because of the ad- 1 Landcare Research, PO Box 40, Lincoln, New Zealand. 2 Landcare Research, Private Bag 92170, Auckland 1142, New Zealand. ditional expense of experimental releases (McFadyen, 3 University of Bristol, School of Biological Sciences, Woodland Road, 1998). This paper reviews developments with release Bristol BS8 1UG, UK. strategies, for example research on the effect of release 4 Landcare Research, Private Bag 11052, Palmerston North 4442, New size on establishment success. Inevitably, our review Zealand. Corresponding author: S.V. Fowler
495 XII International Symposium on Biological Control of Weeds release strategies with arthropod agents. However, Small populations are perceived as vulnerable to many of the issues also affect pathogen agents, and poor performance that may result in extinction. If this we highlight areas where pathogen release strategies vulnerability is not a feature of large populations, then have received research attention. We also ask what the this is often termed an Allee effect, after this issue was establishment rate is for new agent releases in well- first raised by Allee et al. (1949). Allee effects could resourced biocontrol programmes and whether there is become particularly important if the population size room for improvement. Lastly, we use the release of drops because of say bad weather soon after the re- heather beetle in New Zealand to illustrate some of the lease. Allee effects could also cause, or be enhanced by, ecologically interesting challenges that a partially suc- a released population that fails to grow rapidly because cessful release programme can face. of poor individual performance. A corollary of this is that, for a biocontrol agent to be successful, any poor How to make releases of weed performance at low densities will need to be improved at higher densities. For example, if predation causes biological control agents? major problems for agent releases, then continued ef- In biological control releases, a multitude of factors fort with that agent might only be worthwhile if this can be varied, some of which may have a dramatic ef- predation is suspected to be an Allee effect (and thus fect on the probability of the agent establishing a self- will decline as an issue once populations rise above a perpetuating population (Day et al., 2004). Factors that certain threshold size). can be manipulated in biocontrol release programmes With any new agent species, there will be uncer- can broadly be divided into the following: tainty over the optimal way to release. Consequently, mixed strategies are probably best at the early stages 1. Agent stage/quality/release timing: Releases can of- of a release programme. Ideally, the effect on establish- ten be made at a variety of seasons, using a variety ment rates of various release strategies will be explored of life history stages of an agent (e.g. eggs, larvae, using rigorous experimental techniques (and/or adap- pupae and adults for insects, various spore types or tive management approaches), but political or funding hyphal suspensions for pathogens and releases using realities may interfere (McFadyen, 1998). Uncertainty entire plants for pathogens or insects). The number in optimal release strategies has been dealt with explic- of individuals, and their genetic make-up, is obvi- itly in modelling optimal release sizes (see below). ously open to manipulation, but these areas have re- ceived sufficient research attention to warrant their own section below. Optimal release size 2. Habitat/environmental quality: Release sites are se Retrospective analyses of arthropod agents show lected in biocontrol ‘invasions’, and apart from the that establishment rates improve with increased size requirement for the host weed to be present, many of releases (Hall and Ehler, 1979; Beirne, 1985; Sim- other factors may be important (including some berloff, 1989; Hopper et al., 1993), but even large re- form of climate match if the weed occupies a wider leases can be exterminated by unusually severe, but range of climatic zones than are optimal for the local, stochastic environmental effects such as floods, agent). Releases may be made initially into cages droughts or inadvertent weed control. Therefore, there or other protected environments. This may be to is a trade-off between making few large releases, and protect low initial release numbers from abiotic fac- risking chance local extinction, and spreading this risk tors, such as bad weather events, from biotic fac- (over time and/or space) among a larger number of tors, such as predators or competitors, or simply to smaller releases (Grevstad, 1999b). However, small prevent unwanted dispersal of mobile stages. Other populations are also clearly at much more risk of ex- mechanisms to discourage dispersal might involve tinction from chance events (both environmental and supplemental food for agents or treating the target demographic stochasticity) than larger populations. plants with fertilizer to improve the habitat. Demographic stochasticity refers to chance changes Given the ease with which many of these factors in say birth and deaths, which are unlikely to cause could be explored experimentally, it is surprising that extinction in anything other than very small popula- so little practical science has been done. However, re- tions. In contrast, Allee effects on small populations cent studies have experimentally varied release size are considered to involve specific mechanisms reduc- (see section below), and in other cases, the effect of ing individual fitness at small population sizes. The simulated rain (Norris et al., 2002) or rain protection most significant for biological control are likely to be (Hill et al., 1993) on releases has been quantified. There genetic inbreeding and loss of heterozygosity in small are also examples where different release strategies for populations leading to decreased individual fitness (see pathogens have been compared (e.g. Sheppard et al., the next section) and problems in finding mates when a 1993). Recent research, including quantitative field ex- population is small. In general, the importance of Allee perimentation, under the CRC for Weed Management effects has only recently been recognized (Courchamp in Australia has been reviewed by Day et al. (2004). et al., 1999; Stephens and Sutherland, 1999). Allee ef-
496 Release strategies in weed biocontrol: how well are we doing and is there room for improvement? fects can be powerful, as demonstrated by deliberate current release strategies for insects in New Zealand extinctions of pest species by mass release of sterile use a range of release sizes early in a programme. males, which induces major Allee effects in relatively This maximizes efficiency while reducing the risk of large populations (Courchamp et al., 1999). complete failure in case there is a currently unknown Simulation models (Grevstad, 1999b) suggest that threshold release size below which establishment is demographic stochasticity alone is not likely to be im- unlikely. As a result, a dataset of release size and es- portant in the establishment of biocontrol agents, while tablishment success across a range of species is slowly Allee effects and environmental variability are crucial. being accumulated. In a variable environment, a large number of very small releases will maximize the chance of overall establish- ment because environmental variability reduces the Genetics and releases of likelihood of establishment over the entire range of col- biological control agents ony sizes (Grevstad, 1999b). In contrast, when an Allee effect is present (in a constant environment), a single There have been several reviews on genetic issues large release is optimal because the presence of an Al- concerning rearing/culturing and releasing biocontrol lee effect results in a population establishment thresh- agents, concentrating mainly on arthropods (Mackauer, old. For colony sizes below the threshold, a population 1976; Roush, 1990; Hopper et al., 1993; Hufbauer and will become extinct, while those above the threshold Roderick, 2005). Genetic issues, which are acute in will establish (Grevstad, 1999b). small populations, include the loss of alleles through Memmott et al. (1998) investigated the relationship genetic drift and inbreeding depression, especially between release size and the probability of establish- where the mating of siblings increases the likelihood ment of gorse thrips, Sericothrips staphylinus Haliday, of homozygous deleterious recessive alleles. Another in New Zealand. A higher proportion of small releases genetic issue, not related to population size, is the pos- became extinct: Thrips were recovered from 100% of sible selection for laboratory-adapted populations, par- releases of 270 and 810 thrips but from only 33% of ticularly when colonies are maintained for too long in releases of 10, 30 and 90 thrips. This suggested that artificial conditions. the optimum release size for gorse thrips in New Zea- The significance of inbreeding depression in field land might be fewer than 100 (Memmott et al., 1998), populations has been contentious, but in a landmark in contrast to the previous strategy of 1000 thrips per study, Nieminen et al. (2001) found that the egg- release. hatching rate in laboratory experiments with the but- In a 5-year experiment in New Zealand, different re- terfly, Melitaea cinxia (L.), was significantly lower in lease sizes of the broom psyllid Arytainilla spartioph- egg batches laid by inbred females compared to cross- ila (Förster), were monitored (Memmott et al., 2005). bred females. In field studies, using released inbred Local extinction was greatest in the first year, and al- and crossbred populations, fewer inbred populations though the probability of extinction in the first year was survived after 1 year (Nieminen et al., 2001). related to release size, several releases of just one pair The only experimental test on how laboratory rear- of psyllids resulted in established populations. Similar ing affected a biocontrol agent that we could find used results were obtained by Grevstad (1999a) using two the geometrid moth, Chiasmia assimilis (Warren), a chrysomelid beetles in the United States. Thus, releases biocontrol agent against Acacia nilotica (L.) in Austra- of even very small numbers can result in establishment, lia. In this study, Wardill et al. (2004) reared a range of albeit with a higher probability of early extinction com- isofemale lines from several sites. From generation 3, pared with releases of larger numbers of individuals. they mixed the lines and sites, creating several control These conflicts were further investigated using a lines to mimic the ‘bulk-rearing’ typical of biocontrol modelling approach by Shea and Possingham (2000). programmes. At generation 8, they created replicate- They found that it was always possible to find one opti- mixed isofemale lines within and across sites. Genetic mal release size that was better than any mixed release variation was reduced in single isofemale lines in gen- size strategy but only if the relationship between estab- eration 8 compared with generation 1. However, the lishment rate and release size was known. Early in a within- and across-site hybrid lines maintained high release programme, it is better to use a range of release genetic variability. Fecundity in generations 8 to 11 sizes to help determine the optimum. However, spatial showed a similar pattern. Thus, hybridizing isofemale variability in suitability of release sites, if unknown a lines appeared to restore genetic variation and fitness priori, encourages a more risk-adverse release strategy (Hopper at al., 1993). However, ‘bulk-rearing’ over (i.e. more small releases). four to six generations also maintained genetic vari- Overall, the issue is of inherent uncertainty, whether ability and fitness, perhaps because of the minimum in defining an optimal release size or in the problems population size of 30 females and 30 males. likely to be encountered with spatial variation between Overall, the significance of genetic issues -in re sites or just plain bad luck in the stochastic events that leasing biological control agents remains unresolved. could cause local extinction of a release. Consequently, Crucially, we have little evidence that releases fail to
497 XII International Symposium on Biological Control of Weeds establish, or do well, because of genetic factors such as What establishment rates are strain/provenance selection, inbreeding depression or lack of adaptability. Experimental research during the programmes achieving? release and establishment of biological control agents There are different ways of measuring establishment could offer considerable insights. In the meantime, it rates, each subject to biases. For example, the quoted is probably prudent to follow some or all of the guide- establishment rates per weed biocontrol agent species of lines suggested by earlier comprehensive reviews (e.g. 71% (Julien et al., 1984) is a simple and probably quite Hopper et al., 1993) such as: robust statistic, but it ignores whether establishment of • Collect from a wide geographic range and match an agent has failed at different sites or within countries. weed biotype and agent pathotype/provenance where Rates per agent/country can also be calculated, with known. Julien et al. (1984) showing this to be 64%. This takes • Maintain separate lines in laboratory rearing (but into account different experiences in different countries release as hybrid mixtures). but again misses establishment failure in areas within • Rear for a limited number of generations in artificial countries. Rates of establishment per release appear conditions (e.g. no more than five). to be seldom known, although the specific release size • Keep rearing/culturing conditions similar to those in experiments discussed earlier are an exception to this. the field (e.g. fluctuating temperatures) to minimize Syrett et al. (2000a) also report that establishment rates adaptation to laboratory conditions. per release for nine agent releases across a range of • Avoid genetic bottlenecks or ensure that they are pasture weeds in Australia varied from 12% to 92%. short (one generation) and then expand the popula- In New Zealand, it is apparent that the effort put into tion rapidly to maintain heterozygosity. monitoring of any given agent can vary through time, Collecting from a range of sites to maximize the adding variability to overall establishment rates per re- genetic variability of agents has often been used with lease. In addition, if some strains were better than the arthropod biocontrol agents, but care is needed to en- others or local adaptation was important, then differ- sure that, for example, host range is consistent between ences in establishment rates might emerge over time source populations. In contrast, releases of pathogens within a biocontrol agent species. for biocontrol of weeds have typically used material In New Zealand, the overall establishment rate per from single lines in part because of unjustified con- species of weed biocontrol agent has increased from a cerns over the potential evolution of expanded host reported 44% (Cameron et al., 1993) to 76% (Fowler ranges (Morin et al., 2006b). As a result, the biocontrol et al., 2000; Syrett et al., 2000a). Indeed, if the estab- programme against blackberry, Rubus fruticosus L., in lishment rate is calculated for releases after the nation- Australia had, until recently, only used rust strains that wide technology transfer programme was set up in the were effective against a limited number of biotypes of 1990s, then it increases to 95% (Hayes, 2000). Another the weed (Morin et al., 2006a). Matching weed bio- region that has invested strongly in release and redistri- types and agent pathotypes is clearly a vital issue with bution strategies is Oregon, USA, where the establish- selection of pathogens as weed biocontrol agents (e.g., ment rate per species is 81% (McEvoy and Coombs, Morin et al., 2006a,b). With both arthropod and patho- 1999). If pathogens are considered separately, then the gens agents, there is a need to show that the tactic of establishment rate is 100% for both New Zealand and releasing genetically variable weed biocontrol agents Australia (Morin et al., 2006b). On the basis of these to encourage adaptation (e.g. to additional biotypes of statistics, it would seem that there is not a great deal of a target weed or to a wider climatic range) is not going room for improvement in establishment rates for agent to increase the risk of agents expanding their host range species for weed biocontrol in areas where substantial to include non-target plant species. Indeed, a range of effort is invested into release, redistribution and moni- studies outside biological control show that rapid evo- toring. However, failure to establish any agent species lutionary changes are possible (Orr and Smith, 1998; that is released in large numbers can be frustrating. In Thompson, 1998). However, the role of adaptation in New Zealand this has been the case with the sawfly, the success of biological control agents remains un- Monophadnus spinolae Klug., which is one of the few resolved and open to debate (Hufbauer and Roderick, adequately host-specific agents available for the seri- 2005). From retrospective studies, we can say that ex- ous environmental weed Clematis vitalba L. The spe- pansion of the host range of weed biocontrol agents as cies establishment rate can also hide examples where a result of evolutionary adaptation does not seem to establishment has been achieved but only at a poor rate have happened (van Klinken and Edwards, 2002), and, per release: The next section discusses such a case ex- for example, the substantial surveys to check for at- ample. Establishment per se is not the aim of weed bio- tack on non-target plants in New Zealand are showing control programmes, and it appears that despite high that host-range testing procedures dating back to the rates of agent establishment, the proportion of agents late 1920s were generally very reliable (Paynter et al., that appear to contribute to weed suppression remains 2004; Landcare Research, unpublished data). stubbornly low. For example, McFadyen (1998) gives
498 Release strategies in weed biocontrol: how well are we doing and is there room for improvement? agent establishment rates of 74% (in programmes that from releases made from captive-reared heather beetle were rated as successful overall), but a proportion of imported in 1995 is only 6% (Table 1). agents established and contributing to control of only The microsporidian pathogen was detected in one 55%. In New Zealand, establishment rates for all agents captive-rearing line in May 1999. All populations of released exceed 80%, but of those established, we rate heather beetle in captivity were then destroyed. At Te only around a third as contributing to weed suppression Piripiri, heather beetle numbers increased exponen- (Landcare Research, unpublished data). A similar pat- tially from 1999/2000 to 2001/2002 (Fig. 1), causing tern is emerging for pathogens, with 100% establish- severe damage to heather. We urgently redistributed ment in Australia, but only 33% of species contributing beetles from this localized, established population and to weed control (Morin et al., 2006b). also set up a secondary captive-rearing facility nearby. We trialled other release methods, including releasing larvae (in one case with a release size of 6000) and, in A case study: heather beetle one case, used a much larger number of releases of just releases in New Zealand ten adults per release. We also made three releases in heather infestations near Rotorua, about 130 km north Heather, Calluna vulgaris (L.), was deliberately intro- of Tongariro National Park, and at an altitude of 400 m duced in Tongariro National Park in 1912. It now in- (compared to 650–1300 m for sites in and around the fests 50,000 ha, including one third of the park. Heather national park). displaces native vegetation (Williams and Keys, 1994), Unfortunately, the releases in the national park us- and biological control is the only practical management ing beetles collected from the one site where beetles option in the park. The biocontrol programme (Syrett were causing severe damage to heather suffered from et al., 2000b) relied on just one agent species, the heather a similar very low establishment rate as the original beetle Lochmaea suturalis (Thompson), because this releases (Table 2). However, all three releases at Ro- beetle was such a well-studied species, which causes torua established and caused severe damage to heather major damage to valued indigenous heather in Europe within 2 years. Varying release parameters (±caging, (Cameron et al., 1944; Brunsting, 1982). In 1995, bee- early vs late summer, using larvae, release size) did not tles were collected from a range of sites in the UK to appear to improve establishment in the national park. ensure a climatic match to the national park (Emberson, After 2001/2002, the population numbers of beetles at 1986). A debilitating microsporidian disease was com- the Te Piripiri declined markedly (Fig. 1). This collapse monly found in heather beetles imported from the UK. in numbers occurred over an area of about 1 ha and To eliminate this disease, beetles in quarantine were included outlying heather patches that were not notice- reared from isofemale lines, with lines being destroyed ably damaged, so food shortage was not a viable expla- if sampled offspring were diseased (Smith et al., 1998). nation. The population of beetles was being carefully Lines were mixed before main set of 16 releases in the monitored, and this, together with a set of experimental national park in Summer 1997/98, but two releases in studies, showed that predation, parasitism or diseases summer 1995/96 were from one or two lines (second could not be implicated in the population collapse (Pe- generation after field collection) each from the single terson et al., 2004). Although we had done simple cli- geographic areas of Oakworth, England and Glencoe, mate matching before collecting and releasing heather Scotland. These first two releases were from lines that beetle, local weather conditions were the most likely ex- were doing particularly well, so beetles were avail- planation for both the collapse in numbers at Te Piripiri able without disrupting the progress of the main rear- and the dramatically improved establishment rate at ing effort. All release sites were sampled intensively the lower altitude Rotorua sites. The spring weather in for beetle adults or larvae at 1- to 2-year intervals. No the year when the beetle populations at Te Piripiri col- recoveries were made until summer 1999/2000, when lapsed was unusual, with what appeared to be a rela- five adult beetles and 20 larvae were found at Te Pirip- tively warm end to the winter in September, followed iri, a site that had received a covering of volcanic ash in by an exceptionally cold October/November with late June 1996, 5 months after the release. Despite intensive snow. Nearby weather station data confirmed that Oc- collecting efforts, no other recoveries have been made tober 2002 was the coldest since records began 18 years from any of these releases. Thus, our establishment rate previously (Peterson et al., 2004). We hypothesized
Table 1. The first releases of heather beetle in Tongariro National Park, using captive-reared beetles from the UK. Release season/year Number of releases Size of releases Number of releases established Summer 1995/96 2 250 1 Summer 1997/98 13 100–800 0 Summer 1998/99 2 250 0 Totals 17 5700 1 (6%)
499 XII International Symposium on Biological Control of Weeds
160 140 s le
et 120 be
t 100 ul
ad 80
of 60 er 40 mb
Nu 20 0 7 8 9 2 5 6 0 1 4 3 1996/0 1997/0 1998/9 2001/0 1999/0 2000/0 2004/0 2005/0 2003/0 2002/0 Summer Season
Figure 1. The numbers of adult heather beetles in a standardized sweep net sample at Te Piripiri. Numbers increased exponentially from the first detection in 1999/2000 until 2001/2002 but then crashed, ap- parently due to winter/spring weather conditions (further details in text).
Table 2. The second set of releases of heather beetle in New Zealand, using beetles redistributed and/or captive-reared from the first establishment site. Release region Release season/year Number of releases Size of releases Number of releases established Tongariro 2000/2001–2005/2006 49 10–6000 (total, 7500) 2 (4%) Rotorua 2000/01 3 250 (total, 750) 3 (100%)
that beetles emerging from overwintering and their off- just two isofemale lines, and one of these lines only spring would not be able to survive such major temper- contributed 29 beetles to the release. Unfortunately, ature fluctuations. In support, a Danish study reported we do not have preserved specimens of the original fe- that heather beetles had difficulty surviving winters males or of their offspring in subsequent generations. If with fluctuating temperatures, and individuals with the size difference is phenotypic, then we have to find smaller than average body size suffered particularly why. One hypothesis is that the leached volcanic soils high mortality (Jensen and Nielsen, 1985). When we typical of heather infestations in North Island, New measured the body size of beetles collected from Ton- Zealand, lead to nutritionally poorer heather for the gariro National Park or Rotorua and compared them to beetle. Increased soil nitrogen (e.g. from air pollution) field-collected beetles from a range of sites in the UK, has been implicated as a factor causing heather beetle to our surprise, we found that the New Zealand bee- outbreaks in Europe, resulting in conversion of valued tles were significantly smaller than those from the UK heather heathland into grassland (Heil and Diemont, (Peterson et al., 2007). Perhaps, small body size also 1983). As an experimental test of the importance of increases beetle vulnerability to prolonged winters; fur- nitrogen, we have been releasing heather beetle into thermore, winters in Tongariro National Park are longer paired plots at Tongariro National Park with or with- than those in Rotorua and Oakworth, England where out fertilizer application since Spring 2005. By sum- surviving beetles were sourced from (Peterson et al., mer 2006/2007, we had the second outbreak of heather 2007). Currently, we do not know whether this size dif- beetle in Tongariro National Park, which so far has de- ference is genetic or phenotypic. If genetic, then it is stroyed heather in an area of about 1 ha. It may be a co- likely to be caused by genetic drift (founder effects) incidence, but this outbreak started at the first fertilized or inbreeding depression. A recent check of rearing release site we set up. This outbreak has spread beyond records showed that the Te Piripiri release of 250 adult the small fertilized patch, so there could a threshold heather beetles was of second-generation beetles from effect whereby high beetle populations act to increase
500 Release strategies in weed biocontrol: how well are we doing and is there room for improvement? host plant nutritional quality as suggested for in Salvin- Cameron, A.E., McHardy, J.W. and Bennett, A.H. (1944) The ia molesta Mitchell biocontrol in the 1980s (Room and heather beetle (Lochmaea suturalis). British Field Sports Thomas, 1985). It may also be significant that the origi- Society, Petworth, UK, 69 pp. nal establishment site, Te Piripiri, was unique among Cameron, P.J., Hill, R.L., Bain, J. and Thomas, W.P. (1993) all release sites in receiving a heavy fall of volcanic Analysis of importations for biological control of insect pests and weeds in New Zealand. Biocontrol Science and ash in the winter after the release, which might have Technology 3, 387–404. produced a nutrient flush. It is now over 11 years since Courchamp, F., Clutton-Brock, T. and Grenfell, B. (1999) In- the first releases of heather beetle in New Zealand, and verse density dependence and the Allee effect. Trends in a multitude of research questions have emerged; plus, Ecology & Evolution 14, 405–410. to date, we have only suppressed heather over a minute Day, M.D., Briese, D.T., Grace, B.S., Holtkamp, R.H., Ire- percentage of the infested area. son, J.E., Sheppard, A.W. and Spafford Jacob, H. (2004) Improving release strategies to increase the establishment Conclusions rate of weed biocontrol agents. In: Sindel, B.M. and John- son, S.B. (eds) Proceedings of the 14th Australian Weeds Experimental biocontrol releases are being carried out Conference. Weed Society of New South Wales, Sydney, in some cases, after a plethora of papers calling for this. Australia, pp. 369–373. Emberson R. (1986) Factors limiting heather beetle popula- With arthropod agents at least, this has resulted in some tions. In: Green, P. (ed) Heather Control Workshop, Whaka- neat practical science to add to the range of retrospec- papa, Mount Ruapehu, Tongariro National Park, 1–2 May tive analyses and theoretical models aimed at optimiz- 1986. Unpublished report. Department of Conservation, ing release strategies. Multiple strategies to reduce risk Tongariro District, Turangi, New Zealand, 68 pp. of getting something wrong are the norm with arthro- Fowler, S.V., Syrett, P. and Hill, R.L. (2000). Success and pod agents, and commonsense and modelling both sug- safety in the biological control of weeds in New Zealand. gest this is a sensible approach. However, establishment Austral Ecology 25, 553–562. success rates per agent species are quite high, so in Grevstad, F.S. (1999a) Experimental invasions using biologi- terms of getting species established, this research may cal control introductions: the influence of release size on not help much (although some failed establishments the chance of population establishment. Biological Inva- can still be very frustrating). The major ‘log-jam’ ap- sions 1, 313–323. Grevstad, F.S. (1999b) Factors influencing the chance of pop- pears not to be establishment per se but getting agents ulation establishment: implications for release strategies that are effective at weed suppression. The key issues in biocontrol. Ecological Applications 9, 1439–1447. with release strategies (used in a broad sense to include Hall, R.W. and Ehler, L.E. (1979) Rate of establishment of provenance/strain and rearing/culturing) are those that natural enemies in classical biological control. Bulletin of might improve agent performance. These are probably the Entomological Society of America 25, 280–282. mostly genetic factors, such as strain/provenance se- Hayes, L.M. (2000) Technology transfer programmes for bi- lection, avoiding inbreeding depression, allowing for ological control of weeds – the New Zealand experience. sufficient genetic variability to allow for post-release In: Spencer, N.R. (ed) Proceedings of the X International selection (although its importance has yet to be dem- Symposium on Biological Control of Weeds. Montana onstrated) and ensuring that laboratory adaptation does State University, Bozeman, USA, pp. 719–727. not impinge on agent performance. Individual release Heil, G.W. and Diemont, W.H. (1983) Raised nutrient levels change heathland into grassland. Vegetatio 53, 113–120. programmes will nearly always begin with uncertainty Hill, R.L., Gourlay, A.H. and Winks, C.J. (1993) Choosing regarding the best way to make releases, and some of gorse spider mite strains to improve establishment in dif- the scientific challenges are illustrated by the case study ferent climates. 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