Biocontrol introduction
Target pest: Clematis vitalba (Ranunculales: Ranunculaceae), old man's beard
Agent introduced: Phytomyza vitalbae (Diptera: Agromyzidae), old man's beard leaf miner
Imported:
1994, 1996
Import source:
Switzerland
Import notes:
Hill et al. (2001) - Phytomyza vitalbae were imported into New Zealand as pupae and reared for several generations in containment.
Gourlay (2007k) - Phytomyza vitalbae are native to Europe and were first imported into New Zealand from Switzerland by Landcare Research in 1994. The flies were mass-reared for release in the summer of 1996-97.
Landcare Research (2007a) - specimens of P. vitalbae from Switzerland, dated 1996, are present in the Biological Control Voucher Collection of the New Zealand Arthropod Collection [indicating an importation from that source in that year].
Released:
1996
Release details:
Landcare Research (1998c, 1999d, 2000c) - one release was made in the year to Jul 1998, 13 in the year Aug 1998 - Aug 1999, 11 in the year Sep 1999 - Aug 2000.
Gourlay et al. (2000) - since 1996, 42 releases of P. vitalbae have been made at 33 sites throughout New Zealand.
Hill et al. (2001) - Phytomyza vitalbae was first released at Lyttelton in the South Island, on 26 November 1996. Over the next 18 months, 28 further releases were made throughout New Zealand. Releases were as adult flies, 350-1,000 per site.
Gourlay (2007k) - released throughout New Zealand in the summer of 1996-97.
Establishment:
Landcare Research (1998a, 1998b) - surveys at release sites from the previous year to see if P. vitalbae has survived the winter showed it was thriving in Bay of Plenty, Gisborne, Hawke’s Bay, Manawatu-Whanganui and Wellington in the North Island, and Tasman, Marlborough, Canterbury, and Southland in the South Island, i.e. all old-man’s-beard-infested regions except Taranaki (North Island). The fly has been found up to 30 km from release sites.
Landcare Research (1999a) - in only 2 years P. vitalbae has dispersed from release sites at Nelson and Blenheim to colonise all old man’s beard infestations in between (>100 km). Similar rapid dispersal patterns are being noticed in the North Island too. Even small, isolated clumps of C. vitalba have not escaped attack. The number of mines per leaf seems to be increasing rapidly too.
Landcare Research (1999c) - Phytomyza vitalbae seems to be dispersing faster than any previous biocontrol agent released in New Zealand. It has been found near Greymouth on the West Coast of the South Island, over 200 km from the nearest release site, and in the North Island in Auckland, where C. vitalba is rare, the nearest major infestations being in Waikato and Bay of Plenty.
Landcare Research (1999e) - Phytomyza vitalbae is dispersing extremely rapidly; it is established in all C. vitalba-infested regions and is becoming common even on isolated infestations.
Gourlay et al. (2000) - Phytomyza vitalbae is now well established. Monitoring confirms that establishment had occurred at 24 sites and in some areas mined leaves have been found up to 200 km from the release point after just 20 months. Populations are building rapidly, and C. vitalba foliage can contain up to four mines per leaflet in some areas.
Hill et al. (2001) - by 1 April 1999, P. vitalbae had been found at 21 of the initial 29 releases sites [see Hill et al. (2001) entry in ‘Release details’ section above], and 20 of these populations are already considered viable. At the Ashburton (South Island) site, P. vitalbae had spread over 5 km from the point of release within 15 months.
Landcare Research (2002c) - in the short time since P. vitalbae was first released (1996) they have spread well and colonised C. vitalba infestations up and down the country.
Gourlay (2007k) - Phytomyza vitalbae soon established but is attacked by native and exotic parasitoids and so are now less common at sites in both the North and South Islands.
Impacts on target:
Hill et al. (2001) - three parasitoid species (all eulophid wasps) have been reared from P. vitalbae mines since its release. The dominant species, Proacrias sp., appears to be indigenous; the other two (Diglyphus isaea and Pnigalio soemius) are exotic and were deliberately introduced as biocontrol agents. It is not known whether parasitism will be a key factor in the population dynamics of P. vitalbae in New Zealand, but it seems unlikely given the fly’s reproductive capacity.
Landcare Research (2001e) - laboratory trials with small (5-15 cm high) C. vitalba plants showed even one insignificant-looking P. vitalbae mine per leaf was enough to reduce plant growth by 17%, suggesting that 2-3 mines per leaf would reduce growth by 50%. This suggests that the current levels of attack commonly seen in the field (1-2 mines per leaf) might be enough to reduce the vigour of C. vitalba, particularly small plants invading cleared areas. Its role in suppressing large plants is less certain, as they may be more resilient.
Landcare Research (2002c) - it is common to find one or two P. vitalbae mines per C. vitalba leaf throughout the country. Laboratory studies have suggested that one mine per leaf can reduce the growth of small plants by about 17%. Extrapolation of these results indicates that two or three mines per leaf might reduce growth of small plants by as much as 50%. How these results might translate to large plants in the field has remained a mystery until this autumn, when surveys in Marlborough (South Island) indicated what might be possible. Higher than usual levels of mining were recorded, around 10-15 mines per leaf. At one site there were 20-30 mines per leaf, host plants were stunted and unhealthy and opportunistic native plants were already beginning to take advantage of the reduced C. vitalba vigour.
Landcare Research (2002d) - one severely damaging outbreak of P. vitalbae has been seen so far.
Landcare Research (2003e) - last autumn serious damage to C. vitalba by P. vitalbae was reported from Marlborough in the northern South Island [see Landcare Research (2002c) entry above in this section]. Early last spring heavy C. vitalbae attack was observed on new growth at nearby Nelson, significantly impacting young leaves. Owing to mild winters in the Marlborough/Nelson region, C. vitalba does not lose its leaves and it is suspected P. vitalbae breeds all year round; in most parts of the country they overwinter as pupae, at which time it is suspected mortality may be quite high. Later in spring at the Nelson site P. vitalbae damage became less noticeable with a major C. vitalba growth spurt; however, damage to new growth early in the season is still likely to be having some impact on C. vitalba.
Landcare Research (2004e) - insecticide and fungicide exclusion trials at Blenheim (Marlborough, South Island) showed the biocontrol agents P. vitalbae and the fungus Longididymella clematidis were not having a significant effect on C. vitalba [see Paynter et al. (2006) entry below for details]. It is not clear why P. vitalbae is not performing as well as hoped, but at least four species of parasitoids have been reared from it in the field. Although these agents may not significantly affect mature plants, it is possible they may still have beneficial effects if they can affect the growth or survival of seedlings [see Landcare Research (2001e) entry above in this section].
Paynter et al. (2006) - insecticide and fungicide exclusion experiments were performed in the field near Blenheim in the South Island between September 2003 and April 2004 to determine the impact, both separately and in combination, of two biological control agents, P. vitalbae and a fungal pathogen, Longididymella clematidis, released against C. vitalba. Although both agents were common on non-treated plants, they damaged only a small fraction of the total leaf area, with most damage occurring in late autumn after the main period of stem growth. There was no impact on growth and only a minor reduction in percentage cover (8-10%) of non-treated plants. Most of the damage to leaves was caused by P. vitalbae rather than the fungus. Up to 25% of leaves of non-treated plants were mined by P. vitalbae in this study, compared to an average of 47% (range 6.4 to 96%) from surveys at Mangaweka (central North Island) in January 2004 and at 12 sites throughout New Zealand in 2005.
Landcare Research (2006b) - Phytomyza vitalba and the native leaf-miner P. clematadi appear to be sharing parasitoids. Parasitoids identified so far attacking both species are Neochrysocharis spp. (possibly two undescribed species and by far the most common attacking both leaf-miner species), Opius spp. (possibly two undescribed species and moderately common attacking both leaf-miners) and two introduced species, Diglyphus isaea and Pnigalio soemius. In addition, the parasitoid Proacrias sp. has been found attacking P. vitalbae but not P. clematadi. Parasitoid pressure on P. vitalbae may explain why it has not had the desired impact on C. vitalba.
Gourlay (2007k) - some damaging outbreaks have been observed, but the agent seems to be limited by parasitism.
Landcare Research (2008d) - a nationwide survey and literature review investigating the toll that parasitism is taking on weed biocontrol agents has found several agents, including P. vitalbae, for which parasitism levels of more than 60% have been measured. These agents are likely being adversely affected by such levels. Phytomyza vitalbae is parasitised by eight native species - mainly Neochrysocharis and Proacrias spp. [eulophid wasps], also Opius spp. [a braconid wasp] - and two exotic species, Diglyphus isaea and Pnigalio soemius [eulophid wasps].
Landcare Research (2016d) - while mining of leaves is now common, six native and two exotic parasitoids generally keep the leaf miner populations too low to impact on growth. However, damaging outbreaks do sometimes occur, e.g. heavy mining this autumn at Ashburton and on Banks Peninsula.
Paynter et al. (2018) - impacts have been trivial, with P. vitalbae heavily parasitised and predated by eulophid parasitoids (the adults of which feed on P. vitalbae larvae within leaf mines). Combined parasitism and predation rates average at least 58%.
Gourlay (2021c) - the effectiveness of P. vitalbae in New Zealand is not significant and limited by parasitism by six native and two exotic parasitoids. A laboratory study showed that 2-3 mines per leaf can reduce the growth of small plants by 50%. However, while some damaging outbreaks have been seen in the field anecdotal evidence suggests that the number of mines per leaf has remained low so the leaf miner is not having enough impact overall or early enough in the season to be useful.
Paynter (2024) - factors influencing the success of weed biocontrol agents released and established in New Zealand were investigated. Each agent’s impact on the target weed in New Zealand was assessed as ‘heavy’, ‘medium’, ‘variable’, ‘slight’ or ‘none’, where a ‘heavy’, ‘medium’ or ‘variable’ impact have all been observed to reduce populations or percentage cover of their target weed in all or part of their respective target weed ranges in New Zealand. Results showed that: (i) agents that are highly damaging in their native range were almost invariably highly damaging in New Zealand; (ii) invertebrate agents with a closely related ‘native analogue’ species are susceptible to parasitism by the parasitoids that attack their native analogues and failed to have an impact on the target weed, and (iii) agent feeding guild helped predict agent impact - in particular, agents that only attack reproductive parts of the plant (e.g., seed and flower-feeders) are unlikely to reduce weed populations. Damaging impacts of P. vitalbae, a defoliating fly, have not been reported in its native range, it does have a New Zealand native ecological analogue and its impact in New Zealand is assessed as ‘slight’.
Impacts on non-targets:
Gourlay et al. (2000) - host specificity of P. vitalbae to 40 species of plants, including eight Clematis species native to New Zealand, ornamental Clematis species and other species within the family Ranunculaceae, was tested in Switzerland. Oviposition occurred on the European species Clematis viticella, C. tangutica, C. montana, C. jackmanii and C. orientalis and on the New Zealand species C. marata, C. cunninghamii and C. foetida. Occasional mines were observed on other European Clematis species in natural populations in Switzerland. From field observations, and further tests in outdoor cages and in containment in New Zealand, it was considered that the risk of damage to New Zealand native Clematis species was low. The 1996 application for release of P. vitalbae into New Zealand made clear the potential for incidental damage to non-target Clematis species; permission to release was granted nonetheless. Since the release of P. vitalbae in New Zealand, ongoing systematic searches for non-target attacks have so far found the leaf miner at one isolated site on non-target, exotic Clematis species.
Hill et al. (2001) - host range testing was carried out in Switzerland and in containment in New Zealand. Only plants within the family Ranunculaceae were attacked, and only species within the tribes Ranunculeae and Anemoneae. In no-choice tests, feeding and oviposition occurred on a number of test species, mostly Clematis species [see Gourlay et al. (2000) entry above in this section], including several New Zealand natives, of which C. foetida was most severely affected. Feeding on C. foetida reached 10% of the level on C. vitalba and oviposition 14% of the level on C. vitalba. In choice tests, and outdoor cage and field tests, attacks on non-target plants reduced considerably or did not occur. In Swiss tests, with flies closely confined with the test plant, some eggs were also laid on two New Zealand Ranunculus species (the only species outside the tribe Anemoneae attacked) and one New Zealand Anemone species. However, in tests in New Zealand, in a larger experimental arena, these species did not attract feeding or oviposition. Phytomyza vitalbae has been recorded on other Clematis species in Europe; these host range tests confirm this species is narrowly oligophagous but poses no significant risk to New Zealand native plants. Previous work has showed that unless adult P. vitalbae could feed on C. vitalba, life was short, and fecundity was low, suggesting that occasional mines may occur on some exotic Clematis species in New Zealand, but probably only in the vicinity of C. vitalba.
Paynter et al. (2004) - surveys have recorded P. vitalbae attacking the native Clematis foetida at one site on Banks Peninsula (Canterbury, South Island). However, the attack was very minor and unlikely to impact C. foetida populations, and C. foetida at other sites was not attacked. The sporadic nature and low level of attack appears to justify the prediction from host-range testing that P. vitalbae may occasionally mine other Clematis species but can only persist on C. vitalba.
Landcare Research (2005a) - host range testing indicated P. vitalbae could complete development on several native Clematis species, although survival through to adulthood was extremely poor, except on C. foetida, which, in choice tests had an attack rate that was about 6% of that recorded on C. vitalba. Other tests showed that adult female P. vitalbae had poor survival and their ovaries did not develop properly unless they had first fed on C. vitalbae. It was predicted that some minor spill-over might occur if old man’s beard and native Clematis species, mainly C. foetida, were growing in close proximity. In extensive surveys, non-target attack has been found occasionally on C. foetida and twice on the native C. forsteri, but not on any other species. A survey this year looked at the role of the proximity of C. vitalba in non-target attack. In the North Island there were low levels of spill-over at two sites where C. vitalba and C. foetida plants were close together (within 200 m) but not at another four sites when they were more widely separated (6-50 km). The role of the proximity of C. vitalba was less clear-cut in the South Island, due to difficulties finding sites where C. vitalba and native Clematis were widely separated. Non-target damage was not routinely found at all C. foetida sites, and where there was damage the levels were low and inconsequential compared to the damage caused by the native leaf miner Phytomyza clematadi.
Landcare Research (2006b) - shared parasitoids between P. vitalbae and the native P. clematadi [see Landcare Research (2006b, 2008d) entries in ‘Impacts on target’ section above] raises the possibility of indirect non-target impacts of P. vitalbae through increased parasitism of native leaf miners living in close proximity to C. vitalba infestations. Preliminary data indicate that percentage parasitism of P. clematadi by the most commonly shared parasitoids (Neochrysocharis and Opius spp.) does decline with increasing distance from C. vitalba infestations. However, there is no correlation between the abundance of P. clematadi on its host Clematis foetida and the distance from the nearest C. vitalba plants, so if there are indirect non-target impacts going on there is no detectable impact on the abundance of the native leaf-miner.
Gourlay (2007) - Phytomyza vitalbae may cause slight damage to some of the ornamental Clematis species and has occasionally been recorded attacking native Clematis plants (mostly C. foetida, but also C. forsteri on one occasion). Because the females need to feed on C. vitalba before they can lay eggs, this non-target attack tends to occur on plants growing in close proximity to C. vitalba.
Paynter et al. (2008) - broad-scale surveys from November 2002 to August 2005 and quantitative surveys between March 2005 and mid- to late-February 2006 of native Clematis species recorded P. vitalbae attacks on one C. forsteri plant and 15 C. foetida plants. Most non-target attack occurred within 4 km of, and none over 30 km from, the nearest C. vitalba infestation. Trials showed that survival of P. vitalbae was low and oviposition did not occur on C. foetida unless flies had previously fed on C. vitalba, indicating that non-target attack was a ‘‘spill-over’’ effect that is unlikely to have a major detrimental impact on the non-target plants. Host-range testing prior to the release of P. vitalbae predicted this non-target attack but underestimated its prevalence. This study has also shown that spill-over non-target attack can occur at relatively large distances from the target plant, if the biocontrol agent is a good disperser. Additional ad hoc surveys of exotic ornamental Clematis showed the presence of P. vitalbae mines on C. connata C. orientalis, C. rehderiana, C. serratifolia and probably C. montana. Parasitoid rearing indicated that P. vitalbae shares parasitoids with a closely related native leaf-miner Phytomyza clematadi, raising the possibility of indirect non-target impacts through increased parasitism of P. clematadi living close to C. vitalba infestations. However, parasitism rates and abundance of P. clematadi mines on its host C. foetida did not vary with distance from the nearest C. vitalba infestation, indicating that the presence of P. vitalbae does not appear to affect P. clematadi through the presence of shared parasitoids.
Paynter (2008) - Phytomyza vitalbae will occasionally spill-over onto the native Clematis foetida (and on one occasion C. forsteri) but the damage is not significant. This non-target attack mostly occurs within 4 km of C. vitalba; further than normally expected for spill-over attack, owing to the exceptional dispersal abilities of P. vitalbae.
General comments:
Landcare Research (2024c) - a recent population genetics study identified five distinct C. vitalba genotypes throughout its native range in the UK and Europe, with varying prevalence in different regions. Remarkably, all five genotypes appear to be present in New Zealand, with different regions hosting a mixture of genotypes. This suggests that C. vitalba was introduced into New Zealand on multiple occasions from different sources, followed by deliberate spread across the country. The most prevalent old man’s beard genotype in New Zealand shares similarities with genotypes from the UK and Italy, while other genotypes correspond to those from Germany, Sicily and Serbia, with the least common one originating from other European countries. Identifying this genetic diversity in New Zealand presents both challenges to, and opportunities for, developing tailored biocontrol strategies that account for genotype-specific interactions.
References
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