Simon V. Fowler

Are invasives bigger? A global study of seed size variation in two invasive shrubs.

We explored the spatial structure of seed size variation and tested whether seed size differed between native and exotic populations in two invasive species. Seed of Cytisus scoparius (Scotch broom) is significantly heavier in its exotic range, whereas seed of Ulex europaeus (European gorse) is no different between ranges. This result suggests that seed size in C. scoparius is either adaptively or phenotypically responsive to conditions in its exotic range or that plants with large seeds were preferentially introduced. We found that modern ornamental broom seed was no bigger than seed from natural or naturalized populations, suggesting that large seed size in the exotic range is not due to preferential introduction of ornamental varieties with large seeds. Most previous studies of trait differences between native and exotic ranges in invasive species have not taken variation throughout the ranges into account. This is the most comprehensive survey of seed size variation in any species, and the first time that variation in a trait of an invasive species has been studied from individual plant level up to global ranges. Demographic rates can be affected by seed attributes making this study an important first step in understanding how population processes may differ between native and exotic ranges.

Apparent competition can compromise the safety of highly specific biocontrol agents

Despite current concern about the safety of biological control of weeds, assessing the indirect impacts of introduced agents is not common practice. Using 17 replicate food webs, we demonstrate that the use of a highly host-plant specific weed biocontrol agent, recently introduced into Australia, is associated with declines of local insect communities. The agent shares natural enemies (predators and parasitoids) with seed herbivore species from native plants, so apparent competition is the most likely cause for these losses. Both species richness and abundance in insect communities (seed herbivores and their parasitoids) were negatively correlated with the abundance of the biocontrol agent. Local losses of up to 11 species (dipteran seed herbivores and parasitoids) took place as the biocontrol agent abundance increased. Ineffective biocontrol agents that remain highly abundant in the community are most likely to have persistent, indirect negative effects. Our findings suggest that more investment is required in pre-release studies on the effectiveness of biocontrol agents, as well as in post-release studies assessing indirect impacts, to avoid or minimize the release of potentially damaging species.

The effect of release size on the probability of establishment of biological control agents: gorse thrips (Sericothrips staphylinus) released against gorse (Ulex europaeus) in New Zealand

Many biocontrol agents released against alien weeds and pests fail to establish in the field. Here, we ask whether better release strategies could improve the likelihood of successful establishment. A manipulative field experiment was used to investigate the relationship between the probability of establishment and the number of individuals released for a weed biocontrol agent. In this experiment, replicated releases of 10, 30, 90, 270 and 810 gorse thrips, Sericothrips staphylinus Haliday, were made on to isolated gorse bushes in New Zealand. The sampling eY ciency was determined using a further experiment in which known numbers of thrips were released on to bushes. The data obtained showed that in approximately nine out of 10 releases of 10 thrips, at least one thrips would be found. The thrips in the size of release experiment were sampled 1 year after their release. A higher proportion of the small releases became extinct during this time period: thrips were recovered from 100% of the releases of 270 ...

Potential effects of climate change on biological control systems: case studies from New Zealand

Biological control systems are integral to New Zealand’s success as a nation reliant on exporting quality agricultural, forestry and horticultural products. The likely impacts of climate change projections to 2090 on one weed and four invertebrate management systems in differing production sectors were investigated, and it was concluded that most natural enemies will track the changing distributions of their hosts. The key climate change challenges identified were: disparities in natural enemy capability to change distribution, lack of frosts leading to emergence of new pests and additional pest generations, non-target impacts from range and temperature changes, increased disruptions caused by extreme weather events, disruption of host-natural enemy synchrony, and insufficient genetic diversity to allow evolutionary adaptation. Five classical biological control systems based on the introduced species Longitarsus jacobaeae, Cotesia kazak, Aphelinus mali, Microctonus aethiopoides and Microctonus hyperodae are discussed in more detail.

Does transmission of the rust pathogen, Puccinia punctiformis, require stem mining vectors?

Abstract Recent research in Europe has suggested that stem mining insects may be important for vectoring the pathogen Puccinia punctiformis, and largely responsible for its systemic infection in the weed, Cirsium arvense. However, here we present comparative survey data showing that the level of systemic disease is the same in Europe and New Zealand, with and without stem miners, respectively, casting doubt on the idea that these insects are necessary for transmission of the fungus.

Effects of pasture competition and specialist herbivory on the performance of Cirsium arvense

Abstract Combining specialist herbivory with interspecific plant competition can be an effective means of controlling pasture weeds. Cirsium arvense (Canada thistle, Californian thistle, creeping thistle) is one of the worst weeds of pastoral production systems in New Zealand (NZ). The oligophagous leaf-feeding beetle, Cassida rubiginosa, was recently released in NZ for control of C. arvense. To assess the impact of this biocontrol agent we conducted an outdoor potted-plant experiment with low and high densities of Cassida larvae combined with different levels of interspecific competition from typical NZ pasture species. Secondly, we carried out a field-release experiment to quantify the impact of high densities of Cassida under more natural conditions. Interspecific competition reduced all measured plant parameters of C. arvense except mean shoot height and base diameter. Herbivory by Cassida only reduced root biomass, and showed a weak additive response when combined with competition. All other measured parameters of C. arvense showed a substitutive response, with competition being the only factor having a significant impact on the weed. There were no significant synergistic interactions with competition and herbivory on C. arvense. Interestingly, the number of root buds per plant was significantly greater in the presence of herbivory by Cassida, suggesting that C. arvense may compensate for defoliation. Similar to the potted-plant experiment, Cassida had no significant effect on shoot growth and development in the field-release experiment. The results of this study indicate that competition from typical NZ pasture species is a more important factor than herbivory by Cassida, and unless Cassida reaches outbreak densities, it will likely have an insignificant impact on this weed.

Post-release investigations into the fi eld host range of the gorse pod moth Cydia succedana Denis & Schiffermüller (Lepidoptera: Tortricidae) in New Zealand

The gorse pod moth Cydia succedana was released in New Zealand as a biological control agent against gorse Ulex europaeus L. in 1992 and is now widely established. Post-release evaluations of the host range of C. succedana were undertaken using both laboratory assays and field collections on native and exotic plants related to gorse. Field surveys did not detect any attack on native New Zealand plant species. However, contrary to predictions based on pre-release host-range testing, several species of exotic Genisteae, including Scotch broom Cytisus scoparius, Montpellier broom Teline (Genista) monspessulana, and tree lupin Lupinus arboreus, as well as lotus Lotus pedunculatus (Loteae) growing in the vicinity of infested U. europaeus plants, were shown to be hosts of C. succedana in both the North and South Islands of New Zealand. Hypotheses to explain this unexpected non-target attack include a seasonal asynchrony between C. succedana and gorse fl owering phenology, or that the original biocontrol introduction accidentally consisted of either two cryptic species or two populations with different physiological host range.

Invertebrate fauna of boneseed, Chrysanthemoides monilifera ssp. monilifera (L.) T. Norl. (Asteraceae: Calenduleae), an invasive weed in New Zealand

Boneseed, Chrysanthemoides monilifera ssp. monilifera, is a serious invasive weed in New Zealand, particularly in coastal habitats throughout the North Island, northern parts of the South Island, and in the Port Hills near Christchurch. The invertebrate fauna associated with boneseed in New Zealand was surveyed in 1999-2000 as part of a biological control programme for the weed. Boneseed is attacked by a wide range of native and exotic insects but damage is usually not severe. Potential biological control agents that feed on foliage or seeds are unlikely to meet with significant competition from species already resident in this country. However, the combined effect of generalist predators, such as Argentine ants and spiders, and parasitoids could inhibit the effectiveness of some of the invertebrates that would be potential biological control agents. In New Zealand, most (if not all) herbivore niches on boneseed are under-utilised, and there is considerable scope for the introduction of specialised invertebrates that could reduce the vigour of this invasive weed.

Making weed biological control predictable, safer and more effective: perspectives from New Zealand

A persistent problem in weed biocontrol is how to reliably predict whether a plant that supports development in laboratory host-specificity testing will be utilized in field conditions, and this is undoubtedly preventing releases of safe and effective agents. Moreover, the potential for unanticipated undesirable indirect effects of weed biocontrol on ecological networks has raised concerns by policy-makers and the general public. The key to minimizing risks of non-target impacts is prioritizing candidate agents that are both host-specific and effective, such that the number of agents required to bring the weed under control is minimized. As a consequence both the weed and its biocontrol agents become minor components of the local biota. Here we review recent attempts in New Zealand to improve the predictive ability of host-range testing, to avoid potentially safe and effective agents being rejected. Research in New Zealand aimed at predicting whether an agent is likely to experience enemy-release (i.e. reduced parasitism and predation) could assist agent prioritization, potentially making biocontrol both environmentally safer and more effective.

Breaking and remaking a seed and seed predator interaction in the introduced range of Scotch Broom (Cytisus scoparius) in New Zealand

1. Introduced plants may initially experience enemy release, but some of those interactions may be reinstated through biological control. These cases provide opportunities to explore the dynamics of broken and remade consumer-resource interactions.