Quentin Paynter

Biological control of Scotch broom: modelling the determinants of abundance and the potential impact of introduced insect herbivores.

Simulation and analytical models are developed for the European shrub Scotch broom Cytisus scoparius Link (Fabaceae). The simulation model is spatially explicit and allows us to explore not only changes in population size but also the proportion of ground covered by the weed. The simulation model incorporates spatially local density-dependent competition, asymmetric competition between seedlings and established plants, a seed bank, local seed dispersal and an age-structured established plant population. This model is designed to incorporate much of the known population biology of broom. The analytical models are simple approximations of the simulation. The basic model contains nine parameters: the probability a site is disturbed, p dist ; the probability a seed becomes a seedling, g; the probability a seedling survives the first year, s; the probability a seed is lost from the seed bank, d; the minimum age for reproduction, A min ; maximum plant age, A max ; seed production per site, F; the probability a seed is retained in the parental site, fh; and the probability a site becomes suitable for colonization after broom senesces, p so . We review published data on the demography of broom from studies around the world, and also present some previously unpublished data. These data suggest that broom in some exotic habitats can achieve higher fecundities and live longer than in its native range. Analytical approximations provide a good description of the simulation results over a wide range of biologically reasonable parameter values. Specifically, the analytical modekls work well when plants are long-lived or highly fecund. Analysis of the models indicates that when broom colonizes all suitable sites with probability one, that the fraction of sites occupied by broom is determined by only three parameters: the probability of disturbance, p dist the probability a site becomes suitable for colonization following plant senescence, p so ; and maximum longevity, A max . In exotic habitats, where individual broom plants can produce several thousand seeds, differences in these parameters are the most likely reason why broom populations are more weedy than in the native range. The impact of insect herbivores, which reduce plant fecundity, on broom abundance is explored for several environmental scenarios. This analysis suggests that potential biological control agents are most likely to have a substantial impact if the disturbance rate is high, plant fecundity is low, and seedling survival is low. Even herbivores that reduce seed production by only 75% can have a dramatic impact on broom abundance, in contrast to several published predictions. Extensions to the models to allow for arbitrary patterns of age-dependent senescence, and site-specific probabilities of disturbance are presented.

Worth the risk? Introduction of legumes can cause more harm than good: an Australian perspective

Weeds are serious threats to Australias primary production and biodiversity conservation. For example, a recent Australia Bureau of Statistics survey found that 47% of farmers across Australia have a significant weed problem. A literature review revealed that legumes represent a significant proportion of the national weed problem and most serious Australian legume weeds are exotic thicket-forming species that were deliberately introduced for their perceived beneficial properties, such as for shade and fodder, or even quite trivial reasons, such as garden ornamentals. The low economic value of the rangelands most of these species infest, compared with control costs, hinders chemical and mechanical control of these weeds, such that biological control, which takes time, is expensive to implement and has no guarantee of success, may represent the only economically viable alternative to abandoning vast tracts of land. We argue that, because the behaviour of an introduced species in a novel environment is so hard to forecast, better predictive techniques should be developed prior to further introductions of plant species into novel environments. We also discuss the potential of legumes currently being promoted in Australia to become weeds and suggest the recent trend of exporting Australian Acacia spp. to semiarid regions of Africa risks history repeating itself and the development of new weed problems that mirror those posed by Australian Acacia spp. in southern Africa.

Integrated weed management: effect of herbicide choice and timing of application on the survival of a biological control agent of the tropical wetland weed, Mimosa pigra

Abstract Herbivory by Neurostrota gunniella stunts mimosa (Mimosa pigra) plants, which should diminish seedling survival during wet-season floods, reduce competition with regenerating grasses and, therefore, reduce the need for repeated herbicide applications to control mimosa regenerating from the seed bank following clearance of dense stands. Few or no mimosa plants may survive the use of fire to control dense thickets, so N. gunniella populations will be unable to persist. Therefore, for damage to regenerating seedlings to be maintained over subsequent seasons, N. gunniella populations must be rapidly reestablished. Results presented in this manuscript indicate treating regenerating mimosa seedlings with herbicide does not necessarily prevent N. gunniella larvae from completing their development, and optimal timing of herbicide application during ground spraying operations can minimize the negative impact on N. gunniella. Delaying application until approximately 2 weeks from the first appearance of leaf mines should ensure that a high proportion of N. gunniella emerge and can infest any surviving mimosa nearby. The data do not indicate any major differences in the choice of herbicide and N. gunniella survival. However, 33% of seedlings treated with tebuthiuron survived.

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.

Invasive Alternanthera philoxeroides (alligator weed) associated with increased fungivore dominance in Coleoptera on decomposing leaf litter

Invasive Alternanthera philoxeroides (alligator weed), when controlled by biocontrol agent Agasicles hygrophila, is known to differ from native vegetation in its decomposition dynamics. This study investigated whether this difference would have indirect effects for fungal-feeding Coleoptera. The study tested the hypothesis that fungivores would be more abundant and species rich on A. philoxeroides than on native vegetation. The study also tested the hypothesis that fungivores would be more affected than other functional groups. The study was conducted in a northern New Zealand lake. Litterbags were placed beneath A. philoxeroides and two native sedge species (Schoenoplectus tabernaemontani and Isolepis prolifer). Coleoptera communities were examined from the litterbags. Fungivorous Coleoptera were more abundant and species rich beneath A.philoxeroides than beneath either sedge species. No other functional groups differed between cover types. Fungivorous Coleoptera also comprised a greater proportion of total Coleoptera catch from litterbags beneath A.philoxeroides cover than from beneath native sedge cover. Four of the six fungivorous Coleoptera species collected in the study were present beneath A.philoxeroides cover. Of these, the two most abundant species (on native, one exotic) were both from the family Corylophidae. Alternanthera philoxeroides invasion is thus associated with altered Coleoptera communities in this ecosystem.

Assessing the biosecurity risk from pathogens and herbivores to indigenous plants: lessons from weed biological control

Some potentially invasive herbivores/pathogens in their home range may attack plants originating from another geographic area. Methods are required to assess the risk these herbivores/pathogens pose to these plants in their indigenous ecosystems. The processes and criteria used by weed biological control researchers to assess the impact of potential biological control agents on a plant species in its non-native range provide a possible framework for assessing risks to indigenous plants. While there are similarities between these criteria such as the need for clear objectives, studies in the native range of the herbivore/pathogen, good knowledge of the ecology of the target plant and taxonomy of the plant and herbivore/pathogen, and modelling of the interaction between the two organisms, there are some important differences in approach. These include the need to consider the threat classification of the plant, the likely greater risk from polyphagous herbivores/pathogens than oligophagous or monophagous species, and the need to consider the impact of an additional natural enemy in conjunction with a suite of existing natural enemies. The costs of conducting a risk assessment of a herbivore/pathogen in another country that damages plants indigenous to another geographic area means that criteria will be needed for deciding which foreign herbivore/pathogen species should be assessed. These criteria could include the threat classification of the plant, the amount of damage to the particular plant organs affected, and the importance in key ecosystems.

Risk posed by the invasive defoliator Uraba lugens to New Zealand native flora

1 The risk posed to New Zealand native flora by the recently‐established pest of Australian origin Uraba lugens Walker (Lepidoptera: Nolidae) (gum leaf skeletonizer) was assessed. Weed biological control host range testing methods were applied to identify those New Zealand plant species potentially at risk. Native plants tested were primarily in the Myrtaceae, the family that contains all the Australian hosts of U. lugens. 2 Experimental methods included no‐choice larval feeding assays and field cage and laboratory oviposition trials. Difficulty in ascertaining reliable oviposition preference data from cage oviposition trials limited the confidence with which the invaders field host range could be predicted. 3 Field surveys of plants attacked by U. lugens in the infested area supported the initial predictions but only some of the at‐risk native Myrtaceae were present in the area. 4 The risk to native New Zealand plants is presented in terms of two mechanisms: development of self‐sustaining populations and temporary spill‐over of solitary larvae. Development of self‐sustaining populations of the pest within native forests is deemed to be highly improbable. 5 Temporary spill‐over impacts are most likely in urban areas within mixed species plantings or boundaries between native/exotic forests and coinciding with the mobile larval life stages. Spill‐over impacts from U. lugens have only been recorded to date on Metrosideros excelsa.

Effect of artificial shading on growth and competitiveness of Alternanthera philoxeroides (alligator weed)

Terrestrial Alternanthera philoxeroides is difficult to control with current tools. Shading as a control tool depends on the relative shade tolerances of the target weed and co-occurring species. This study examined A. philoxeroides’ shade tolerance with inter-specific competition from pasture species. In glasshouse conditions, shade and competition reduced A. philoxeroides growth but did not cause senescence. Grasses senesced in response to shade. In the field, competing species’ biomass reduced by over 90% in shade. A. philoxeroides biomass was unaffected by shade, contrasting with glasshouse results and resulting in increased dominance in shade. Connection to unshaded ramets in the glasshouse reduced some, but not all, of the differences between shaded and unshaded plants. Thus, resource sharing likely facilitated A. philoxeroides growth in the field, but may have been less influential than release from inter-specific competition. This research suggests shading may have limited use for controlling A. philoxeroides unless shade-tolerant inter-specific competition is provided.

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.