Saville Statistical Consulting Ltd.

Lincoln, New Zealand

Saville Statistical Consulting Ltd.

Lincoln, New Zealand
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Nboyine J.A.,Lincoln University at Christchurch | Saville D.,Saville Statistical Consulting Ltd | Boyer S.,Lincoln University at Christchurch | Boyer S.,Unitec Institute of Technology | And 2 more authors.
Journal of Pest Science | Year: 2016

The effects of the association between grasses and fungal endophytes on orthopterans are very poorly studied although they are important grassland pests. Here, the endemic New Zealand weta, Hemiandrus sp. ‘promontorius’, and Festulolium loliaceum infected with Epichloë uncinata, were used to study the effect of endophyte-mediated resistance in grasses on this large orthopteran insect in the laboratory, and the effects of this interaction on the grass. The insect was presented with F. loliaceum with and without E. uncinata infection in no-choice and paired choice experiments. Other controls were Epichloë festucae-infected Festuca rubra and endophyte-free Lolium perenne. In no-choice experiments, persistent attempts by the insect to graze the endophyte-infected grasses (but promptly abandoning them) resulted in a significantly higher number of plants lost due to excision at their stems after the first bite (P = 0.004). The inability of affected grasses to compensate for the lost biomass resulted in a lack of significant difference between the dry biomass of endophyte-infected and endophyte-free controls (P = 0.206). However, in choice experiments, there was a preference for the endophyte-free controls when they were paired with the endophyte-infected grasses (P < 0.05). The current work shows that endophyte-infected grasses can sustain high plant losses when attacked by an orthopteran insect in the absence of an alternative food source. This contrasts other endophyte/herbivory experiments in which high herbivory occurs because chemical plant defences are at a low concentration or the endophytes have other non-toxin roles in the plant. © 2016 Springer-Verlag Berlin Heidelberg

Cripps M.G.,Lincoln University at Christchurch | Edwards G.R.,Lincoln University at Christchurch | Bourdot G.W.,Agresearch Ltd. | Saville D.J.,Saville Statistical Consulting Ltd. | And 2 more authors.
Plant Ecology | Year: 2010

Cirsium arvense (L.) Scop. (Californian, Canada, or creeping thistle) is an exotic perennial herb indigenous to Eurasia that successfully established in New Zealand (NZ) approximately 130 years ago. Presently, C. arvense is considered one of the worst invasive weeds in NZ arable and pastoral productions systems. A mechanism commonly invoked to explain the apparent increased vigour of introduced weeds is release from natural enemies. The enemy-release hypothesis (ERH) predicts that plants in an introduced range should experience reduced herbivory, particularly from specialists, and that release from this natural enemy pressure facilitates increased plant performance in the introduced range. In 2007, surveys were carried out in 13 populations in NZ (7 in the North Island and 6 in the South Island) and in 12 populations in central Europe to quantify and compare growth characteristics of C. arvense in its native versus introduced range. Altitude and mean annual precipitation for each population were used as covariates in an attempt to explain differences or similarities in plant traits among ranges. All plant traits varied significantly among populations within a range. Shoot dry weight was greater in the South Island compared to Europe, which is in line with the prediction of increased plant performance in the introduced range; however, this was explained by environmental conditions. Contrary to expectations, the North Island was not different from Europe for all plant traits measured, and after adjustment for covariates showed decreased shoot density and dry weight compared to the native range. Therefore, environmental factors appear to be more favourable for growth of C. arvense in both the North and South Islands. In accordance with the ERH, there was significantly greater endophagous herbivory in the capitula and stems of shoots in Europe compared to both NZ ranges. In NZ, capitulum attack from Rhinocyllus conicus was found only in the North Island, and no stem-mining attack was found anywhere in NZ. Thus, although C. arvense experiences significantly reduced natural enemy pressure in both the North and South Islands of NZ there is no evidence that it benefits from this enemy release. © 2010 Springer Science+Business Media B.V.

Cripps M.G.,Lincoln University at Christchurch | Bourdot G.W.,Agresearch Ltd. | Saville D.J.,Saville Statistical Consulting Ltd | Hinz H.L.,CABI Europe Switzerland | And 2 more authors.
Biological Invasions | Year: 2011

Introduced weeds are hypothesized to be invasive in their exotic ranges due to release from natural enemies. Cirsium arvense (Californian, Canada, or creeping thistle) is a weed of Eurasian origin that was inadvertently introduced to New Zealand (NZ), where it is presently one of the worst invasive weeds. We tested the 'enemy release hypothesis' (ERH) by establishing natural enemy exclusion plots in both the native (Europe) and introduced (NZ) ranges of C. arvense. We followed the development and fate of individually labelled shoots and recorded recruitment of new shoots into the population over two years. Natural enemy exclusion had minimal impact on shoot height and relative growth rate in either range. However, natural enemies did have a significant effect on shoot population growth and development in the native range, supporting the ERH. In year one, exclusion of insect herbivores increased mean population growth by 2. 1-3. 6 shoots m-2, and in year two exclusion of pathogens increased mean population growth by 2. 7-4. 1 shoots m-2. Exclusion of insect herbivores in the native range also increased the probability of shoots developing from the budding to the reproductive growth stage by 4. 0× in the first year, and 13. 4× in the second year; but exclusion of pathogens had no effect on shoot development in either year. In accordance with the ERH, exclusion of insect herbivores and pathogens did not benefit shoot development or population growth in the introduced range. In either range, we found no evidence for an additive benefit of dual exclusion of insects and pathogens, and in no case was there an interaction between insect and pathogen exclusion. This study further demonstrates the value of conducting manipulative experiments in the native and introduced ranges of an invasive plant to elucidate invasion mechanisms. © 2011 Springer Science+Business Media B.V.

Bourdot G.W.,Agresearch Ltd. | Hurrell G.A.,Agresearch Ltd. | Saville D.J.,Saville Statistical Consulting Ltd.
New Zealand Plant Protection | Year: 2011

A gel preparation of mycelia of the plant pathogen Sclerotinia sclerotiorum was applied to the pasture weeds Carduus nutans L., Carduus tenuiflorus L., Cirsium arvense (L.) Scop., Jacobaea vulgaris L. and Ranunculus acris L. Doses ranging from 0 to 1,000 μl gel/plant were applied as 10 or 50 μl droplets, one per leaf axil. Statistically significant responses to dose were evident in C. arvense, J. vulgaris and R. acris, leading to 90, 74 and 100% reductions in living tissue respectively at 200 μl/plant compared to the control 29 days after treatment. For C. nutans and C. tenuiflorus, where even low doses gave good control, the reductions (averaged over all doses) were 98 and 88% respectively indicating that these two annual thistles were exceptionally susceptible to this S. sclerotiorum formulation. For C. nutans, 1 litre of the gel would, if precision applied, be sufficient to control at least 20,000 plants, suggesting broad-acre use may be commercially viable.

Bourdot G.W.,Agresearch Ltd. | Bourdot G.W.,Saville Statistical Consulting Ltd | Saville D.J.,Agresearch Ltd. | Saville D.J.,Saville Statistical Consulting Ltd
Weed Technology | Year: 2010

Broad- host-range pathogens are appealing as candidates for commercial development as bioherbicides because of the wider market potential of a product that is effective against a range of weeds. But when these pathogens are able to spread in space (or time), a risk analysis is necessary. Here we test the hypothesis that a safety zone around a bioherbicide application site is adequate so long as it is wide enough to ensure that dispersing inoculum has diluted sufficiently that the density of inoculum occurring naturally in a susceptible crop is no more than doubled by the influx of bioherbicide spores. To this end the plant diseasepathogen inoculum density relationship using data from nine published experiments was modeled using the logistic equation. This revealed that a doubling of the natural spore density of a plant pathogen in the range of 103.4 to 106.7 spores/ml may generally be expected to result in unacceptable increases in disease in a susceptible crop. A doubling outside this range (<103.4 or >106.7) is less likely to do so. Therefore when the natural density of inoculum in a crop's environment occurs outside this range, an "acceptable" safety zone for the pathogen's use as a bioherbicide can in most cases be defined by the 11 ratio of addednatural inoculum. However, if a more "risk averse" safety zone is desired, it can be defined using a 110 ratio of addednatural inoculum. © 2010 Weed Science Society of America.

James A.,University of Canterbury | Brown R.,University of Canterbury | Basse B.,Agresearch Ltd. | Bourdot G.W.,Agresearch Ltd. | And 3 more authors.
Ecological Modelling | Year: 2011

Optimising the management of invasive plants requires the identification of the population size outcomes for alternative management strategies. Mathematical models can be useful tools for making such management strategy comparisons. In this paper we develop a generic landscape meta-population model and apply it to the weedy grass, Nassella trichotoma, an invasive species occupying approximately 800 land parcels, predominantly pastoral farms, in the Hurunui district, North Canterbury, New Zealand. Empirical evidence reveals that this meta-population is currently stable (at a median density of 6plantsha-1) under a community strategy requiring manual removal (termed 'grubbing') of plants annually from all land parcels. Reduction in population size requires an alternative management strategy. Field data, collected over a 12 year period, were used to provide stochastic parameter values for land parcel size, carrying capacity, rates of local population growth and grubbing.The model reveals that at steady state, the most significant contribution to population growth on a land parcel comes from within the land parcel itself; the expected annual per capita growth on an individual land parcel is 4 orders of magnitude greater than the expected annual contribution from plants arising from other land parcels. This result implies that many of the farms currently supporting N. trichotoma may pose little or no threat to, nor are threatened themselves by, other farms infested by the weed. However, the steady state distribution (of the weed's population density) was sensitive to the spread rate, revealing a need for data on this process. It was also sensitive to how any increase in the grubbing rate is distributed; increasing it via a uniform distribution U(0, 1) where all rates between 0 and 100%year-1 are equally probable did not affect the steady state, whereas increasing the rates via the uniform distribution U(0.25, 0.75) resulted in fewer farms with high population densities. In general the model provides a basis for exploring the effects of a wide range of alternative grubbing strategies on population growth in N. trichotoma. © 2010 Elsevier B.V.

Lamoureaux S.L.,Agresearch Ltd. | Basse B.,Agresearch Ltd. | Bourdot G.W.,Agresearch Ltd. | Saville D.J.,Saville Statistical Consulting Ltd
Weed Research | Year: 2015

The weedy grass Nassella trichotoma (nassella tussock), historically an economically damaging invader of modified tussock grasslands in New Zealand, currently causes little if any reduction in farm production. This is a result of successful historical regional management programmes in which plants have been removed manually (by grubbing) each year before they seed. To inform a debate about the need for ongoing regional management, we developed a stage-structured spatially explicit integrodifference equation population model and linked this to a cost analysis. We used the model to compare the weed's future population trajectories and related regional control costs over 50 years under three alternative management scenarios. The total discounted (3% p.a.) costs of no management, three-yearly grubbing and continued annual grubbing were NZ $417 million, $736 million and $131 million respectively. These analyses indicate that annual grubbing of N. trichotoma returns a net benefit of $286 million ($417 - $131 million) compared with doing nothing and a net benefit of $605 million ($736 - $131 million) compared with a 3-yearly grubbing programme. These results support the continuation of annual grubbing as the long-term economically optimal management strategy for N. trichotoma on pastoral farms infested by the weed in New Zealand. © 2015 European Weed Research Society.

Bourdot G.W.,Agresearch Ltd. | Hurrell G.A.,Agresearch Ltd. | Saville D.J.,Saville Statistical Consulting Ltd.
New Zealand Journal of Agricultural Research | Year: 2014

To determine the peak cover of Cirsium arvense (Californian thistle) on pastoral farms throughout New Zealand (NZ), a postal and telephone-based questionnaire survey of 600 pastoral farms using a stratified-random sampling procedure was conducted. Three key results emerged from the analysis of data received from 502 responding farms: (1) overall, 89% of NZ farms support C. arvense (79%, 87%, 85%, 97% and 94% for beef, dairy, deer, sheep and sheep/beef farms, respectively); (2) the weed is spread across the entire farm in 45% of cases (35%, 40%, 56%, 50% and 53% for beef, dairy, deer, sheep and sheep/beef farms, respectively); (3) the peak percentage grazed area covered by C. arvense is overall 6% (4%, 6%, 6%, 12% and 6% for beef, dairy, deer, sheep and sheep/beef farms, respectively). This analysis provides the first quantitative information on the occurrence and cover of C. arvense on pastoral farms throughout New Zealand. © 2013 The Royal Society of New Zealand.

PubMed | Saville Statistical Consulting Ltd and Lincoln University at Christchurch
Type: | Journal: Insect science | Year: 2016

The efficacy of different combinations of under-vine and inter-row treatments for managing a soil-dwelling orthopteran pest, weta (Hemiandrus sp.), in vineyards was investigated over two seasons. This insect damages vine buds, thus reducing subsequent grape yield. The under-vine treatments comprised pea straw mulch, mussel shells, tick beans (Vicia faba Linn. var minor (Fab)), plastic sleeves on vine trunks (treated control) and control (no intervention), while inter-rows contained either the existing vegetation or tick beans. Treatments were arranged in a randomized complete block design with 10 replicates. Data were collected on weta densities, damage to beans and components of yield. The latter were numbers of bud laid down per vine, shoots per bud, clusters per shoot, grape bunches per vine, bunch weight and yield. The under-vine treatments significantly affected all variables except the number of shoots per bud. In contrast, none of the variables was significantly affected by the inter-row treatments or their interaction with under-vine treatments, apart from weta density. At the end of the experiment, weta density in the shell treatment was c.58% lower than in the control. As a result, there was c.39% significant yield increase in that treatment compared to the control. Although the under-vine beans and sleeves treatments increased yield, there were no reductions in weta density. With under-vine beans, the insect fed on the bean plants instead of vine buds. Thus, yield in that treatment was c.28% higher than in the control. These results demonstrate that simple agro-ecological management approaches can reduce above-ground damage by soil-dwelling insects. This article is protected by copyright. All rights reserved.

Saville D.J.,Saville Statistical Consulting Ltd
Agronomy Journal | Year: 2015

Multiple comparison procedures (MCPs), or mean separation tests, have been the subject of great controversy since the 1950s. Essentially, these procedures are an attempt at simultaneously formulating and testing pairwise comparison hypotheses using data from a single experiment. An unacceptable operating characteristic of most MCPs is their “inconsistency,” an idea that is illustrated in this article. This characteristic led to the development of a “practical solution” to the MCP problem, which is to “cut the Gordian knot” by abandoning any attempt at simultaneous formulation and testing. Instead, I recommend using the simplest multiple comparison procedure, the unrestricted least significant difference procedure, to (i) formulate new hypotheses at a known “false discovery rate” (in the null case) such as 5%, and (ii) independently test interesting new hypotheses in a second experiment. I also discuss the implications for sample size calculations of the choice of MCP. © 2015 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved.

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