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Management of the Diamondback Moth and Other Crucifer Insect ...

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Predicting outbreaks <strong>of</strong> amigratory pest: ananalysis <strong>of</strong> DBMdistribution <strong>and</strong>abundance revisitedZalucki, M.P.SCHOOL OF BIOLOGICAL SCIENCES, THE UNIVERSITY OFQUEENSLAND, ST LUCIA 4072, QUEENSLAND, AUSTRALIA.m.zalucki@uq.edu.auFurlong, M.J.SCHOOL OF BIOLOGICAL SCIENCES, THE UNIVERSITY OFQUEENSLAND, ST LUCIA 4072, QUEENSLAND, AUSTRALIA.m.furlong@uq.edu.auABSTRACTOur estimates <strong>of</strong> <strong>the</strong> effect <strong>of</strong> climate on <strong>the</strong> worldwidedistribution <strong>and</strong> relative abundance <strong>of</strong> diamondbackmoth have been revised. Using <strong>the</strong> known limits <strong>of</strong> <strong>the</strong>species’ range, experimental observations <strong>and</strong> <strong>the</strong>known <strong>and</strong> inferred responses <strong>of</strong> <strong>the</strong> pest to temperature<strong>and</strong> moisture as <strong>the</strong> base data, we have parameterized aCLIMEX model to predict temporal abundance <strong>and</strong>spatial distributions <strong>of</strong> diamondback moth. We test ourmodel by comparing <strong>the</strong> predicted diamondback mothdistribution with its “known” distribution; for such amajor pest <strong>the</strong> latter is very poorly defined indeed. Wefur<strong>the</strong>r analyze changes in relative abundance amongyears due to variable climatic conditions, using a series<strong>of</strong> long-term diamondback moth population data setsfrom China <strong>and</strong> Engl<strong>and</strong> <strong>and</strong> associated wea<strong>the</strong>r data.Models such as this are crucial to our underst<strong>and</strong>ing <strong>of</strong><strong>the</strong> reasons for changes in DBM abundance. They arealso essential for <strong>the</strong> development <strong>of</strong> long-term pestpressure forecasts <strong>and</strong> allow us to begin to disentangle<strong>the</strong> effects <strong>of</strong> various management practices from <strong>the</strong>normal variation in abundance due to climate alone.KeywordsBioclimatic modelling, forecasts, prediction, populationoutbreaks, Plutella xylostellaINTRODUCTIONThe diamondback moth, Plutella xylostella L.(Lepidoptera: Plutellidae), or DBM, is <strong>the</strong> most widelydistributed major pest <strong>of</strong> crucifers. For such a majorpest surprisingly little research has been conducted onits long term population dynamics <strong>and</strong> <strong>the</strong> forecasting <strong>of</strong>outbreaks (but see Zalucki <strong>and</strong> Furlong 2008). For apest manager, being able to predict pest speciesabundance <strong>and</strong> distribution (‘pest pressure’), <strong>and</strong> itstiming <strong>and</strong> level, is crucial to both strategic planning<strong>and</strong> tactical decision-making (e.g. Maelzer et al. 1996).Phenological models, based on insect physiological timescales, have been relatively successful at predicting <strong>the</strong>timing <strong>of</strong> insect population peaks (e.g. Moh<strong>and</strong>ass <strong>and</strong>Zalucki 2004; Collier <strong>and</strong> Finch 2004) <strong>and</strong> may beuseful for strategic timing <strong>of</strong> sampling <strong>and</strong> controlmeasures. Forecasting pest levels is more problematicbecause many factors influence abundance (see e.g.Yonow et al. 2004; Zalucki <strong>and</strong> Furlong 2005;Muthuthantri et al. 2010). For all major pest species,effective <strong>and</strong> timely forecasts would be useful fordetermining insecticide budgets, hiring additional cropscouts or making strategic decisions on which crops toplant. Long-term forecasts <strong>of</strong> abundance would beespecially useful for major pest species such as DBM.Abundance can vary greatly among years. Outbreaks <strong>of</strong><strong>the</strong> increasingly insecticide-resistant DBM can makepest management difficult; a situation which can beexacerbated if <strong>the</strong>re are insufficient supplies <strong>of</strong>insecticide for distribution (Zalucki et al. 2009).Zalucki & Furlong (2008) analyzed <strong>the</strong> seasonalabundance <strong>of</strong> DBM in Gatton, Australia <strong>and</strong> relativelyrecent outbreaks <strong>of</strong> <strong>the</strong> species in <strong>the</strong> British Isles <strong>and</strong>developed a CLIMEX model which predicted <strong>the</strong>worldwide geographic distribution <strong>of</strong> DBM. Here werefine that model <strong>and</strong> make more specific predictions <strong>of</strong><strong>the</strong> poorly known geographic distribution for what issuch a ‘well-known’ insect pest. We predict <strong>the</strong> speciesseasonal phenology for a greater number <strong>of</strong> sites across<strong>the</strong> species range <strong>and</strong> present a simple analysis <strong>of</strong> longtermabundance data for Hangzhou, China. Wellconstructedmodels that realistically describe <strong>the</strong> effects<strong>of</strong> climate on abundance are not only useful forforecasting, but also aid in <strong>the</strong> interpretation <strong>of</strong>population changes <strong>and</strong> impacts <strong>of</strong> managementpractices on pest populations.If climate is <strong>the</strong> main determinant <strong>of</strong> where a species islikely to be found, we might expect a strong influence <strong>of</strong>climate variability or wea<strong>the</strong>r on <strong>the</strong> temporal variationin abundance at a given site (Zalucki <strong>and</strong> Furlong 2005,2008; Zalucki <strong>and</strong> van Klinken 2006; Lawson et al.2010). CLIMEX is generally used to predict <strong>the</strong>suitability <strong>of</strong> a site for a species based on long-termaverage conditions <strong>and</strong> estimated responses <strong>of</strong> <strong>the</strong>species to seasonal variation in temperature <strong>and</strong>moisture. If we have <strong>the</strong> long-term daily wea<strong>the</strong>r datafor a site (<strong>the</strong>se are <strong>the</strong> data on which a site’s climaticcharacteristics are based) we can use <strong>the</strong> estimatedspecies response to climate variables to infer <strong>the</strong>variation in suitability (fluctuations in <strong>the</strong> variousindices) at <strong>the</strong> site over time, based on <strong>the</strong> observedclimate record (see also Zalucki <strong>and</strong> van Klinken 2006).Essentially we generate a model <strong>of</strong> likely seasonalpopulation growth <strong>and</strong> mortality rates based on climatealone, using information on a species’ geographicdistribution <strong>and</strong> its seasonal phenology at given sites toderive <strong>the</strong> species responses to climatic conditions, or itsclimatic “niche”.MATERIALS AND METHODSThe rationale behind CLIMEX has been described manytimes <strong>and</strong> we do not repeat it in detail here (see e.g.Yonow <strong>and</strong> Su<strong>the</strong>rst 1998; Zalucki <strong>and</strong> Furlong 2008).8 AVRDC - The World Vegetable Center

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