LOCAL DISPERSION OF THE EUCALYPTUS LEAF BEETLE CHRYSOPHTHARTA-BIMACULATA (COLEOPTERA, CHRYSOMELIDAE), AND IMPLICATIONS FOR FOREST PROTECTION

1. Chrysophtharta bimaculata is a native chrysomelid species that can cause chronic defoliation of plantation and regrowth Eucalyptus forest s in Tasmania, Australia. Knowledge of the dispersion pattern of C. bi maculata was needed in order to assess the efficiency of an integrated pest management (IPM) programme currently used for its control. 2. Us ing data from yellow flight traps, local populations of C. bimaculata adults were monitored over a season at spatial scales relevant to comm ercial forestry: within a 50-ha operational management unit (a forestr y 'coupe') and between coupes. In addition, oviposition was monitored over a season at a subset of the between-coupe sites. 3. Dispersion in dices (Taylor's Power Law and Iwao's Mean Crowding regression method) demonstrated that C. bimaculata adults were spatially aggregated withi n and between coupes, although the number of egg-batches laid at the b etween-coupe scale was uniform. Spatial autocorrelation analysis showe d that trap-catches at the within-coupe level were similar (positively autocorrelated) to a radius distance of approximately 110 m, and then dissimilar (negatively autocorrelated) at approximately 250 m. At the between-coupe scale, no repeatable spatial autocorrelation patterns w ere observed. 4. For any individual site, rapid changes in beetle dens ity were observed to be associated with loosely aggregated flights of beetles into and out of that site. Peak adult catches (> the weekly me an plus standard deviation trap-catch) for a site occurred for a perio d of 2.0 +/- 0.22 weeks at a time (n = 37), with normally only one or two peaks per site per season. Peak oviposition events for a site occu rred on average 1.4 +/- 0.11 times per season and lasted 1.5 +/- 0.12 weeks. 5. Analysis of an extensive data set (n = 417) demonstrated tha t adult abundance at a site was positively correlated with egg density , but negatively correlated with tree damage (caused by conspecifics) and the presence of conspecific larvae. There was no relationship betw een adult abundance and a visual estimate of the amount of young folia ge on trees. 6. Adults of C. bimaculata are show n to occur in relativ ely small, mobile aggregations. This means that pest surveys must be b oth regular (less than 2 weeks apart) and intensive (with sampling poi nts no more than 150 m apart) if beetle populations are to be monitore d with confidence. Further refinement of the current IPM strategy must recognize the problems posed by this temporal and spatial patchiness, particularly with regard to the use of biological insecticides, such as Bacillus thuringiensis, for which only a very short operational win dow exists.