A spatially replicated study of Argentine stem weevil Listronotus bonariensis (Kuschel) population dynamics was carried out in six dryland Canterbury, South Island ryegrass Lolium perenne L./white clover Trifolium repens L. pastures over three seasons between 1994 and 1997. The aims were to provide information on the weevil’s population dynamics in dryland environments and to determine population regulatory mechanisms. In dryland Canterbury pastures, L. bonariensis adults and larvae exhibit phenology similar to that reported elsewhere in New Zealand. Population density was generally low, probably reflecting the loss of endophyte-free ryegrass tillers as the pasture aged. Density-dependence relationships were found in both halves of the life cycle: 1) between densities of overwintered adults in spring and those of the next generation in summer; and 2) densities of adults over winter, from the summer peak to those remaining the following spring. Endophyte-free tiller density also played a part in determining summer adult abundance. Partitioning the first relationship into two components, spring (overwintered) adults to first generation summer larvae, and from those larvae to peak summer adults, showed density dependence in both, but an endophyte-free tiller effect only in the first. In all cases, endophyte-free tiller densities provided more significant contributions to predictive relationships for weevil densities than did total tiller densities, suggesting that weevil abundance is partly determined by the endophyte-free tiller resource in dryland habitats.
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