Population dynamics of the argentine stem weevil ( Listronotus bonariensis) in pastures of Waikato, New Zealand

During the period 1980–1984, a field and laboratory study was carried out on the population dynamics of the grass-infesting weevil, Listronotus bonariensis (Kuschel). Life tables were constructed for populations at 11 field sites in the vicinity of Hamilton, New Zealand, covering 45 generations. Populations were multivoltine. The stage-interval mortalities that contributed most to variance in population density were overwinter mortality of adults, variations in natality, egg losses and first larval instar mortality. Results of key-factor analysis and laboratory experimentation indicated that stabilization of population size was attained through density-related regulatory processes operating in overwinter mortality and competition for resource early in the life cycle. The three processes shown below were apparently involved. 1. (1) Pathogen-induced mortality of overwintering adults was density dependent, and led to reduced variation in density at the onset of the spring reproductive period. The fungal pathogen Beauveria bassiana (Balsamo) Vuillemin appeared to operate in an epizootic manner, and the protozoan Microsporidium itiiti Malone enzootically. 2. (2) Density-dependent regulation of recruitment could be mainly attributed to density-induced scramble competition amongst reproductive adults for tiller resource. High weevil numbers per unit resource, as measured by the number of available grass tillers free of infection by Acremonium lolii Latch, Christenson & Samuels, resulted in reduced fecundity per individual and shifts in oviposition behaviour. Aberrant oviposition behaviour in conditions of limited resource was associated with increased mortality of eggs and neonatal larvae. 3. (3) Increased larval combat and mobility apparently occurred amongst second instar larvae searching for favourable food resource. These processes resulted in population growth trends being dependent on the number of favourable grass tillers ( A. lolii-free) available per individual over the generation interval. The dynamics of the populations are discussed in relation to the practical aspects of damage control in grazed pastures.