Population dynamics and larval establishment of Sitona discoideus (Coleoptera: Curculionidae) in New Zealand lucerne.
Abstract
Population data on Sitona discoideus collected from young lucerne in Canterbury, New Zealand, in 1982-86 were analysed using life tables. Manipulated curculionid densities in 1983-84 allowed intra-seasonal and inter-seasonal relationships to be studied. In general, there was a high degree of density-dependent mortality, probably among the neonate larvae during their establishment in the root nodules. There was considerable variation in the pattern of larval survival in wet compared with dry seasons. In the latter, density-dependent effects occurred after little cumulative egg laying (750-2400 eggs/m2) compared with higher levels (11 770/m2) in the wet season. Under wet conditions, there was greater mortality among the later instars. Egg-laying in the field was modelled on laboratory-derived temperature/rate relationships and field air temperatures. Model predictions suggested that 30-50% of the maximum fecundity was reached, indicating some maladaptation. The range of feeding habits of Sitona spp. is reviewed with regard to the degree of dependence of the genus on root nodules.