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Abstract
The genus Sitona Germar is associated with papilionoid legumes of the tribe Trifolieae, and Medicago spp. in particular. The larvae eat root nodules, and the adults eat the foliage. In 1975, populations of S. discoideus Gyllenhal were found in Hawke’s Bay and Canterbury. Since then they have spread through the lucerne-growing areas of New Zealand, and are often found in high densities. This study was initiated in 1976 to provide basic information (for integrated control procedures) should this species ever become a pest in New Zealand. Adult S. discoideus feeds on M. sativa L. (lucerne), annual medicks (e.g., M. polymorpha L. and M. lupulinus L.), and Trifolium striatum L. The morphometric and gravimetric characteristics of all stages are recorded. These data are extended to show that 1000 completed larval developments/m2 remove 5.7 kg N ha-1 per generation from their host crop. Larvae spend 90% of their development period in nodules. Data were collected after carrying out general observations and regular field sampling. Initially, the latter concentrated on the stages that live in the soil but went on to include foliage sampling and pit fall trapping, followed by a 4 year period of population monitoring. Sweep netting was calibrated with a D-Vac suction sampler so that adult densities could be discussed as number m-2. Populations of overwintering weevils can be reinforced in October and November by adults that developed from eggs laid during the previous autumn. However, most eggs were laid in November. Adults emerging in December and January were in a reproductive diapause and their gonads did not develop until March. Most eggs were laid during April to December. A small amount of oviposition was detected during the December-March diapause period. Adults undertook a dispersal flight in April–May. Migrating females were sexually mature and would have mated before they took off. This strategy is bionomically sound as a migration of tenerals could result in the isolation of unmated females. The combined effect of wing muscle development and gonadal maturation is likely to induce maximal adult feeding activity in April and May. Carabid, staphylinid, and elaterid larvae (and probably adults) eat larvae. Hippocampus coccinellae (Braconidae) parasitises adults at very low levels. Microctonus aethiopoides (Braconidae) is a potential biological control agent. S. discoideus is subject to protozoan and fungal pathogens. Sitona spp. have the bionomic characteristics of colonisers (r-strategists). S. discoideus has been able to proliferate in Australasia because its preferred hosts are available and often abundant throughout the year.
