The seasonal biology ofSitona discoideusGyllenhal (Coleoptera: Curculionidae), an introduced pest of New Zealand lucerne

Abstract

Sitona discoideus Gylh., an introduced pest of lucerne in New Zealand, exhibits univoltine, aestivatory seasonality in Canterbury and Otago, with each generation appearing in late December. Some two weeks after the December emergence of adults, flights to aestivation sites commenced. These sites were commonly remote from the lucerne stands and included such places as under stones or at the base of trees, fence posts, etc. At the commencement of aestivation, the indirect flight muscles atrophied rapidly, only to redevelop some 6-8 weeks later when autumnal post-aestivatory return flights to lucerne began. During the winter, S. discoideus fed on lucerne foliage and became reproductively mature. By October, age-related adult mortality had begun, and for about two weeks in December no adult weevils were present at all in one Canterbury site. In Otago, there was a distinct generation overlap of 2-3 weeks. The eclosion of new generation weevils was closely synchronised; it is suggested that this may have been the product of differential egg development and egg-laying temperature thresholds and a high fourth instar and pupal develoment threshold. Synchronised emergence of adults tended to accentuate the problem of defoliation caused by their feeding.

A spatial model for the successful biological control of Sitona discoideus by Microctonus aethiopoides

“…Post‐aestivation dispersal occurs throughout March and April, with parasitized weevils returning earlier on average than healthy ones (Goldson et al . 1984).…”

“…1984). Thermal summation, using Darfield temperatures (New Zealand Meteorological Service data) and parasitoid development rates (Goldson et al . 1984), suggests that those parasitoid larvae present in the crop at the end of March should reach adulthood by May.…”

“…Post‐aestivatory weevils disperse widely in early autumn (Goldson & French 1983), and autumn adult densities in the crop peak in May when all surviving weevils have re‐established. Eggs are laid in the crop throughout winter, but particularly in early spring (Goldson et al . 1984).…”

Determination of the number of larval instars ofSitona discoideusGyllenhal (Coleoptera: Curculionidae) using probit analysis

“…In Canterbury, New Zealand, S. discoideus larvae are found in the soil from late September (Goldson et al 1984); therefore, at approximately weekly intervals during September 1983 to January 1984, larval and pupal populations were sampled from a one-year-old lucerne stand at Darfield, Canterbury. On each sampling occasion 25, 140 X 140 X 250 mm soil blocks were removed from within lucerne rows.…”

“…Although the biology of S. discoideus has been described by several authors (e.g., Allen 1971' Aeschlimann 1979Goldson et al 1984), the larval stages remain comparatively little studied. However, Kwong Sue et al (1980) have suggested from an incomplete data set, that there may be four laral instars.…”