The population dynamics of the mediterranean snail, cernuella virgata (da costa, 1778) (hygromiidae), in continuous-cropping rotations in South Australia

Abstract

The population dynamics of the mediterranean snail, Cernuella virgata (Hygromiidae), which is a pest of grain crops and pastures in southern Australia, was studied in three adjacent fields used for continuous cropping on Yorke Peninsula, South Australia, for 20 years. The cropping sequence was commonly a rotation of faba beans, wheat and barley. Population densities were measured every month for the first 4 years and thereafter only in autumn and spring each year. Autumn and spring were the respective breeding season for adults and the resultant juvenile population that infested grain harvests. Populations varied greatly in abundance between years and fields. Spring abundance was correlated, albeit weakly, with prevailing spring and previous autumn rainfall. The latter can thus be used as an early warning of harvests suffering from snail contamination. Snails grew largest in faba bean crops and were more abundant in the wheat crops that followed faba beans than they were in the subsequent barley crop. No evidence was found of increased snail abundance near field edges, which would suggest biased seasonal movements of snails between adjacent fields, as has been reported for other studies of C. virgata in different agricultural systems (interfaces between crops and pastures, pastures and woody/weedy roadside vegetation). It seems probable, in this case, that the similarities in vegetative form of adjacent crops provided limited stimulation for differential dispersal between fields. The frequencies of banded (darkly coloured) shells also did not vary with distance from field edges, suggesting that the fence posts and tall weeds that occurred at the interfaces between fields were insufficient to provide (cooler, above-ground) shelter in summer and enhance local survival of banded C. virgata relative to locations further into fields. © The Author 2012. Published by Oxford University Press.