The biology and control of Riccadoella limacum (Schrank), a mite pest of farmed snails
The differences between the similar species, Riccardoella limacum and Riccardoella oudemansi we're examined using SEM techniques. The two species may be separated by differences in setal structure and number. The internal anatomy of R. limacum was examined using plastic sections for light microscopy and found to be similar to previous descriptions. Computer-aided three dimensional reconstruction was carried out on serial sections and the relative positions of various organs noted. The life cycle of R. limacum was confirmed as having a hexapod larva and three nymphal stages. The occurrence of an overwintering egg was observed in hibernating snail hosts and there was no relationship between hatching and snail hibernation. No such egg is present in R oudemansi, perhaps related to the death of their hosts during the winter. Mites were found to spread rapidly to colonise a new host and quickly laid numerous eggs, large numbers of mites have been seen to build up inside a snail. R. limacum was found to contain a substance which reacted with keyh6le limpet haemocyanin antibodies, this was likely to be haernocyanin. The mite obtains haemocyanin as a form of nourishment from the hosts blood, though the possibility that mucus is also a source of food cannot be eliminated in this study. The effect of different mite numbers on the snail was examined. High mite infestations cause poor quality shells, slow growth and delayed reproductive maturation. A low number of mites can be tolerated seemingly with little in effect except on reproduction. The concentration of proteins and glycogen/free glucose in the snail tissues was the same in snails with and without mites. However, as the final weights of snails were dramatically different, the total amount of protein and glycogen/free glucose in each snail was also different. Some interest in R. limacum was shown by the predatory mite, Macrocheles muscaedomesticae though it is not certain whether deaths of R. firnacum can be attributed to the predator. R. limacum is attracted to certain stimuli more than others, snail odour being the preferred stimulus. In the light of the present study recommendations for controlling R. limacum on a commercial snail farm have been made.