"MEDIC PASTURES appear to offer a better micro-habitat for snails and the population density is always greater in pastnres than crops," says Vanessa Cavagnaro who recently completed her PhD stndy on the 'ecology of Mediterranean snails'.
Round and conical Mediterranean snails were introduced to Australia from Europe in the early part of the twentieth century. Since then their populations have grown and moved, and now they are a major pest of graiu crops in South Australia and parts of Victoria. Snails can cause crop damage, but the greater problem is grain contamination at harvest.
As the first stage in her PhD thesis at the University of Adelaide (supported by growes through the GRDC), Ms Cavagnaro took 20 years of data by Geoff Baker, CSIRO Entomology, and analysed it to see if triggers for changes in population density could be determined.
"Rainfall is the next key driver of popnlations and this interacts with temperatnre and season. Mild conditions in autumn and spring favour population growth, providing there is also moisture, while temperature extremes result in more stable or declining populations."
Population density is also affected by the Southern Oscillation Index and soil type. Snails have a high demand for calcium to build their shells. It had been assumed that snails favoured high calcium soils, such as the grey calcareous soils found on the southern Yorke Peninsula. But Ms Cavagnaro's work demonstrates this is not necessarily correct. For example, she found round snails (C.virgata) laid more eggs in a red-brown earth, from the mid-north of SA, which has no traceable calcium, compared to· a grey calcareous sand from the Yorke Peninsula, an area where snails are considered a problem in the field.
"What we did find was that, if the soil is dry, snails will not lay eggs and that the number of egg clusters increases as the soil moistnre increases from wilting point to satnration."
She says it is therefore likely that the risk of grain contamination from round snails will be greater following a relatively wet autnmn and spring. The survival of eggs and juveniles is likely to be dependent on continued moist conditions. Therefore, population explosions are less likely in a year with a wet autnmn followed by a dry winter/spring.
Snail youngsters on the go
Another question was: what triggers movement and the distance moved? Two thousand individually numbered snails - both conical and round - provided the answers.
"The most important finding was that juveniles generally travel twice as far as adult snails in the same period. "Until now scientists had believed that, for the four common pest species of snail, the opposite was true, as juvenile snails are less likely to move to baits.
"Therefore, something else is stopping juveniles from taking the bait."
Differences were found between the movements of the two species in crops and pastnre and the triggers for movement in adults and juveniles.
The difference climate changes can make
Temperatnre and rainfall were found to be the key elements for all groups assessed. For example, the model, which was found to closely match observed data, predicted that on a rainy 10°C day a snail would move 69 cm, but on a 10°C day with no rain the distance moved would be only about 10 cm. At lower temperatnres, for example 4°C with rainfall, movement would be about 30 cm, but without rain it would be about 18 cm.
Ms Cavagnaro emphasises that these numbers are dependent on species and age. However, they give growers an indication of the effect that climate changes can have on snail movement.
"These fIgures may help growers predict the area over which baiting may be required, if small dense populations of snails are identifIed."
The work indicates that, if populations are to be controlled successfully, new management strategies are required for the different species and ages of snails.
"We know adult round snails eat bait while the conicals and juveniles are less attracted to it, perhaps because it is not a chosen food source or is too solid for the snails to eat."
Ms Cavagnaro hopes that her data models will provide a useful tool for analysis of new information from current research on the impact of the parasitic fly as well as for potential new research on bail formulations. She herself will be pursuing the issues of the influence of soil characteristics on snail populations and the calcium metabolism of snails. This knowledge could· result in completely new management tools.
Program 3 Contact: Dr Geoff Baker 02 6246 4406 email Geoff.Baker@csiro.au