Timing is everything when it comes to effective baiting of snails. And the optimal time to bait snails is in autumn before egg laying begins - a message to grain growers that has been reinforced through ongoing research.
To inform growers' baiting practices, the seasonal activity patterns of snails in different conditions across eight sites in South Australia and Western Australia continues to be investigated through a GRDC investment.
This research, led by the South Australian Research and Development Institute (SARDI) - the research division of Primary Industries and Regions SA - has found that:
- for common white snails in SA and WA, reproduction generally occurs from April to mid-spring;
- the onset of reproduction can vary greatly from year-to-year and is largely dependent on rainfall - a wet start can lead to earlier snail reproduction;
- snails 'shut down' their breeding activities between August to October, depending on the finish to the season; and
- snail baits appear to kill snails more efficiently during times of the year coinciding with snail reproduction.
SARDI entomologist Helen Brodie says the aim of baiting is to control snail populations by 'knocking out' mature snails before significant reproduction has occurred.
"Snails are most efficiently killed by baiting in early autumn. At this time, prior to and during reproduction, snails are active, hungry and the most vulnerable to the bait active ingredients," Ms Brodie says.
Spreading the word
Ms Brodie says of the few molluscicide active ingredients available, metaldehyde is the most commonly used.
"Unfortunately, snail baits in general are not attractive to snails over distance," she says.
"They will not seek the baits out, so the baits need to be placed at a time and sufficient density to ensure snails encounter them before the baits degrade and become less effective.
"Baits must be applied just before prolonged periods of snail activity to ensure pellet encounter.
"If growers are unsure whether snails are active, then it is best to bait a small area and check for dead snails after a few days."
It is important to gain an even spread of snail bait by calibrating spreaders for the selected bait product, and monitoring outputs, Ms Brodie says.
She says optimal baiting efficacy requires at least 30 pellets per square metre.
In areas of higher snail densities, up to 60 pellets/m2 may be required to ensure adequate rates of encounter. If this is above the registered rate of the product, repeat applications may be necessary - ensuring label directions are followed.
Ms Brodie says it is important for baits to be stored correctly and to be deployed at the right time to ensure maximum effectiveness.
"Environmental conditions will affect bait efficiency," she says.
"We found that exposure of baits to high temperatures did reduce the active ingredient concentration over time, but UV exposure had no effect.
"Iron chelate products are less effective if exposed to rainfall above 35 millimetres. However, extended high rainfall will break down most bran-based pellets - regardless of active ingredient.
"It is important to be aware of the temperature and rainfall forecast before spreading baits, and to store snail pellets in cool and dry conditions.
"Storing baits in a shed where the temperature regularly exceeds 40°C can cause bait degradation."
Ms Brodie is also involved in GRDC-invested collaborative research, led by CSIRO in partnership with SARDI, looking at biological control of snails.
This work has involved evaluation of a Moroccan parasitic fly strain, which scientists hope to deploy as a biological control tool targeting the introduced conical snail.
The fly species is a natural parasitoid (parasite that eventually kills its host) of the snail species in its native habitat across Europe and northwest Africa and was chosen for its genetic and climatic credentials.
GRDC Research Codes: DAS00134, DAS00160, CSE00061, DAS300, DAS00174, YPA0002
More Information: SARDI-PIRSA Communications, PIRSA.Media@sa.gov.au