PestFacts March 2021
Timing is everything: when to bait snails
Now is the ideal time to bait snails before most egg-laying occurs. Most snail baiting should occur between March and June. Within that range, seasonal conditions will determine snail activity and when best to apply bait.
Latest snail research
A major GRDC-invested research program on snails, led by SARDI, in collaboration with DPIRD, University of South Australia, University of Adelaide, and farming systems groups, was recently completed. The research increased knowledge of snail biology to better inform the timing of controls, focusing on the vineyard snail (Cernuella virgata), the Italian snail (Theba pisana) and the small pointed snail (Cochlicella Barbara).
At multiple sites across SA and WA, the reproductive status of snails was monitored monthly, movement behaviour was monitored continuously using time-lapse video, and micro-climate variables were measured. Data was analysed to determine links between weather conditions and snail activity (movement, breeding) to help growers identify potential baiting opportunities. For more details, see the 2021 GRDC Advisor Updates paper Movement, breeding, baiting, and biocontrol of Mediterranean snails (Perry et al. 2021). An updated snail management manual is in preparation for launch later in 2021.
Breeding seasons
For all target species, the main breeding season extended from autumn to spring. This pattern was consistent between sites and years, although the onset or finish of reproduction varied by 1–2 months depending on local seasonal conditions. On average, breeding seasons were March to late September for vineyard snail (SA – 3 sites, WA – 1 site), late February to late July for Italian snail (SA – 1 site), and March to October sometimes extending into late November for small pointed snail (WA – 3 sites). At a given location, a small proportion of the snail population may breed somewhat earlier or later.
Video footage revealed periods of high snail movement following rainfall events in early-mid summer, however, reproductive organs remained immature and breeding did not occur at this time.
Movement
Snails typically aestivate during late spring to summer, with some intermittent movement. From late summer through autumn, the main period of interest for baiting, snails increasingly moved in response to increased relative humidity at ground level. Most movement occurred at a relative humidity of at least 90% in March (or of at least 80–85% in April and May) for round snails, or of at least 95% through March to May for small pointed snails (Perry et al. 2021). Initial increase in movement was mostly overnight, between midnight and soon after sunrise, associated with dew formation.
When to apply bait
To maximise effectiveness, snail baiting should be concentrated during early autumn before most eggs are laid. Approximately March to June is a useful guideline. Baiting at other times is less effective. Snail baits kill snails more efficiently during periods when snails are breeding (e.g. autumn, winter) than at other times (e.g. spring and summer) (Brodie et al. 2020, Perry et al. 2020). Most eggs are laid in the early part of the breeding season.
We recommend that growers commence monitoring for baiting opportunities in late summer from approximately late February onwards. To detect overnight movement and confirm that snails are feeding (in hotter months they sometimes collect moisture), growers can apply small areas of bait in snail-infested areas and check for the presence of dead snails after a few days. A sequence of several consecutive nights with forecast conditions moist enough for snail movement (see Movement subheading above) is ideal to give a reasonable level of mortality.
How to apply bait
Tips for effective baiting strategies:
- Select a bait product according to forecast conditions.
- Calibrate your bait spreader for your chosen bait product to ensure even spread.
- Avoid application before heavy rainfall or at soil surface temperatures > 35oC. Exposure to high heat can degrade metaldehyde active ingredients, although UV exposure does not, and rain can break down bran-based pellets. Store unused bait in cool, dry conditions, avoiding hot sheds.
- Pellet densities of at least 30 pellet points per m2 (up to 60 per m2 where snail densities exceed approx. 120 snails per m2) are required to ensure an adequate chance of encounter. Re-apply bait as required according to label directions.
For more information, refer to the SARDI snail and slug baiting guidelines.
Further reading
Perry KD, Brodie H, Baker GJ, Nash MA, Micic S, Muirhead K (2021). Movement, breeding, baiting and biocontrol of Mediterranean snails. Proceedings of the GRDC Advisor Updates, Adelaide, 9–10 February 2021, pp 251–255.
Brodie H, Baker GJ, Muirhead K, Perry KD (2020a). Snail management: learnings from recent studies. Proceedings of the GRDC Advisor Updates, Adelaide, 10-11 February 2020.
Perry KD, Brodie H, Fechner N, Baker GJ, Nash MA, Micic S, Muirhead K (2020b). Biology and management of snails and slugs in grains crops. Final report for GRDC (DAS00160). South Australian Research and Development Institute, December 2020.
Baker GJ, Brodie H, Nash MA, Cunningham N, Perry KD (2017). Improved management of snails and slugs. Final report for GRDC (DAS00134). South Australian Research and Development Institute, December 2017.
Enhancing biocontrol of conical snails
Almost 20 years ago, a biocontrol agent was introduced into parts of South Australia to control conical snails. New research by CSIRO and SARDI, funded with GRDC investment, has investigated ways to enhance biocontrol and you can get involved.
A brief history
The fly Sarcophaga villeneuveana (aka. S. penicillata) (family: Sarcophagidae – flesh flies), is a specialist parasitoid of the conical snail, Cochlicella acuta and small pointed snail, Cochlicella barbara. In the early 2000s, this species was cleared for release in Australia as a biocontrol agent for C. acuta. Between 2001 – 2004, the fly was sourced from the Montpellier region in France, mass-reared at SARDI, and released at 22 locations in Yorke Peninsula and South East regions of South Australia (Leyson et al. 2003). The fly was successfully established on southern Yorke Peninsula but had limited impact and spread, with overall levels of C. acuta parasitism estimated at less than 2% (Muirhead, 2018).
Evaluating a new genetic lineage to improve biocontrol
More recently, it was discovered that Australian C. acuta snails form a genetic lineage mostly likely derived from Morocco, Spain and Portugal, rather than from the Montpellier region (Jourdan et al. 2019). It was thought that a genetic mismatch between the parasitoid fly and snail host may contribute to the low parasitism success in Australia.
Since 2019, a GRDC research program led by CSIRO, together with SARDI, has investigated whether parasitism success can be improved by introducing into Australia a new biotype of the fly sourced from the same region as Australian C. acuta. Testing at the CSIRO laboratory in France found that flies from Morocco parasitised Australian C. acuta snails more efficiently than flies from France, Spain, or Australia. It was hoped the Moroccan fly strain would be better adapted to the local host snails and climate in Australia, leading to improved performance (see renewed attempt at biological control of pointed snails).
In 2020, flies from Morocco were imported and reared in quarantine at SARDI for host specificity testing. In no-choice tests, the Moroccan flies attacked several Australian native snail species, and consequently cannot be released in Australia. Instead, a new mass-rearing and release program has been established at SARDI to increase the impact of the established fly strain on conical snails.
Native vegetation promotes biocontrol
As part of this work, SARDI performed four surveys on Yorke Peninsula (January and April 2019 and 2020) to understand current levels of biocontrol. Thousands of snails were collected and examined for parasitism. The parasitoid fly was detected at 13 of 19 sampled locations, indicating that the fly has spread approximately 20 km from the original release sites on southern Yorke Peninsula. Where the fly occurred, overall parasitism was approximately 3% for both C. acuta and C. barbara of suitable size (> 5mm).
Encouragingly, higher parasitism levels of up to 48% (C. acuta) or 27% (C. barbara) were observed at sites with summer-flowering native vegetation nearby. These levels of parasitism are expected to help suppress conical snails. In contrast, nil or low parasitism was observed at locations without such vegetation. It is likely that floral resources provide a food source for adult flies which is necessary to maximise populations, an area that SARDI is investigating further. These data show the parasitoid fly can perform well against conical snails in the Australian climate in local areas with favourable habitat. For more details, see the 2021 GRDC Advisor Updates paper, Movement, breeding, baiting, and biocontrol of Mediterranean snails (Perry et al. 2021).
Biology of the parasitoid fly
The fly attacks aestivating conical snails bigger than 5 mm in size. After mating, females lay a live larva near the shell opening. The larva crawls inside the shell and consumes the flesh of the resting snail, killing it. At maturity, the larva pupates inside the shell and emerges as an adult fly. Approximately 5–6 generations are possible during spring and summer. Flies overwinter in the pupal stage inside the snail shell.
Adults of S. villeneuveana are similar in appearance to bush flies. Adults are 5–7 mm in length with reddish eyes, and grey and white parallel stripes on the thorax between the wings. Larvae are white maggots up to 6 mm in length. Pupal cases are brown in colour and easily visible with the naked eye inside the shell opening of parasitised snails.
A new mass-rearing program: how you can be involved
Following high parasitism levels found at some sites, a new mass-rearing and release program has been established at SARDI, with continuing GRDC support, to increase the distribution and impact of the established fly strain. In summer 2020/2021, SARDI released > 20,000 flies on areas of western Eyre Peninsula and Yorke Peninsula where the fly was previously absent. Further mass-rearing and releases are planned for this coming spring and summer (2021/2022) in the Eyre Peninsula and South East regions of South Australia.
If you are interested in having flies released on your property this spring to assist in suppressing conical snails, please contact:
Dr Kym Perry
Phone: (08) 8429 0738
Mobile: 0421 788 357
Email: kym.perry@sa.gov.au
Dr Kate Muirhead
Phone: (08) 8429 2741
Email: kate.muirhead@sa.gov.au
Acknowledgements
This article was written by Dr Kym Perry (SARDI), Dr Kate Muirhead (SARDI) and Dr Valerie Caron (CSIRO).
Further reading
Perry KD, Brodie H, Baker GJ, Nash MA, Micic S, Muirhead K (2021). Movement, breeding, baiting, and biocontrol of Mediterranean snails. Proceedings of the GRDC Advisor Updates, Adelaide, 9–10 February 2021, Adelaide, pp 251–255.
Perry KD, Brodie H, Muirhead K (2020). New methods for snail control. Final report for GRDC and SARDI (UOA1903-014BLX (9177340)). South Australian Research and Development Institute; University of Adelaide, September 2020.
Muirhead KA, Baker GJ and Nash MA (2017). Biological control of pointed snails: surveys of Sarcophaga villeneuveana parasitism of Cochlicella acuta in South Australia. Report for GRDC (DAS00134). South Australian Research and Development Institute, November 2017.
Renewed attempt at biological control of pointed snails. GRDC Ground Cover Online, October 2020.
Jourdan M, Thomann T, Kriticos DJ, Bon MC, Sheppard A, & Baker GH (2019). Sourcing effective biological control agents of conical snails, Cochlicella acuta, in Europe and north Africa for release in southern Australia. Biological Control, 134, 1-14.
Leyson M, Hopkins DC, Charwat S, Baker GJ (2003). Release and establishment in South Australia of Sarcophaga penicillata (Diptera: Sarcophagidae), a biological control agent for Cochlicella acuta (Mollusca: Hygromiidae). In: Dussart, G.B.J. (Ed.) Slugs and Snails: Agricultural, Veterinary & Environmental Perspectives. 295–300. Proceedings 80th BCPC Symposium, British Crop Protection Council, Thornton Heath, U.K.
