Tuesday, November 28, 2023

Common white or vineyard snail


Cernuella virgata (Da Costa) Eupulmonata:


Distribution: European–Mediterranean Basin in origin, now in Australia (Baker 1986), where it occurs in NSW, SA, Tas., Vic. and WA but is most prevalent in SA (Hopkins et al. 2003).

Pest status: Major, restricted, regular.

The shell of the common white snail (shell diameter 15 mm). Note the completely round hole (umbilicus) in the shell on the right.

Identification: The diameter of the shell of a mature snail ranges from 10 to 15 mm. The coiled white shell has a brown band around the spiral in some individuals while others completely lack this banding. The umbilicus is open and circular (Hopkins and Miles 1998). Under magnification, regular straight scratches are visible across the shell.

May be confused with: The white Italian snail, Theba pisana. They can be separated by the differences in the umbilicus and the scratchings/etches on the shell.

Host range: Includes field crops such as wheat, barley, oats, field peas, faba beans, canola and also pastures. It feeds mainly on organic matter on the soil surface but may damage young plants. It is an important pest because it contaminates grain crops at harvest and clogs and damages harvest machinery.

Common white snail aestivating on a fence post

Life cycle on cereals: This species aestivates over summer by climbing on to crop stubble and residues, various weeds and fence posts to avoid high summer temperatures at the soil surface. Autumn rains trigger activity down onto the ground, mating and egg-laying. Most eggs are laid in late autumn or early winter but some egg-laying continues through to early spring. Eggs hatch in about 2 weeks and immature snails grow steadily throughout the winter and spring before they aestivate through the next summer. It can have an annual or biennial life cycle.

Risk period: Cereals, particularly barley, and pulses and canola are susceptible to direct feeding damage shortly after crop emergence in the autumn. The risk of machinery clogging and grain contamination occurs at harvest.

Damage: Moderate to high densities of snails can cause serious defoliation of emerging barley, pulse and canola crops to the point where re-sowing of the worst-affected areas is necessary. At harvest, snails can clog and damage harvest machinery and cause frustrating delays in the harvest period.

They can also contaminate harvested grain samples and lead to the downgrading of grain classification, or total rejection of the contaminated load at the point of delivery.

Monitoring: Successful management depends on regular monitoring of snail numbers across the whole farm. Square sampling quadrats (30 × 30 cm) can be placed on the ground, and snails counted and usually converted to numbers of snails per square metre. It is important to count only live snails; the shells of dead snails persist for many years and should be excluded from the count. As snails often move from adjacent roadside verges, it is important to also monitor these areas to assess the risk of snail movement in adjacent paddocks.

Action level: In cereals, 20 per square metre or more at the time of crop-sowing.

Chemical control: A number of baits are registered for snail control. Controls should be applied prior to or immediately after seeding of cereals. The objective is to control adult snails prior to major egg-laying for the season and prevent major increases in numbers that pose a risk at harvest time later in the season. Use the label rate of bait for moderate snail numbers, but if snail densities exceed 80 per square metre this rate should be increased.

The shell of the white Italian snail (shell diameter 17 mm). Note the hole (umbilicus) in the shell on the right is half closed off.

Cultural control: Stubble management (slashing, rolling or cabling) in January and February is an important control tactic for snails. Snails dislodged from stubble and crop residues onto the soil surface on hot summer days (maximum temperature greater than 35°C) may desiccate and die; snail numbers can be reduced by 50 to 70% using stubble management techniques. Burning stubble residues provides excellent snail control but should only be practiced where the risk of soil erosion is low and when burning is allowed. Burning may reduce snail numbers by up to 99%. Both of these cultural control tactics should be followed with baiting if snail numbers still exceed the established threshold levels.

Host-plant resistance: No host-plant resistance is recorded for cereals or other host plants.

Natural enemies: Apart from some predation by birds and lizards, there are no known natural enemies of the common white snail across

southern Australia.

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