Very few weeds occurred in this vineyard, and they were mainly in the vine row. The snail activity over the two seasons followed a similar pattern (Fig. 1a & b). Snail numbers on weeds, although relatively low, peaked in autumn and early winter but declined sharply towards the end of July. During the very wet and cold winter months snails were inactive and hibernated on weeds and cover crops. Very little snail activity was recorded from August to October (data not shown). After the cover crop and winter weeds were treated with herbicide in October/November, both the white dune snail and brown garden snail rapidly moved on to the grapevines. From November onwards, snail activity on the grapevines increased to reach a peak in December (Fig. 2a & b). As conditions became hotter and drier, snail numbers decreased and very little snail activity was observed from February to April as the snails aestivated (data not shown). Very little feeding damage was noticed throughout the season, which could be due to the fact that the snails were mostly aestivating on the vines and very few snails were present on the grapevines. Very few snails were observed mating and laying eggs throughout the whole season.
White dune snails and brown garden snails were observed feeding on small mallow (Malva parviflora) and musk herons bill (Erodium moschatum), the only weeds present in this vineyard (Table 1).
Seasonal occurrence: Vineyard with natural weed cover
The white dune snail, brown garden snail and the tower snail were all recorded in this vineyard, but the tower snail occurred in much lower numbers and was never found on grapevines. The three-banded garden slug was recorded on weeds several times, but never on grapevines.
Eighteen weed species were recorded in this vineyard with feathertop chloris (Chloris virgata), narrow-leaved ribwort (Plantago lanceolata), sowthistle (Sonchus oleraceus) and wild radish (Raphanus raphanistrum) being the most abundant (Table 1). During autumn snail activity on the weeds reached a peak in May (Fig. 3a & b). Several pairs of all three snail species were recorded mating, but most egg-laying was recorded in mid-April to mid-May after the first good autumn rains. During the cold, wet winter months, snail activity declined as snails went into hibernation.
Very few snails were recorded on vines from August to mid-October (data not shown), after which white dune snail numbers increased (Fig. 4a & b). When herbicides were applied to the ridges (vine rows) a steep rise in white dune snail numbers was noticed on the grapevines. White dune snails moved rapidly from the ground and surrounding weeds on which they were hibernating, into the vines, where they stayed almost throughout the season. Snail bait was put down by the producer in mid-November 2011, after which white dune snail numbers were reduced by approximately 50%. However, one month later the snail numbers increased to the initial level and kept on increasing. Moderate to severe feeding damage to grapevines was noticed from véraison to harvest. The white dune snail was the major snail pest throughout the vineyard, while the brown garden snail only occurred in small numbers from August to May. The brown garden snail was very effectively controlled by the bait treatment applied in mid-November.
The number of snails was overall substantially higher where weeds grew actively in the work row throughout the year (site 2) than where cover crop management was applied with triticale in the vine row during winter (site 1).
Narrow-leafed ribwort was the preferred host for feeding and shelter of all three snail species. Wild radish and sowthistle were also favoured as a food source by the white dune snail, while the brown garden snail also preferred wild radish.
Efficacy of snail bait
Site 1: Due to the relatively dry conditions and low snail population in the drip-irrigated vineyard with triticale as cover crop, very few dead snails were observed. All the bait treatments controlled both the white dune and brown garden snails (Table 2). Within twenty one days after administering the snail bait, the number of snails started increasing slowly as the effectivity of the snail bait started to diminish.
Site 2: Although relatively dry conditions prevailed in the micro-irrigated vineyard with natural weed cover, good control of both the white dune and brown garden snail was recorded for all commercial snail baits tested (Fig. 5). White dune snail numbers in grapevines and snail damage were drastically reduced even in the following season, proving that snail control is attainable when applied correctly and at the right time. Where snail numbers are very high, bait applications may have to be made regularly over several seasons. To maintain control, bait applications should be maintained as a standard practice.
Fewer snails were present at the site where the cover crop triticale was combined with full surface chemical control from bud break than at the site where opportunistic weeds were abundant, even during the growing season. It was shown that weeds served as hibernation sites and food sources for snails, resulting in much higher snail numbers at the start of the growing season. Brown garden and white dune snails showed a preference for narrow-leaved ribwort, wild radish and sowthistle as food sources. Although both major snail pests were found hibernating and aestivating on the cover crop (triticale), they were never observed feeding on it.
When herbicides were applied to the weeds and cover crop, the snails present on the ground and cover crop moved rapidly into the grapevines.
These results demonstrate that uncontrolled weeds are an enormous problem for effective snail control, particularly if they are allowed to continue growing in the work row during the growing season. Cover crop management with a grain species such as triticale, on the other hand, would be more beneficial for snail control, as snails do not feed and breed on it. Where cover crops are sown in alternate rows only, the presence of weeds preferred by snails in the other rows will largely negate the beneficial effect of the cover crop in terms of reducing snail numbers. In areas where snails are a serious problem, a cover crop that is not a food source for snails, such as triticale, should be sown in every row and weeds favoured by snails should be controlled in the vine rows.
All the commercial bait products tested, gave effective snail control, confirming that accurate timing is paramount for successful snail control. It is also very important to control the weeds in the vine row before they reach a height of 300 mm, to prevent the buildup of too much plant material on the soil surface. At pruning, canes should be placed in the work row and not in the vine row. The aforementioned steps will reduce the risk of snails getting into the vines via alternative routes, thereby forcing them to go past the bait.
Bait application must coincide with the control of weeds and/or cover crops. Weed and/or cover crop control should preferably be done when conditions are favourable for snail activity to ensure maximum efficacy of snail control. In cases where weed control needs to be applied before bud break, the bait application should be followed up when conditions are favourable for snail activity. Follow-up treatments are ineffective once the snails have moved into the grapevines.
Guidelines for sustainable snail control:
- Monitor snail activity throughout the year.
- Sow a grain cover crop such as triticale during April/May in every row – soil tillage during sowing will damage snail eggs laid in the soil and it will also reduce the weeds that serve as a food source for snails.
- Control snails in autumn, or shortly after harvest, when conditions are favourable for snails to start feeding and mating. This is a very important time for snail control.
- Pruned canes should be left in the work row rather than in the vine row.
- Control weeds in the vine row before they reach a height of 300 mm throughout the year.
- Put snail bait out just after the application of the herbicides.
- If high numbers of snails are present, a follow-up application of snail bait will be necessary for at least two seasons.
- Continue snail control even if numbers are low, as numbers can increase rapidly over a short period.
For further information contact Abraham Vermeulen at firstname.lastname@example.org.
Abraham Vermeulen, Elleunorah Allsopp & Johan Fourie