Snail and slug bait technology has come a long way since the traditional bran-based dry process pellets. In Europe, the need to improve delivery of active ingredients to increase the number of snails and slugs killed while reducing environmental impacts has seen the launch of new products with enhanced delivery systems of old active ingredients, resulting in greater return on investment due to less kg/ha of product applied. For example, new formulations of metaldehyde products kill a greater number of snails and slugs using less active under a greater range of conditions.
There are now only two modes of action available on the Australian market, since methiocarb has been withdrawn from sale. The development of new chemical active ingredients is unlikely. Choosing the right snail and slug bait for the situation in which snails and slugs are to be managed is critical to ensure continued access to these crop protectants.
For snail and slug baits to work, some basic principles are relied upon. Pests must first encounter a pellet, which requires:
Individual activity – snails or slugs must be actively searching for food, not just moving about to rehydrate; the complexity of habitat is thought to influence snails’ ability to find pellets.
Even number of baits per unit area – the focus on an increased pellet density alone has seen the manufacture of small pellets that increase the likelihood of individuals consuming a sub-lethal dose (see below). Pellets have to be evenly applied across the full width of application area. Uniform pellet size, weight and density ensure no area is missed. Patchy control can occur when products with high variation are used and/or application equipment is not calibrated.
Attractiveness of bait – snails and slugs display non-random movement towards attractive pellets (the true definition of a bait) compared to seedlings. South Australian Development and Research Institute data demonstrated a significant (χ2 = 129.62, df = 2, p < 0.001) number (87%) of Italian snails first touched a Metarex Inov pellet compared to a wheat seedling or not making a choice.
Attractive products result in a greater likelihood individuals will encounter a bait than a seedling.
Once pests have encountered a snail and slug bait, they must consume a lethal dose, which requires:
Palatability – addition of feeding enhancers (e.g. COLZACTIVE) ensures individuals consume enough active ingredient to ingest a lethal dose. In the case of metaldehyde that causes paralysis, consumption of a sub-lethal dose can be an issue with some products because individuals can’t continue to ingest enough to destroy their mucous cells.
Enough bait for the target population – if product does not remain after a couple of days following application, it is usually due to large numbers of snails and slugs consuming it all. Re-application to those “hot spots” will be required.
Enough toxicant in the bait – the loading of active ingredient determines the amount consumed; hence low loading pellets require more total product to be applied. In wet conditions, small pellets with greater surface area to volume ratios lose more active ingredient; hence less toxicant will be consumed. For products containing metaldehyde, it is generally recommended that 30 to 40 g/kg is the optimum concentration.
Having a persistent bait, that snails and slugs will consume to receive a lethal dose, allows for application before individuals are active. This timing often coincides with rainfall. Bran-based products that breakdown after rain and have low initial loadings of active ingredient need to be reapplied after heavy rainfall, whereas modern rainfast products continue to kill for up to a month after application and rainfall. Some products contain components that make the pellet more resistant to weathering, however snails and slugs are not inclined to feed on these hard, yet rainfast, pellets.
Combining what is known about the factors that make a good snail and slug bait has led to the delivery of products that have faster and more efficient mortality, with greater persistence. The continued improvement of delivery technologies has seen less active applied, hence lower environmental loadings, yet better crop protection and snail and slug control, leading to better returns on growers’ investment in snail and slug bait.