This summer has been a great season for the snail and slug population; a damp and winter, dry spring and wet warm summer has led to an over abundance of these voracious creatures.
For the majority of us when we see the slug and snail eating our flowers or vegetables we kill them: or throw them into the neighbours’ garden. Salt applied to them is instant death or a slower swim to oblivion in a beer trap can solve the problem and make the gardener feel better when emptying the bodies away in the morning. In towns across the country the tap tapping of a thrush at its anvil is a rare sound; the sight of even house sparrows or tits is worth a comment today as our garden birds are declining in number. There appear no natural predators for snails or slugs.
Garden centres and other shops offer slug pellets: the packaging states that they are rain resistant so they can be spread about, away from pets, knowing they will not just dissolve in the first shower. In reality though at the first shower the lethal ingredient, Metaldehyde, begins leaching out of the pellet and enters the soil with the rain. After a day or so the slug pellet is still visible on the soil surface but it is only a coloured husk that is near harmless. The Metaldehyde is on its way to the water table, groundwater or stream.
Now consider the farmer or market gardener; their problems with the slithering hoards that attack their crops can not be treated by hands on methods. The costs of the problem are too great and the scale, both in acreage and creature numbers too vast to do what we normal gardeners might do and this year we are struggling. The use of Metaldehyde is the farmers’ only option at present. The dressing of the land with Metaldehyde has had to be regular and repeatedly carried out this summer; earlier that normal due to the climactic conditions. Record rain has led to record runoff and record amounts of snail and slug ‘bait’ being put on the land.
The accepted 5 metre buffer strips where agro-chemicals are kept away from rivers, streams and other water courses has not really worked this year; the volumes of rain have seen to that.
The full effect of the weather conditions of the last months are yet to be realised or understood. The rain falling onto parched, baked fields saw above average runoff events take dressing after dressing of Metaldehyde into ditches and streams.
This issue over the years has become a problem for the water industry; Metaldehyde is appearing as a trace constituent in surface waters used for potable abstraction. The levels are extremely low but may still take these supplies outside the legal permissive value of 0.1 parts per billion. This is not a purely British problem but international one. British water companies have though been the first organisations to admit that the issue can not be conjured away. The industry is researching for a method to remove it from the water as part of its raw water treatment prior to entering the public mains; but a solution appears a long way off. This is neither scaremongering nor being an apologist for the water companies; there are many worse things that our own homes can adulterate our water with; from old lead piping, backwash from poorly maintained central heating boilers or bacteria from a badly kept header ta0nk in the loft.
The majority of the farmers who need to use these slug and snail preparations do not wish to do so; the costs of both the pellets and their application are high; particularly in a year such as is being experienced at the present. The risk of loosing crops is the driving force. The ease in which a field of potatoes can be destroyed by Black Keeled slugs is remarkable and can be achieved in a matter of days. Trials carried out in the early 2000s showed that the optimum times for treatment were at planting, canopy emergence or join up and at defoliation. The type of pellet used was found to be critical; mini ones which are considerably cheaper to purchase break down after only a short period of rainfall whilst the larger are more robust to bad weather.
Sugar beet and most other root crops are at risk from mollusc attack as are some brasica. The damage to crops annually can run into the tens of millions of pounds annually and can be more. The agricultural industry and the water companies are in constant dialog regarding these issues and a considerable sum of money has been invested by the water utilities in educating the farming fraternity in best practise and safer methods of pesticide application. The Environment Agency, DEFRA and the National Farmers Union have also been involved with educational initiatives in this area.
The water companies are looking at river water quality data to understand timings of high usage and identify ‘hot spots’ of adulteration. This data collection is not just from abstraction points but in real time flow and quality monitoring along rivers that are abstracted from to gain a more detailed understanding of specific reaches of the rivers. The water industry is also working with the manufacturers of pesticides to find viable alternatives to those already in use.
The use of nematodes as a control option, there are proprietary brands and home made variations. Some of the proprietary assassins are based on microscopic eel worms which kill the slug and are harmless to crops such as potatoes, others that are made in house rely on captured slugs and snails and the concentration of their own toxicity which can be developed into a small scale nematode like poison that has worked satisfactorily on small scale trials. The issue of nematodes is under international scrutiny; the eel worm nematode may be used only in certain counties of California and not in others. Their crossing of state boundaries is not allowed due to concerns of moving and releasing non native species in some areas where they are not licensed as pest control methods.
Strangely in the UK the use of nematodes is a lot less formal than in the United States with internet purchases jetting in from around the world incorrectly described customs declarations as to their nature being not uncommon.. The fact that the term nematode is considered a coverall for correct environmentally sound horticultural practise is a further concern; there are over 20 thousand such creatures and 16 thousand are known parasites. The concern is that some will develop a parasitic interest in other creatures if their food target were to be so depleted by their actions and other, human interventions.
This short piece has been written to inform in a straightforward manner some of the issues that farmers and the water industry have to contend with and are not generally known about by the main body of the population. A more scientific piece will be published shortly.
27th August 2012.