Giant slugs and giant problems

23 Sep 2014

I’m sure I heard it first on BBCR4’s Today programme but I can’t find a record of it. Then I met a person outside our house who confirmed it. In fact, he said that his dad had mistaken one for a large stick when taking his dog for a walk late one evening. What am I talking about? Garden slugs: they are ENORMOUS this year.Giant slugs

Over the last couple of weeks I have upped my anti against slugs in the garden. This has meant rather than just relying on metaldehyde pellets, I’ve roamed the lawn in the dark picking slugs and transporting them to a large open area for the birds to eat the next morning. It’s Integrated Pest Management and encouraging bird numbers all rolled into one.

I hope that my metaldehyde use is not contaminating water. The garden is not underdrained and slopes away from any hard surfaces. However, metaldehyde in water has been an issue ever since the river companies developed a method of measuring it with sufficient accuracy to say when levels in water exceed the Drinking Water Directive limit of 0.1 parts per billion (ppb).

The problem with metaldehyde is that the processes installed at waterworks to remove small Directive exceedances of nearly all pesticides don’t do much at all in reducing the levels of metaldehyde. It’s neither strongly adsorbed by the activated carbon beds nor oxidised by the ozone treatment. Hence, unlike most pesticides, the raw water going into the treatment works has to comply with the Directive.

The very active Metaldehyde Stewardship Group has for the past few years been providing guidance in its Pelletwise campaign in order to reduce metaldehyde movement to water. This autumn’s guidance document reflects the increasing pressure on the future use of this pesticide. Quite rightly the guidance has adopted a targeted and conditional approach. There is little doubt in my mind that adopting best practice, particularly in identified high risk fields, will significantly reduce pesticide movement to water.Get Pelletwise

It comes as a surprise to most people that pesticide movement to water through the soil is not influenced by the solubility of the pesticide. Glyphosate is one of the most soluble pesticides available but does not move through the soil to watercourses. Its presence in water is largely determined by spray drift and its wide usage on hard surfaces.

The ability of pesticides to move through the soil to water is mainly determined by how strongly they are adsorbed by the soil (glyphosate is strongly adsorbed by the soil) and their persistence in the soil. There is a relatively simple calculation based on these two measurements which indicates the level of risk of movement through the soil. It’s called the Groundwater Ubiquity Score (GUS). Despite being relatively simple it’s surprisingly accurate, but of course more precise models are used in pesticide registration. It was developed by Gustafson (another reason for the abbreviation GUS), a Monsanto scientist who published the approach in 1989.

The GUS score for metaldehyde suggests that (marginally) it should not move through the soil in sufficient quantities to cause a problem. There are perhaps three possible explanations as to why it is occurring in water.

Firstly, that it is moving to water through different routes. There is little doubt that run-off from fields and the washings from contaminated equipment are a source of metaldehyde in watercourses. However, I understand that movement through soil has been confirmed as a route by the Metaldehyde Stewardship Group.

Secondly, this is an exception that proves the rule for the GUS score. This is a possibility but somehow I doubt it.

Finally, there may be something about the use of metaldehyde that the GUS score does not take into account. This may be the explanation. The GUS score assumes that the pesticide is applied directly to the soil but this is not the case with metaldehyde. It’s necessarily applied in pellet form and so is not immediately adsorbed by the soil. This may influence how long it persists in the soil environment and possibly undermines the methodology used in calculating its GUS score.

I must dash as it is now dark and I must continue with my slug patrol.