The curious case of the toxic slugs

If you’re in anyway tuned into the world of environmental news, you will have most certainly heard of neonicotinoids. Neonicotinoids (or neonics for short) are a class of chemicals which have become increasingly popular for controlling pest insects in agricultural systems. Neonics are highly toxic to insects, but usefully have low toxicity to birds and mammals.

While neonics have been widely adopted by farmers, they have less than a sterling reputation among many groups. Neonics work through a mode of action known as systemic delivery. This is achieved by coating the seed with the pesticide treatment. This active ingredient is then incorporated into the plant tissue, and is delivered directly to an insect feeding on any part of the plant. By applying neonics to seeds, riskier methods of pest control – like widespread spray programs – can be avoided during later stages of production. Due to their efficacy and relatively targeted delivery, neonics have become an important tool for suppressing insect pests.

The largest problem with the use of neonics is that they are applied  before their use is known to be required, a process known as ‘prophylactic application’. This process is analogous to taking aspirin to relieve a headache, you could foreseeably have later in the day. Over-reliance on a substance can eventually lead to reduced efficacy through pest resistance, and when little pest pressure is present – can contribute to the loss of beneficial invertebrates through non-target effects. When these two factors are considered together – potential damage caused by pest species is further exacerbated.

A new open access study published in the Journal of Applied Ecology has demonstrated a novel pathway through which neonics can impact the natural environment. This pathway involves a herbivore unaffected by the insecticide, which through feeding on treated plants become toxic prey for its predators.

No-till soybeans were the study system used within this paper. In the particular region studied, the dominant pest problems are caused by slugs. These slugs feed on the soybean seedlings, causing mortality and reducing yields. While the neonic treated plants are highly toxic to insect herbivores, slugs are unaffected by the presence of neonics. However, these slugs are kept in check by a diverse group of organisms including: ground beetles and farmland birds. We call these predators ‘natural enemies’. One of these natural enemies is Chlaenius tricolor – a wonderfully charismatic ground beetle. In the study region, these beetles are common within arable fields,  are highly mobile, and have a particular fondness for eating slugs.

In a series of laboratory experiments, the team confirmed that slugs were unaffected by consuming soy plants treated with thiamethoxam (a type of neonic commonly used to treat soybeans). However, when beetles were offered slugs which had fed on treated plants – more than 60% of individuals died or were seriously impaired.

The team then scaled this up to a field level, and found a similar pattern. In field plots grown from neonic treated seeds they found significantly higher densities of slugs, and significantly lower captures of natural enemies. Amazingly, the team even found five percent higher yields from non-treated seed (although pressure from insect pests was very low).


The rapid movement of a pesticide through this food-chain is an important reminder of how interconnected our food production systems are. It beautifully demonstrates sustainable agricultural production demands healthy ecosystems. While some would use the findings of this study as evidence for the condemnation of neonics and other insecticides within agricultural ecosystems, I wouldn’t agree. Insecticides remain an important tool within agricultural production. To me, the findings from these experiments serve to demonstrate just how cautious we must be with our application of pesticides.

The curious case of the toxic slugs brings to light just how complex, and sensitive our environment is. We need to think about the impacts of our actions on a multitude of scales. Whoever would have guessed that a predaceous beetle would be negatively impacted by chowing down on slugs made toxic by feeding on a neonic-treated soybean plant?

Douglas, M. R., Rohr, J. R., Tooker, J. F. (2015), EDITOR’S CHOICE: Neonicotinoid insecticide travels through a soil food chain, disrupting biological control of non-target pests and decreasing soya bean yield. Journal of Applied Ecology, 52: 250–260. doi: 10.1111/1365-2664.12372


2 thoughts on “The curious case of the toxic slugs

  1. TO: Paul Manning
    FROM: Eric Koperek
    SUBJECT: Correct Pesticide Terminology
    DATE: PM 11:34 Wednesday 22 April 2015

    Nota Bene: There is no such thing as a “systematic insecticide”. The correct term is “SYSTEMIC INSECTICIDE”. (You don’t have to believe me; ask the chemical manufacturers or take a course in agricultural chemicals. Talk to the folks who invented neonicotinoids — they will set you right).

    It is not my intention to nag you (truly), but if you are going to write about agricultural science, get your vocabulary straight.

    Eric Koperek
    Office: 413 Cedar Drive, Moon Township, PA 15108 USA


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