Tuesday, 28 December 2010

Homeopathic Insecticides: a sustainable alternative?

Had I been asked that question before researching this post, my answer would have been 'almost certainly not'.

I recently came across an article which reported that two homeopathic insecticides had shown such good results (“... we have successfully developed homeopathic pesticides which are not only 100 per cent effective, but they have also no side effects”, Professor Dr Iftikhar Waris) that the Pakistani government has approved them for use.

This surprised me on several levels:
1.       I had previously thought that homeopathy was a medicinal con, relying upon the placebo effect to relieve gullible sick and/or hypochondriac people of their cash
2.       I had not seen anything in the scientific literature about the ability of these products to control anything
3.       I could not see what the mechanism of action could possibly be
4.       In my experience, getting 100% efficacy in the field is all but impossible

Homeopathy, a definition [taken from The Skeptic's Dictionary]: "a system of medical treatment based on the use of minute quantities of remedies that in larger doses produce effects similar to those of the disease being treated."

And they really mean minute! For example, a 30C dilution (commonly used in homoeopathic medicine): "1 ml of a solution which has gone through a 30C dilution is mathematically equivalent to 1 ml diluted into a cube of water measuring 1,000,000,000,000,000,000 metres per side, which is about 106 light years." (Wikipedia).

An interesting final paragraph in an entry on Homoeopathy on the Science Based Medicine blog sums it up nicely: "Therefore, according to everything we currently understand about biology, chemistry, and physics homeopathy is highly implausible and should not work. And, when we carefully study homeopathic remedies they in fact do not work." 

Anyway, back to the article. As it gave no real detail on the products or their efficacy, a literature & internet search was clearly required. This showed that there are a number of proponents of this approach, but very little detail. The one paper I found published in a peer reviewed journal (Cavalca et al, 2010) was the only place where there was any real detail on the efficacy of a homeopathic insecticide. This claims a statistically significant effect at 30C – the highest dilution tested. However, the effect size is so small though that, in my opinion, there is no effect. Note though that the effect claimed is not one of mortality but slight changes to the time taken for the test organisms to pass through the various life stages.

Insecticidal Activity

The synthetic pyrethroids (for example) are extremely efficient insecticides and are effective at very low doses (e.g. 5 grams of active ingredient in at least 200 litres of water per hectare to controls aphids in wheat - equivalent to 0.0025% w/w [from a randomly selected link to a product label]) however, it is critical that the active molecule comes into contact with the target pest and is able to enter it's body. Pyrethroids are a class of insecticides that modulate sodium channel activity by binding to certain receptors causing the channels to remain open, leading to paralysis and death. The mode of action of other pesticide classes also require the interaction of molecules of the active ingredient with specific sites in the organism.

The Cavalca (2010) study and the pesticides of Professor Waris appear to use dilutions of botanical products that have insecticidal activity (rather than using the 'law of similars'). However, given the level of dilution, there are no molecules left in the product, so how can it have any activity? One putative mechanism is through the 'memory of water', although the scientific evidence for water having a 'memory' is overwhelmingly negative.  Even if this were not the case, however well the water ‘remembers’ the presence of the AI it would not have the right molecular structure that would allow it to bind to the appropriate receptors.

Residual activity (i.e. molecules of the active ingredient remain on the leaf once the water has evaporated and is picked up by insects coming into contact with it) is an important feature of the success of many insecticides. If the only trace of the AI is in the memory of the water and the water evaporates, does the leaf need to remember the water's memory of the molecules?


In the absence of any data from Professor Dr Iftikhar Waris, are there any possible ways in which they could have found insecticidal activity from a product containing little or no active ingredient? 

Well perhaps…

Many pesticides contain additional chemicals that increase effectiveness. For example, a wetting agent ensures that the surface tension of the product is lowered significantly to allow the it to spread across the leaf surface as much as possible. Direct application of water plus wetting agent (but minus the active ingredient) can still kill many insects (particularly if small and relatively immobile) by drowning and should always be included in any efficacy studies as one of the controls.

The article suggests that it works by increasing plant vigour. A healthier plant will be better able to utilise it’s own defence mechanisms against pests, so if the product contains ingredients that promote the health of the plant they may have reduced pest problems. How a product that is essentially water can show that sort of effect in a properly controlled trial remains a mystery to me. 

I hope that Professor Waris and other proponents of this technique will publish the results of their trials and allow the scientific community to understand the way the treatments work. 

Without a body of work showing positive results, published in the appropriate peer reviewed literature, the answer to my original question remains almost certainly not and I would suggest that reliance on such products is almost certainly a mistake. 

Cavalca, P.A.M., Lolis, M.I.GdA, Reis, B., Bonato, C.M. (2010). Homeopathic and Larvicide Effect of Eucalyptus cinerea Essential Oil against Aedes aegypti. Brazilian Archives of Biology and Technology, 53: 835 - 843. (PDF)