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THE SCIENCE OF NON-REPELLENT CHEMISTRY: PART 1

The first of a three-part series looking at the science behind non-repellent chemistry.

In recent years, a great deal of promotion has focused around the benefits of using non-repellent active ingredients, as opposed to actives that repel insects. A particular benefit of non-repellent insecticide chemistry is that insects cannot detect the presence of the active, which leads to greater exposure, and supposedly, greater horizontal transfer of the insecticide from one insect to the next. This is however only part of the story.

Also impacting on non-repellence is the time it takes for the product to kill the insect, the other ingredients used in the formulation, the method of formulation, the dose rate and the target pest species. For example, performance against termites is frequently different to that against other pest species.

Typically, the market has tended to consider pyrethroids to be repellent, and actives such as the neonicotinoids (imidacloprid, dinotefuran, clothianidin), phenylpyrazoles (fipronil) and anthranilic diamides (chlorantraniliprole, cyclaniliprole) as non-repellent.

Defining repellence

Let’s first consider the concept of pyrethroids representing repellent chemistry. Certainly, the repellence of pyrethroids is well documented, and the consequences of this have been experienced by many professional pest managers, e.g. cockroaches rapidly vacating harbourages after application, and the tingling (paraesthesia) experienced by technicians when applying these products. However, the repellence of pyrethroids is not uniform, and is most strongly associated with the so-called alpha-cyano pyrethroids, as is explained below.

All pyrethroids are esters (organic compounds) of a pentenolone alcohol and a carboxylic acid. Alpha-cyano pyrethroids are characterised by having an alpha-cyano-group, (a carbon atom joined by a triple bond to a nitrogen atom), within the alcohol moiety (Figure 1). This generally provides improved residual performance and insecticidal activity.

It is this alpha-cyano group that has the biggest effect on repellence. The mode of action on the nervous system is slightly different between the alpha-cyano pyrethroids (deltamethrin, beta-cyfluthrin, cypermethrin, lambda-cyhalothrin), and those pyrethroids that do not possess an alpha-cyano group (bifenthrin, permethrin).

Figure 1: Cypermethrin molecule with alpha-cyano group highlighted

Pyrethroids without an alpha-cyano group produce relatively short trains of nerve impulses with a depolarising after-potential with repetitive activity in the peripheral nerves. In contrast, alpha-cyano pyrethroids induce long trains of nerve impulses and do not invoke repetitive activity in the peripheral nerves. Instead, they cause a long-lasting depolarising after-potential (Flannigan et al[i]).

Consequently, non-cyano pyrethroids induce time constants of decay in the order of milliseconds, whereas alpha-cyano pyrethroids result in time constants of decay in the hundreds of milliseconds to more than one second. This generally results in slower knockdown of the insects. More particularly, this subtle difference in the mode of action means that those pyrethroids without an alpha-cyano group are significantly less repellent to insects and can be better considered as low repellent actives e.g. bifenthrin.

Repellence and dose rate

The degree of repellence is also a factor of the dose rate. A product that is non-repellent at low application rates may well be repellent when applied at higher rates.

Working with termites, Dr Michael Lenz from CSIRO Entomology tested the repellence and moulting effects of the chitin synthesis inhibitor, chlorfluazuron, at different rates. It was tested at 0.1, 0.5, 1.0, 1.5 and 2.0g/kg. Results showed there was no repellence at rates up to 1.0g/kg, but that at 1.5g/kg repellence was evident (unpublished, 2000).

Similarly, a study performed by Professor Dr Ahmad Said Sajap from the Faculty of Forestry at Universiti Putra Malaysia showed the repellence of imidacloprid was related to concentration. At 0.01%, imidacloprid was essentially non-repellent, but at rates of 0.03% and higher, repellence was reported (unpublished, 2004).

This was a consideration in the development of Bithor Dual Active Insecticide. Since this product contains two actives from two different chemical classes – bifenthrin (low repellent at typical use rates) and imidacloprid (non-repellent at typical use rates) – the levels of each active could be halved, whilst actually increasing the insecticidal activity. This is due to the ‘potentiation’ that occurs between the two actives, meaning the two actives work synergistically together to increase the potency of each. This allowed the actives to be used at lower dose rates, which greatly reduced the repellence factor.

The importance of formulation

The next key factor is the method of formulation. Typically, when two actives are employed in a formulation, each active is milled separately and added individually into the formulation. To ensure the non-repellence of Bithor Dual Action, the two actives were milled together in a process referred to as ‘Blended active concentrate (BAC) technology’. This means the imidacloprid helps to mask the bifenthrin to produce a product that is non-repellent in use.

The use of different inert ingredients in a formulation has also been shown to affect the repellence of the final end-use product. Similarly, a highly refined inert ingredient may be non-repellent, whilst a cheaper, less refined version, may display repellence.

Repellent or non-repellent? It will depend on the target insect, active ingredient, formulation and dose

The repellence of an active is also affected by the impurities present. Typically, the technical-grade active constituents used in modern insecticides contain about 95-98% pure active and 2-5% impurities. The impurity profile will vary slightly between different sources, and can affect the repellence of the active.

In the study by Professor Dr Sajap (referenced above), the repellence of two different sources of technical-grade imidacloprid were compared against Coptotermes gestroi. A range of methods, including soil tunnelling trials, showed that at typical use rates of 0.01%, measurable differences in the degree of repellence existed between the two different sources of technical-grade imidacloprid.

Similar results were obtained in a later study with fipronil. They confirmed that, particularly with non-repellent chemistry, it cannot be assumed that simply because a product contains the same active, the product will perform the same. Of course, in addition to the active, the formulation itself will impact on performance.

Steve Broadbent, Regional Director, Ensystex