Larvicide substitutes
With no vaccine or etiological treatment, the main means of controlling the chikungunya epidemic, which swept through Reunion Island, is vector control. Measures must be taken to reduce the density of the Aedes albopictus vector.
Control of mosquito larvae targets larvae pools formed by standing water to limit mosquito population growth. From the first signs of the epidemic, vector control operations were conducted: 
initially using temephos-based biocidal products, then with Bti-based products as from February 2006.
|
Moreover, further to the extension of the authorisation to use temephos, the European Commission encouraged the French authorities to develop alternative solutions to this substance. A new solicited request was therefore sent to Afsset on 24 August 2006, on the comparative assessment of the effectiveness and associated risks of larvicide products used for vector control. |
 |
After consulting the French Ministries of Health and Ecology, CEHTRA and the IRD and on the basis of the IRD’s bibliographic work, pyriproxyfene and spinosad were selected for their predicted effectiveness and toxicological profile, favourable in principle. Both substances were assessed comparatively with Bti which is currently the larvicide of reference.
Working method (procedure):
The risks associated with the use of these insecticides were assessed on the basis of firstly European regulatory assessment diagrams used for plant protection products and biocides and secondly reviews published by official assessor bodies. It concerns the production of a review of the information available on the physicochemistry of substances and their dangers for humans and the environment and a brief assessment of the risks for workers, consumers, humans through the environment and the environment, for the use of these substances as a larvicide on Reunion Island. Their effectiveness was assessed using bibliographic data and, given the emergency context, the assessment was simplified to allow for a swift response.
This review and assessment were based on:
General data, available in the literature, on the substances and their dangers, which were collected by Afsset;
Data on the precise methods for use, provided by mosquito control operators or manufacturers of the substance or preparations;
An estimation of exposure levels, with no real measures, based on generic exposure scenarios related to this type of use. Developed initially for use in continental Europe, their appropriateness for vector control on Reunion Island is uncertain. 
Concerning their effectiveness:
Bti stays active for a short time (1 week) because of its rapid sedimentation and adsorption on suspended particles in water. No case of resistance has been reported to date. It has a very specific range of action because the target organism must present an alkaline intestinal pH and toxic receptors.
Pyriproxyfene stays active for a long time (6 weeks) but like any insect development inhibitor, its impact is only visible a few days after treatment. No case of resistance or cross-resistance with other chemical families of insecticides has been reported to date.
Spinosad stays active for a short time (1 week). No case of resistance has been reported to date. However, the lack of data on cross-resistance with other chemical families of insecticides cannot exclude the occurrence of a case of resistance.
Concerning their toxicological properties:
Bti does not present acute toxicity. Although data on chronic toxicity are rare, chronic effects of Bti in humans are not expected.
Pyriproxyfene presents low acute toxicity. It does not irritate or sensitise skin, but is slightly irritant for the eyes. It is not considered to be mutagenic, carcinogenic, teratogenic or toxic for reproduction.
Spinosad presents low acute toxicity. It does not irritate or sensitise skin, but is slightly irritant for the eyes. It is not considered to be mutagenic, carcinogenic, teratogenic or toxic for reproduction.
Concerning their physicochemical properties and behaviour in the environment:
The biological nature of the active substance Bti is variable: it is a mixture containing viable or inactivated spores of Bacillus thuringiensis var israelensis and insecticide protoxins. Also to note are the variable composition of Bti formulations marketed, the identity of producing strains and mixture of spores and toxins. Moreover, Bti persists in the environment in the form of spores which, in certain conditions, preserve the germination potential during several months.
Pyriproxyfene is not very soluble in water and decomposes very quickly in the environment. However, its formulation in granules extends its stability and effectiveness for 6 weeks after application. Its affinity for organic matter is high, which indicates a tendency to soil retention and a high adsorption on suspended particles in water. Its bioaccumulation potential is high, which indicates a potential risk of bioaccumulation in food chains. Pyriproxyfene is not very volatile;
Spinosad is a mixture of spinosyn A and spinosyn D (at a ratio of 86:14). The solubility of spinosyn A is moderate and of spinosyn D low. The speed at which spinosad decomposes is moderate. Its affinity for organic matter is high, which indicates a tendency to soil retention and a high adsorption on suspended particles in water. Its bioaccumulation potential is high, which indicates a potential risk of bioaccumulation in food chains. Spinosad is not very volatile.
Concerning their ecotoxicological properties:
The specific mode of action of Bti means that it is not toxic for any organism without an alkaline intestinal pH or without toxin receptors. As a result, Bti is moderately toxic for aquatic organisms but not toxic for birds, mammals and bees. Moreover, Bacillus thuringiensis is a bacterium present in its natural state in the soil.
Spinosad and pyriproxyfene are very toxic for aquatic organisms, and pyriproxyfene is more toxic than spinosad. Neither are toxic for birds, mammals and earthworms, but both are very toxic for bees.
Concerning risks for humans:
The risk associated with Bti exposure while it is being applied is low for operators and people in the vicinity.
The risk associated with pyriproxyfene exposure while it is being applied is low for operators and people in the vicinity.
The risk associated with spinosad exposure while it is being applied is acceptable for operators and people in the vicinity only when certain recommendations for use are complied with.
Concerning risks for the environment:
The environmental risk after applying Bti is low and limited to target organisms in the aquatic environment. Despite the persistence of spores in the environment and their germination potential, no worrying adverse effect has been reported for the environment in the 25 years it has been used worldwide.
The environmental risk after applying pyriproxyfene only concerns the aquatic environment, particularly because of its very high toxicity for aquatic invertebrates. The potential risks associated with its persistence in the environment may also be a cause for concern.
The environmental risk after applying spinosad concerns bees and the aquatic environment, particularly because of its very high toxicity for aquatic invertebrates. The potential risks associated with its persistence in the environment may also be a cause for concern.
In conclusion, Bti acts in a very specific way and there are few risks for humans and the environment. Although they pose greater risks than Bti, pyriproxyfene and spinosad are interesting alternatives as they would add to the range of substances available to improve management of the possible development of resistance and would enable treatments to be adapted to the specific natures of the areas to be treated.
More information:
Chikungunya epidemic on Reunion Island 
In France, bathing is the most common recreational activity carried out in water. Not requiring any specific level of fitness and enjoyed by most people, it is recommended to everyone as a source of benefits, particularly for children.
>> Read more
