The World Health Organization (WHO) Global Malaria Action Plan promotes indoor residual spraying (IRS) as a primary operational vector control intervention to reduce and ultimately eliminate malaria transmission. In some southern African countries DDT is regarded as the most effective insecticide for this purpose. Depending on the dosage and substrate nature, DDT retains its efficacy against malaria vectors for up to 12 months. In South Africa DDT was temporarily replaced with the pyrethroid deltamethrin between 1996 and 1999. However, DDT was reintroduced in 2000 when malaria transmission reached epidemic proportions. The failure of the pyrethroid was attributed to the return of the major vector mosquito, Anopheles funestus that was shown to be resistant to pyrethroids but fully susceptible to DDT [1, 2]. Other WHO-approved pyrethroid, organophosphate and carbamate insecticides are limited in effective IRS residual life. Furthermore, repeated application of these alternatives is required in order to provide year-round protection and this significantly increases the costs of IRS . Formulations based on micro-encapsulated insecticides have been tested with great success [4–7]. These results show that shielding the insecticides from the outside environment stabilizes them against premature degradation. However, the higher costs associated with such formulations may limit their widespread implementation as replacements for DDT in IRS.
The stability of WHO-approved insecticides for IRS is affected by the pH of the environment [8–14], temperature [15–19], exposure to ultraviolet (UV) light [20–30] and the availability of degrading bacteria [31–36]. Pyrethroids, organophosphates and carbamates degrade via hydrophilic attack of the carboxylic and carbamic ester linkages [11–13]. DDT undergoes alkaline dechlorination to yield DDE .
On thermal exposure, phenyl carbamates representative of bendiocarb and propoxur degrade to the corresponding phenol and methylisocyanate . Pyrethroids transform by isomerization, ester cleavage and primary oxidation of the final products [16, 17]. Organophosphates, e.g. malathion, initially isomerize to S-alkyl organophosphates before they eventually decompose . The primary step in thermal decomposition of p,p'-DDT is the elimination of HCl, resulting in the formation of p,p'-DDE at 152°C . DDE starts to volatilize at the onset of the process. The decomposition temperature is dependent on the type of DDT, for o,p'-DDT the decomposition starts at higher temperatures.
All the insecticides classes, except for pyrethroids to some extent, are degraded by exposure to ultraviolet light. Fenitrothion undergoes photo-oxidation on the benzene methyl group to form a carboxylic acid group. It may also undergo oxidation on the ─P═S moiety group to form the oxon and ester cleavage to form the corresponding phenol . Malathion is generally stable to photolysis. This may be due to a lack of chromophores to absorb radiation in the UV- range . Pirimiphos-methyl photodegrades rapidly forming 2-diethylamino-6-methylpyrimidin-4-ol as the major degradation product . Carbamate photodegradation involves the cleavage of the carbamic acid ester to form the corresponding alkyl phenyl ether or alkyl phenol ether . Carbamates isomerize on irradiation through the photo-Fries re-arrangement. Photo decomposition of carbamates also produces cholinesterase inhibitors . DDT undergoes homolytic cleavage of the C─Cl bond on the trichloromethyl group to form DDD as the product . DDE may be produced due to exothermic effects of the initial photodechlorination stage. The overall photodechlorination is proposed to be dominated by the sequential dechlorination pathway with each successive dehalogenation proceeding more slowly. Apart from etofenprox, all pyrethroids are structurally similar. They all feature the characteristic carboxylic ester moiety, the dihalogen substituted vinyl moiety and the phenoxy ether group. Etofenprox only contains the phenoxy ether group. The carboxylic ester moiety is susceptible to photodegradation and hence decarboxylation of these insecticides is the main route of photodegradation [26, 27]. However photodecarboxylation of these pyrethroids is a minor transformation process accounting for not more than 15% of the transformation products . The amount of decarboxylation depends on the halogen substituents on the vinyl moiety, i.e. the more electronegative the halogen the more the decarboxylation. The main process of transformation of these pyrethroids is photo-isomerization (cis/trans and E/Z) on the triplet diradicals formed on exposure to ultraviolet light (>300 nm), e.g. deltamethrin , cypermethrin and cyhalothrin . Cyfluthrin is subject to photolysis rapidly forming 4-fluorophenoxybenzaldehyde via ester hydrolysis and the release of cyanide ion from the corresponding cyanohydrin .
Extensive work has been done on the fundamentals of insecticide biodegradation to develop bioremediation techniques to detoxify contaminated environments . Microbiological degradation of insecticides relies on the availability of organisms that can secrete specific degrading enzymes. Bacteria and fungi have the ability to produce these enzymes under both aerobic and anaerobic conditions. Carbamates, pyrethroids and organophosphates have a common ester moiety that provides a route for bacterial-mediated enzymatic biotransformation. The enzyme groups that are able to hydrolyse these ester moieties include carboxyesterases for carbamic and carboxylic esters, and phosphotriesterases and carboxylesterases for triphosphate esters . DDT is degraded by oxygenases and dehydrogenase enzymes . A period of adaptation is required before degrading microbes manage to establish themselves and significant degradation of the insecticide is observed . This acclimation period entails the building of viable microbial colonies that have a capacity to induce enhanced degradation . The length of the acclimation period is also affected by the availability of suitable metabolic substrates for growth. These can be a carbon source, a mineral nutrient source, or both. The presence of alternative carbon substrates and nutrients generally increases the rate of biodegradation. This is supported by the observed increase in the rate of DDT degradation in highly fertilized soils .
Mineral powders such as bentonite, gypsum, montmorillonite, kaolin, attapulgite, diatomite etc. have been used as insecticide carriers in granular insecticide formulations [37, 38]. These minerals have the ability to slowly release the adsorbed insecticide into the environment [39, 40]. Lagaly  presented an excellent review on pesticide-clay interactions and formulations relevant to formulating insecticide laden clay formulations.
Impurities can mediate catalytic degradation of insecticides adsorbed on soil surfaces. Stability can be improved by deactivating surface active sites responsible . Additives such as polyethylene glycol are reportedly very effective deactivators. A useful property of mineral powder carriers is that they can stabilize photolabile and thermolabile insecticides [43–45].
Our ultimate aim is to develop long-lasting, "green" and cost effective alternatives to DDT for IRS. This paper reports on investigations directed towards finding ways to extend the active life of insecticides formulated in environmentally stable, pesticide-based "whitewash" or equivalent "paint". The goals of this investigation were to overcome the time-limited effectiveness of current WHO-approved DDT alternatives by evaluating the interactions between selected insecticides with the micro-environment on which they are applied and also with low-cost paint ingredients.