Biocides that attack only insects

Biocides turn out to be less toxic for the environment if they are subjected to microencapsulation, due to the fact that this process forms shell(s) for the substance. This is the conclusion of chemist Ms Mariluz Alonso in a thesis defended at the University of the Basque Country (UPV/EHU). In this research, she chose a number of biocides and other complementary substances, seeking a micro-encapsulation which, besides being compatible with the environment, is more soluble in water, more manageable for the operator, with better conditions of conservation, and effective against airborne insects. Her thesis is entitled Micro-encapsulation of biocides, and has given rise to publications in journals such as the International Journal of Environmental Analytical Chemistry (the article is entitled Advantages of biocides-β-cyclodextrin inclusion complexes against active components).

The aim of the researcher was to obtain a product that overcame the limits of common biocides. These limits correspond to a toxicological profile that restricts their use, poor solubility in water, high viscosity (in some cases) that complicates their handling, and high sensitivity (in most cases) to light and temperature. The product not only complies with these parameters, but has also shown that it is effective against the domestic fly, in tests undertaken in the laboratory.

Cyclodextrine as encapsulating agent

In order to form this product Ms Alonso chose the carbamate insecticides (concretely, bendiocarb) - nicotinoids and pyrethroids. Carbamates are less damaging for the environment than others, more biodegradable and less toxic for humans. Their great disadvantage is that insects can become resistant to them. Nicotinoid and pyrethroid insecticides, on the other hand, do not have any disadvantage with respect to resistance. Nicotinoids are applied particularly in horticulture, because they are safer for bees. Pyrethroids are the most commonly used, being characterised by their low toxicity towards mammals, although are toxic to bees and aquatic organisms.

Moreover, the researcher intensified the activity of the pyrethroids, including piperonyl butoxide (Pb) in the microencapsulation. This is a substance that increases the efficacy of this insecticide between 10 and 150 times. Within the study she also managed to encapsulate tebuconazole fungicide.

As regards microencapsulation itself, the researcher opted for cyclodextrine-type (CD) carbohydrates as an encapsulating agent. Carbohydrates have high aqueous solubility, little tendency to absorb humidity, a high average lifespan at ambient temperature and are low cost. Cyclodextrines, in concrete, are cyclical dextrines obtained from the partial hydrolysis of maize starch. Also, a yield greater than 99 % was obtained in the formation of microcapsules, on applying a method for dissolution with organic solvents optimised in the laboratory where the research was undertaken.

Less detectable for insects

As was concluded in the thesis, thanks to the micrometric size of the encapsulations obtained, the product has the advantage of being able to treat a greater surface area and being less detectable for insects. Moreover, thanks to the CD protecting the biocide, its toxicological profile is neutralised and its degradation through storage reduced. Besides, on enhancing the solubility of the compound (and thus it being possible to prepare the commercial product based on water) the use of petroleum-derived solvents is avoided, enabling the manufacture of products that are more environmentally-friendly. Finally, the difficulty arising in handling liquid biocides at ambient temperature is overcome with the formation itself of these microencapsulations.

Ms Alonso has been able to demonstrate in the laboratory the insecticide efficacy of the product against the household fly. This result acts as a basis for obtaining effective formulae which can be applied to various types of insects in general and to domestic flies in particular, both indoors and outdoors.

About the author

Ms Mariluz Alonso Alonso (Bilbao, 1978) is a Doctor of Chemical Sciences. She drew up her thesis under the direction of Ms Rosa María Alonso Rojas and Ms Rosa María Jiménez Sanz; professor and senior lecturer respectively at the Department of Chemical Analysis of the Science and Technology Faculty at the UPV/EHU. Ms Alonso is currently a member of the research personnel contracted at this department.