Efficacy of homemade botanical insecticides based on traditional knowledge. A review

Homemade botanical insecticides are widely used by subsistence and transitional farmers in low-income countries. Their use is often driven by the limited availability or cost of commercial pesticides. Homemade botanical insecticides are often recommended by agricultural extension services and some development organizations. However, this could be questioned because scientific evidence of their efficacy and safety may not be available or accessible. Although botanicals with insecticidal properties have been widely studied, a synthesis focusing specifically on homemade preparations used in realistic field or storage conditions is missing. In this paper, we review efficacy assessments of botanicals used to prepare homemade insecticides. This covers twelve botanicals recommended by national extension partners in 20 countries within the global agricultural Plantwise program. These are as follows: garlic (Allium sativum), neem (Azadirachta indica), chili pepper (Capsicum spp.), Siam weed (Chromolaena odorata), mother of cocoa (Gliricidia sepium), chinaberry (Melia azedarach), moringa (Moringa oleifera), tobacco (Nicotiana tabacum), clove basil (Ocimum gratissimum), tephrosia (Tephrosia vogelii), tree marigold (Tithonia diversifolia), and bitter leaf (Vernonia amygdalina). This review shows that (1) all the selected botanicals contain active ingredients with insecticidal, antifeedant, or repellent properties, and (2) homemade insecticides based on all the selected botanicals have been used with some success to control pests or prevent damage, although efficacy was variable and often lower than the positive controls (synthetic pesticides). Factors affecting the efficacy of homemade botanical insecticide include variation in active ingredient content and concentration in plant material, as well as variation in the preparation process. In conclusion, there is some evidence that homemade botanical insecticides could contribute to reducing losses in food production. Since further research is needed to better understand their variable efficacy and potential health and environmental risks, those who promote the use of homemade botanical insecticides should also communicate those “unknowns” to the farmers who use such products.