Alternative locust control with a dilutable linseed oil emulsion

Locust swarm control is still a main topic in agriculture and is often based on the extensive use of chemical agents that can be problematic for the environment and human health. Currently, only a few alternative control agents are available on the market, such as neem oil, the fungus Metarhizium sp., and the microsporidian Paranosema locustae. In this study, we improved the formulation of a botanical insecticide that is based on linseed oil and was found to be effective against the desert locust, Schistocerca gregaria. To make sure that linseed oil is most effective, we first tested several natural plant oils in a comparative laboratory study. Linseed oil performed best and resulted in 100% mortality within only 2 days after a single spray treatment. However, this linseed oil formulation contains three expensive essential oils, thus we further improved it to avoid costly essential oils. A much lower concentration of linseed oil was also found to be highly effective and killed adult S. gregaria within 3 days in an experiment performed under semifield conditions. Furthermore, a concentrated stock solution of this botanical insecticide was developed to increase stability and reduce shipping costs in areas where water for dilution is available. Mechanical treatment, applied during the preparation of this linseed oil emulsion, resulted in the formation of micro-colloids, which prevent the separation of phases and enable 1:16 dilution with water containing bicarbonate. Interestingly, only 24 h after single spray treatment, locusts became immobile and most of them were unable to jump, which would make them easy prey for natural enemies. Later, insects died due to the hardening of the linseed oil emulsion. This novel linseed oil emulsion is harmless for humans, rather easy to produce, and remains stable over weeks in its concentrated form, which suggests that it is promising alternative agent for locust control.