It has long been taught that all drugs are poisons - it is just a question of the dose. It would be appropriate to add that it is a question of the individuals susceptibility, and this applies to animals and plants as well as people. Tobacco extracts and nicotine are poisons for people and animals, including insect pests. Spraying with nicotine is a former pest-control measure that went out of fashion when more potent control agents of greater effectiveness and reliability were invented and promoted by industrial manufacturers. Nicotine is mainly known as a plant or crop protector against insects which suck plant juices. However, it proved to be hazardous to users and faded from general use. It may be worth re-evaluating this relatively cheap agent for other uses including water purification. The main issue would be whether the nicotine could be used economically and effectively without posing any hazard to users or consumers. The necessary research could be done, but it is very unlikely that industry would sponsor a high quality study.