Nicotine is an effective pesticide for several small insects, except for plants in the nightshade family, to which it belongs. If you smoke (and, sad to say, I do) always wash your hands very thoroughly before working in your tomatoes, potatoes or eggplant. Tobacco in cigarettes may still harbor pathogens that attack nightshades, and you can spread those deseases to your nightshade crops. Nicotine sulfate in a pesticide manufactured by Bayer may be responsible for Bee Colony collapse.

Informative but incomplete article on nicotine
Good article on   soap.

Soaps and nicotine do not leave lengthy residues and so are probably more benign when used correctly than synthetic pesticides. But they both also have the potential to harm beneficials.

Moderation in all things, I guess.

Over time, I found that using row cover at the proper time, planting trap crops, and good garden hygiene resulted in a substantial decrease in my need to use pesticides of any kind.