<prompt> Malathion is used once every -3 months6 months9 months12 months  <response>: Malathion has the least toxicity of all organophosphorus compounds. Water disperaible powders are available. Malathion is used in doses of 100-200 mgsq.ft. every 3 months. Lindane, malathion OMS-33 duration of effectiveness is 3 months while DDT has effectiveness for 6 to 12 months. Due to low toxicity malathion is recommended as an alternative to DDT. Parks Textbook of Preventive and Social Medicine, 25th Edition, Pg 833 