Least toxic organiophosphorus compound is -

Malathion has the least toxicity of all organophosphorus compounds. Yellow/clear brown liquid used in dose of 100-200 mg.per.sq.ft. every 3 months because of its low toxicity, malathion has been recommended as an alternative insecticide to DDT. as a low volume spray (ULV) spray it has been widely used to kill adult mosquitoes to prevent or interrupt dengue hemorrhagic fever and mosquitoes borne encephalitis epidemics. Park's Textbook of Preventive and Social Medicine, 25th Edition, Pg 844