Effect of Pyrethrum on mosquitoes is described as -

Pyrethrum extract is an excellent space spray. the active principle(pyrethrin) is a nerve poison and kills insect instantly on mere contact. Sprayed at a dose of 1 Oz of the spray solution per 1000 Cu.ft.of space. Doors and windows are kept closed for half an hour. Effective in reducing the number of mosquitoes but the reaction is only temporary since it has no residual action. Park's Textbook of Preventive and Social Medicine, 25th Edition, Pg 833