The Cowpea aphid; Aphis craccivora, adult female was treated for five days, under laboratory conditions, with acetonic crude plant extracts to evaluate their acute effects (during 72hr post treatment) and  efficiency as natural insecticides. The used plant extracts  were Lupine (Lupinus termis), Lemon grass (Cymbopogon citratus), Moringa (Moringa oleifera) and Chillipepper (Capsicum baccatum). The results indicated that Lupine was the  most efficient followed by Lemon grass. Bioassay testes using leaf dipping technique indicated that LC₅₀ of Lupine, Lemon grass, Moringa and Chillipepper was 2.6, 5.2, 13.2, and 38.68gm %, respectively. Lupine and lemon grass extracts were chosen to test their acute effects on aphid adult after 24 and 72 hr post treatment. Treatment of aphids by LC₅₀ of Lupine and lemon grass crude extracts led proteins to be significantly reduced after 24 hr as compared to control. The decrease was continued after 72 hr post treatment. This means that plant extracts exert their effect after relatively short time. Carbohydrates as main energy source were, more or less, affected by the same manner as proteins, while there was no reduction in triacelyglycerols content. On the other hand, treatment activated defensive enzymes like estereses and phenoloxidases. Lupine affected the main metabolites, especially proteins and its metabolizing enzymes more than Lemon grass. Besides, reaction of defensive enzymes was more higher in aphids treated with lemon grass than that treated with lupine. This might explains why lupine was more efficient than lemon grass.