The watermelon and the green alfalfa are important crops in Morocco. The ripe fruit of the watermelon has a high nutritional value, rich in vitamin and mineral salts and used for medicinal purposes (kidneys and cleaning of the urinary tract). The alfalfa green is principally cultivated for livestock feed. A common problem in the production of watermelon and the alfalfa is the infestation by aphids. They transmit viruses of the tile to the plants. They are more likely to be a problem in the time costs. The study presents the effects of products Thyme, Oregano, Neem and their mixture on the aphids of watermelon in comparison with their effects on aphids of alfalfa green. The tests were conducted of the months from May to September on the aphids on the fields of watermelon and alfalfa green. It determines the percentage of deaths of aphids in function of the dose 1% and 5% for these products and the time 3, 5 and 7 hours after treatment. A comparison between the results obtained showed that the extract of Thyme is more active in watermelon and the Oregano is in alfalfa.