Answer: Insecticidal Soap is the treatment of choice for aphid infestations and I would recommend going one step further - spray the leaves to the point of saturation, leave for 5 minutes, then thoroughly rinse the pesticide from the leaves with plain water. This step will ensure the aphids are killed but will also prevent burning of the foliage of your lemon tree. The fatty acids in the Insecticidal Soap can build up and clog the natural openings in the leaves, so rinsing it off will keep everything clean. Aphids love tender new leaves and since they can fly, they can appear on the most unlikely garden plants. I'm sure the aphids hitched a ride while your tree and other plants were being transported. Controlling the population while it is still small should keep your other plants from becoming victims of this pest. Best wishes with your new tree.
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