Aspen trees tend to be aphid magnets during the warm months and aphid egg hatcheries when it's cold. A sure giveaway of the presence of adult aphids is curling of the affected aspen's leaves, caused by the sticky "honeydew" that the insects secrete. They will probably appear in late spring or early summer when conditions become favorable for aphid activity. Accommodating factors include plenty of moisture, warm weather and the presence of few predators. Treat the aspen tree in the morning or early afternoon so that it will dry before nightfall as soon as you discover the presence of aphids. You'll find this home remedy not only environmentally friendly, but highly effective.
Spray the affected aspen with Isopropyl alcohol every three days for two weeks. Wet all surfaces thoroughly. The alcohol will kill aphids on contact. If the infestation is light and hatching adults are caught early enough, this may completely solve the problem.
Spray affected areas of the aspen with a strong jet from the water hose if treating with alcohol isn't doing the job.
Mix up a batch of insecticidal soap and spray the tree with it, covering all surfaces thoroughly. Follow the packaging instructions carefully. Allow the solution to set for about 30 minutes, and rinse off completely.
Repeat insecticidal soap applications every four to seven days as long as your aspen tree is troubled by aphids. The duration can extend right up to frost. Stop regular treatments with insecticidal soap in late fall.
Apply another insecticidal soap treatment in late winter or early spring. Pick a day when the temperature is above freezing. This will help by knocking down aphid eggs which are overwintering on your aspen, preventing a huge hatch-out of adults in the spring.
Spray with horticultural or vegetable oil in February or March. Choose a day when the temperature is above freezing. Apply from the tips of the limbs back about 18 inches. Branch tips are where aphids spend their winters. Coat all surfaces generously with the oil, which will clog the breathing of the aphids and smother them. Follow the manufacturer's recommendations carefully.