Rose of Sharon or althea is a large flowering shrub or a small tree related to the hibiscus via the mallow family. The late spring and summer flowers are an attractant to bees and butterflies in the garden so insect control must be kept to natural, physical means to avoid killing off beneficial insects while removing the troublesome ones. According to the U.S. Forestry Service, aphids are the No. 1 nemesis of Hibiscus syriacus, with Japanese beetles playing runner-up in Northern climates. Physical removal of both bug species is simple to do and can be effective without harm to the plant.
Spray off colonies of aphids that gather on the fresh young tips of the branches. Use a garden hose with an adjustable stream to blast the bugs off of the plant without harming the plant tissues. The hardest jet setting may be too strong but a moderate stream can easily dislodge the aphid colonies.
Snip off the tips of the branches that are infested with aphid colonies and discard or burn them. They can be snapped off with your fingers or cut with scissors or pruning shears. This will not hurt the plant and will immediately reduce the aphid population on the plant.
Refrain from over-feeding your Rose of Sharon as excess fertilizer tends to boost aphid populations and does not do much to help the plant. Follow your fertilizer product dosing directions carefully to prevent inadvertent application of excess fertilizer.
Pick or flick off any Japanese beetles that alight on the plant to feed on the flowers. Patrol the plant in the morning when the beetles are groggy and their reflexes are slow. Flick the beetles into a jar or bucket of soapy water with a finger, dowel or soft paintbrush to capture and drown them. Relocation is futile due to their ability to fly and to attract more beetles to a specific site.