The angel's trumpet plant (Brugmansia) is a South American shrub or small tree that originally hails from Brazil, Columbia and Chile. It is renowned for its trumpet-shaped flowers, hence its name. As a relatively tropical plant, the angel's trumpet can only be grown in USDA hardiness zones 9 through 11. Aphids are a major pest and will cause widespread foliage loss if not treated immediately, according to the University of Wisconsin.
Inspect the plant for aphids. The insects congregate on the underside of the angel's trumpet plant's leaves and at the junction of the leaf and the branch. Though aphids may be present all year, they are most common in March and April, according to the University of Wisconsin.
Spray affected foliage with a standard insecticidal soap, obtained from most garden stores and nurseries. Mist the soap directly on the underside of the leaves where the aphids are located. The soap will kill them upon contact.
Inspect the foliage again after seven to 10 days. Re-treat, as necessary. New aphids may have hatched or moved onto the plant since you last applied the insecticide.