These tiny black bugs are most likely aphids, which are a common hibiscus pest. They are frequently found on or near hibiscus flower buds and new leaves. The sheer number and contrasting color of many aphids makes them easy to spot, particularly during the spring as new growth occurs. Aphids are one of the most prevalent pests and at least 250 different aphid species are pests of important commercial plants within the United States. Act quickly to address this commmon pest. A large population of aphids can injure or even kill hibiscus.
Aphids are very small, pear-shaped insects, ranging in size from 1/32-inch to 1/8-inch in length. Aphids come in just about every color including black, white, yellow and pink. Green aphids are also common on hibiscus plants. While aphids are visible to the naked eye, a magnifying glass is useful to positively identify this pest. All aphids have a pair of small, pointed dual cornicles that protrude from their posteriors. You can be certain the pest is an aphid if these are present, because no other insect has these features.
A small population of aphids will produce cupped or distorted leaves. Leaves will remain this way until they eventually fall off the hibiscus. Flowers buds may also harden, which will result in distorted flowers. Aphids excrete sugary liquid honeydew in their wake, which consists of unused plant sap and any other waste products. This substance is a common growing medium for a visually unappealing black fungus known as sooty mold. Sooty mold may stunt the growth of the plant by interfering with photosynthesis and will attract ants. Aphids also carry viruses that may stunt or kill plants that they infect. This ability to transmit disease is potentially more deadly than the damage inflicted from their feeding.
Aphids are fairly defenseless and easily controlled while their population numbers are low. Unfortunately, the reproductive capacity of aphids is so great that early detection and multiple control strategies are often necessary. Regularly examine new growth for signs of infestation. A small colony of aphids may be hand-picked, crushed or even sprayed off plants using water. There are also numerous insects that feed on aphids including ladybird beetles, green lacewings, syrphid fly larvae and parasitic wasps. These beneficial predators will naturally attract to infestations but, if necessary, more of some species may be purchased. Ladybird beetles are available for purchase at many garden centers.
If natural means of control are not sufficient forms of aphid control, for example if aphids are observed on at least five percent of the hibiscus new growth, consider using insecticides. Insecticidal soaps and horticultural oil can be very effective against aphids and will have little residual effect on hibiscus foliage, people or beneficial insects. Coverage of all parts of the twigs and both side of the leaves is essential because aphids must be directly sprayed with these substances to work. Multiple applications are often necessary.