The black beetles eating your basil (Ocimum Basilicum) are probably flea beetles, which are one of the few pests that attack this fragrant annual herb. These small insects are typically not a serious problem and do little to no long-term damage to basil. Controlling flea beetles is relatively easy, requiring proper care and preventive measures.
Flea beetles are small, active insects with a shiny black body. Their enlarged, black legs allow them to jump like fleas. They measure about 1/10 of an inch long and quickly jump when disturbed. This makes locating these annoying pests a bit difficult. Adult flea beetles overwinter in plant debris and emerge in the spring to feed on the leaves of plants. They lay their eggs in the soil, and — after the eggs hatch in about a week — the larvae feed on plant roots.
Flea beetles and their larvae damage plants during their feeding. Tiny, shot-like holes in leaves is a sign of flea beetle feeding, while a large infestation can result in bleached, ragged leaves, complete defoliation, and stunted growth. The larval stage doesn’t typically cause serious damage but can leave scarring on the roots. Thankfully, damage is not generally serious on established basil plants. Flea beetles can also transmit diseases from one plant to another.
According to the University of California Statewide Integrated Pest Management Program website, insecticides are typically not warranted unless the flea beetle population is high. In these cases, insecticides containing carbaryl can be applied to control the flea beetles without harming the basil plants. However, care must be taken, and you should follow the instructions located on the label to prevent damage to the plants. For example, one brand of ready-to-use carbaryl insecticide recommends spraying the foliage of infected plants every 7 to 14 days until the flea beetles are under control.
Since seedlings are more likely to become damaged by flea beetles, use protective coverings until they reach the sixth leaf stage. Another preventive measure is to apply reflective mulches around the plants before they become infested with flea beetles. Furthermore, regularly remove debris near the basil, especially in the fall. Without the debris, the adult flea beetles won’t be able to overwinter and reinfest the basil plants in the spring.