Basil (Ocimum spp.) is an annual herb that suffers from few pests, but Japanese beetles, aphids and slugs are sometimes tempted by its aromatic leaves. Sweet basil (Ocimum basilicum) and "Pesto Perpetuo" (Ocimum × citriodorum "Pesto Perpetuo") are two commonly affected varieties. Japanese beetles are about 1/2 inch long and metallic, shiny green. These pests can reduce basil to stems. Aphids cluster on shoot tips and the undersides of leaves, sucking basil's sap and causing distorted growth. Repellent sprays and certain plants help repel Japanese beetles and aphids, and diatomaceous earth helps repel slugs. Row covers protect basil from most pests.
A hot pepper spray repels most pests from basil but should be used with caution. To make the spray, place 3 to 4 chopped hot peppers or 2 to 4 teaspoons of cayenne pepper sauce in 1 quart of boiling water. Turn off the heat and leave the mixture for 24 hours. Strain the mixture through a sieve to remove the remains of the hot peppers, and add 1 quart of cold water. To help the repellent stick to the basil leaves, add 2 drops of biodegradable dish-washing liquid and mix them gently into the solution. Before spraying a basil plant, apply the spray to a small area and wait 24 hours. Check the area for harmful effects before spraying the rest of the plant.
Diatomaceous earth repels and controls slugs before they can eat basil plants. Slugs graze on basil at night or on dull, cool, damp days, leaving ragged holes. To help repel slugs, sprinkle a layer of diatomaceous earth 1/4 inch deep around the basil plants. Slugs that try to cross the barrier are scratched by the earth's sharp grains, which causes dehydration and death. Reapply the diatomaceous earth barrier after watering or rainfall. Avoid mulching basil plants suffering from slug attack because slugs hide in the mulch during the daytime.
Japanese beetles and aphids don't like the smell of certain herbs, and growing them near basil can have a repellent effect. According to the University of Illinois Extension, chives (Allium schoenoprasum) repel Japanese beetles, and dill (Anethum graveolens) repels aphids. Grow these herbs next to basil plants for the best effect. Chives are hardy in U.S. Department of Agriculture plant hardiness zones 4 through 8, and dill is an annual herb. Dill can become invasive, so remove its faded flowers to prevent it from self-seeding.
Growing basil in garden borders with catnip (Nepeta cataria) and white-flowered chrysanthemums helps protect the plant from pests. Catnip and white-flowered chrysanthemums, such as Marilyn (Chrysanthemum "Mariyo" Marilyn), repel Japanese beetles and catnip also deters aphids, according to the University of Illinois Extension. Sow or transplant basil plants near these perennials, but not so close that they block sunlight from the basil. Catnip is a perennial in USDA zones 3 through 7, and Marilyn is hardy in USDA zones 5 through 9. Though it repels many insects, catnip may attract cats into the garden.
Row covers prevent many flying and crawling pests from reaching basil. Sheets of lightweight material that allow light and water to pass but block out pests, row covers are sold at most garden stores. Place a piece of row cover material over basil as soon as it goes out into the garden in spring. Weigh the edges down with stones, pulling the edges tight against the ground so that no insects can crawl underneath. Examine the cover for holes every week or so, and lift it to check that no pests have managed to enter.