Southern Purple Mint Moth
One very popular herb for the backyard garden is the evergreen Rosemary, Rosmarinus officinalis which serves as an attractive landscape shrub in addition to its culinary uses. Rosemary grows best in full sun and well-drained soils. When planted in the ground, rosemary can grow 4-5 feet in height and spread. Plants do have beautiful bluish flowers that form in late winter and early spring, providing nectar for bees.
Rosemary is generally a pest free plant but some gardeners may find damage resulting from an insect pest. Rosemary is one of the hosts for the Southern purple mint moth, Pyrausta laticlavia which will lay eggs on several plants in the mint family. Caterpillars feed on leaves along the stems which become matted with silken threads and frass from the insects. As a result of the feeding, leaves are not useable and plants do not look attractive. Heavy populations will stress plants.
Since rosemary is often used to flavor many foods, many people do not want to use an insecticide on the plant. One least toxic option that can be used safely on edibles is Bacillus thuringiensis. The product is specific for caterpillar management and safe for predators that may help manage some caterpillars. Homeowners may also physically remove caterpillars or prune out damaged stems on larger plants.