The most common bugs that harm rosemary plants are aphids, spider mites, whiteflies and mealybugs. Ladybugs are particularly beneficial around rosemary plants, as they feed on the aphids.
An increase in beneficial insects is one indication that harmful bugs may be attacking the rosemary plants. Check the plants for signs of bugs, many of which are visible, particularly on the stems, under leaves and at the base of the plant. Examine the rosemary for dead or brown leaves and stems, another sign that bugs are eating the plant.
Spray the bugs with insecticidal soap or a solution of 1 cup water, 1/2 tsp. dish soap and 1/2 tsp. vegetable oil. Thoroughly rinse the plants off after the bugs die. Immediately trim and remove all dead leaves and stems to prevent reinfestation.