The petunia (Petunia x hybrida) is a herbaceous, short-lived perennial usually grown as a bedding annual. Though petunias are generally considered pest-free, plants that are under stress from lack of care or those that have bad luck may develop an infestation that can threaten the life of the plant. In some cases, an already dying petunia is better off discarded than saved.
Petunias are attacked by several kinds of foliage- and flower-feeding caterpillars, notably the petunia budworm and the variegated cutworm. Caterpillars can kill or stunt the growth of young petunias. Remove any visible caterpillars by hand and destroy them. Eliminate nearby weeds, which can act as hiding places, and provide proper care to help the plant's overall health improve. Sometimes, natural predators and viruses will step in and save the day if the plant is able to survive long enough.
Spider mites are usually just a nuisance on woody plants, but they can easily kill annuals, notes University of California Statewide Integrated Pest Management Program. Water stress is a leading cause of spider mite infestations, so water your petunias deeply and regularly if you've been neglecting them. Applying ready-to-use insecticidal soap and spraying all leaf surfaces thoroughly may be effective. Cyclamen mites, which are not visible to the human eye, cause leaves to curl and flowers to discolor and drop. Sadly, plants infested with cyclamen mites cannot be saved and it's better to remove the plants before the mites spread.
Western flower thrips can wreak havoc on petunias, killing flowers and greatly weakening the entire plant. A thrips infestation can be difficult to diagnose and control, though you may be able to find thrips by setting out sticky blue traps, which you can buy at some garden centers. Improve care by watering the petunias regularly, removing spent flowers and cutting back on high-nitrogen fertilizers. Aphids are easy to spot and may be dislodged with a strong stream of water or by releasing predatory insects, such as lady beetles.
Petunias prefer a bright, sunny area and well-draining, moist soil that rich in organic matter. The plants should be watered regularly to a depth of 6 to 8 inches -- the soil should never be allowed to become bone dry. Petunias may be grown as perennials in U.S. Department of Agriculture plant hardiness zones 9 through 10, though they are generally grown as annuals. You can fertilize petunias by spreading 2 pounds of a 5-10-5 fertilizer over each 100 square feet of garden area and watering it in thoroughly.