Aphids are population explosions on six legs. It's not surprising, then, that aphid-infested petunias (Petunia spp.) face a serious threat from the sap-stealing insects. Whether grown as perennials in U.S. Department of Agriculture plant hardiness zones 10 through 11 or as annuals anywhere, untreated aphid-infested petunias end up covered in goo, black with fungus and listless from nutrient loss. Getting rid of the invaders is easiest before their numbers soar.
Eliminating aphids before they overwhelm petunias requires monitoring the plants at least twice weekly when they produce lots of new growth. The insects are most active when temperatures are 65 to 80 degrees Fahrenheit. Look for groups of them massed on stems and the back of leaves.
Most petunias have naturally sticky leaves, but aphids compound the problem by excreting gooey, undigested sap called honeydew. spores landing on the honeydew germinate into layers of greasy, black fungus. Heavy sooty mold may interfere with photosynthesis, preventing the petunias from manufacturing what they need for vigorous growth.
Stop an early aphid infestation in its tracks by washing the pests off with a blast of water from a hose sprayer adjusted to a strong spray or jet setting. Early-morning spraying is best so the petunias dry quickly and have less risk of fungal infection.
Where conserving water is important, prune the infested stems and leaves, and dispose of them in sealed plastic bags. Rinse your pruning tools in rubbing alcohol before beginning, between cuts and when you finish so they don't spread plant diseases.
Rid petunias of established aphid colonies by first ridding them of ants. Set enclosed, sugar-based, boric acid ant baits along the ants' foraging trails between the petunias and the ants' nests. Some of the ants will transport the bait back to the nests, where the slowly acting boric acid will kill the ants.
Enter, Aphid Predators
Without ants' protection, aphids have no defense against their natural enemies, such as:
Shallow-flowered, pollen- or nectar-producing plants near your petunias will lure aphid predators. Green lacewings swarm to sweet alyssum (Lobularia maritima), which is hardy in USDA zones 5 through 9. Bishop's weed (Ammi majus), an annual, attracts parasitic wasps, and convergent lady beetles can't resist spring-to-fall-blooming fennel (Foeniculum vulgare), perennial in USDA zones 4 through 9. If fennel is invasive in your area, simply cut off its spent flowers to prevent them from setting seeds.
Suffocation by Soap
If nothing else works, try a ready-to-use insecticidal soap. Insecticidal soap suffocates aphids on contact. The non-toxic, organic soap won't harm beneficial bugs that come to the garden after it dries. Spray the product on the petunias during dry, calm conditions, when no rain is expected for at least one day and the temperature is below 90 degrees Fahrenheit. Dress in long pants, a long-sleeved shirt and safety goggles, and water the plants well before spraying them with the insecticidal soap.
Cover the petunias completely with the spray, ensuring it reaches all the stems and both sides of leaves. Treat the plants with the insecticidal soap weekly, or as often as the soap's label recommends, until they are aphid-free. Follow the manufacturer's directions when working with any insecticide.