Petunias (_Petunia x hybrida_) fill many needs in the landscape, including mass plantings, borders, ground covers, hanging baskets and containers. The plants also work well in rock gardens or planted where their trumpet-shaped flowers and ovate leaves cascade over walls. Petunias grow as annuals in all locations but are herbaceous perennials in U.S. Department of Agriculture plant hardiness zones 9 through 10. The plants are relatively pest-free, although several white bugs can create problems. Luckily, they're manageable.
Aphids are small, pear-shaped insects that come in several colors, including white. These insects damage petunias by sucking the sap from the plants with their long, strawlike mouthparts. Aphid eggs overwinter on plants and hatch the following spring, but may go unnoticed during this stage. A severe aphid problem is obvious, though, because the insects gather in masses, covering plant stems, buds and foliage. Small infestations typically don't cause much damage or require treatment. Symptoms of a more serious aphid problem that may call for intervention include the following:
- Yellowing leaves
- Puckering leaves
- Distorted leaves
Once aphids gather the nutrients they need from the petunias, they secrete a concentrated waste called honeydew. These secretions may lead to sooty mold, a fungal problem that can blanket the plants in a velvety black layer, which , if heavy enough, can lower the plants' ability to photosynthesize. You can wash sooty mold off the leaves with a blast of water or wipe it off with a damp cloth.
Many times ants are present where aphid problems exist. You might observe them running up and down plant stems as they herd the aphids together to collect their honeydew. The first line of defense is to treat the ants. An effective way of handling the problem is to place commercial ant stakes or ant bait traps around the affected petunias. These traps are safe for wildlife and beneficial insects and operate by attracting worker ants, which eat the bait and carry bits of it back to the nest to share, usually wiping out the colony in a matter of days or weeks.
Whiteflies may not be noticeable on petunias until they reach their last winged stage of development, so you're not likely to see their tiny eggs underneath the leaves. Once they reach their winged stage, however, you'll see crowds of white gnatlike insects flying around infested plants when disturbed. Similar to aphids, whiteflies in all stages of development suck sap from petunias. They also produce honeydew, which can lead to the development of sooty mold. Treatments for minor infestations of whiteflies usually aren't necessary, but large outbreaks require control, which becomes increasingly difficult during periods of warm weather when whitefly populations are on the rise. Symptoms of a whitefly problem include the following:
- Yellowing foliage
- Dry-looking leaves
- Leaf drop
- Possible death of leaves
Like aphids and whiteflies, thrips have mouthparts designed for sucking juices from the petunia's leaves, buds and flowers, and damage is inflicted by both the adults and nymphs. These insects are elongated, about 1/20 inch in length and, depending on the type, can be white with fringed wings. Because of their small size and feeding habits on unexposed portions of the plants, control is often difficult, and many times, damage occurs before the pests are discovered. Thrips also transmit various viruses that can be deadly to young plants.
You can easily check for thrips by holding a white piece of paper underneath a petunia and shaking the plant. If thrips are present, they'll fall onto the paper. Additional indications of a thrip infestation include
- Distorted leaves
- Leaf curl and scars
- Deformed and discolored flowers
- Flower buds that never open
- Silvery discoloration on leaves and petals
- Leaf drop
- Black to brownish droppings on foliage
Beneficial Predator Insect Control
Unless aphid, thrip or whitefly infestations are severe, many times Mother Nature takes care of the problem with beneficial insects that attack and kill the pests. Often, no other control is necessary. Keep in mind that the use of harsh insecticides can lower the population of beneficial insects, including honeybees. When beneficial insect populations decrease, whitefly, thrip and aphid populations increase. Beneficial insects that help control these pests include the following:
- Predatory wasps
- Lady beetles
- Minute pirate bugs
- Lacewing larvae and adults
- Soldier beetles
- Predatory thrips
You have a variety of management options at your disposal that won't disrupt populations of beneficial insects, but will control thrips, aphids and whiteflies and are environmentally friendly.
Pour on the Water
Water your petunias regularly to keep the soil moist. Drought-stressed plants are more prone to pest infestations. Dislodge whiteflies, thrips and aphids with a strong blast of water from a hose and water the surrounding garden area to keep dust down, which helps control pest populations. If thrips are present, overhead watering will kill them.
Monitor the Nitrogen
Don't overfertilize petunias with nitrogen. It creates excessive new growth on the plants, which in turn attracts aphids and thrips.
Reduce Weed Growth
Keep the garden space clean and free of weeds. Overgrown weed populations attract many pests such as aphids and thrips, which move to the petunias when they're planted in or near weedy areas.
Prune and Discard
Distorted, curled leaves shelter pests, making it harder for treatments like insecticidal soaps to reach and kill them. With heavy aphid, thrip or whitefly infestations, it's best to pull up ravaged plants and discard them in a sealed plastic bag. For lighter infestations, prune out the affected leaves and dispose of them in the same manner.
Use Insecticidal Soap
If the pest populations are large, you may have to control them by using insecticidal soap, which is milder and safer for beneficial insects than synthetic pesticides. Spray a ready-to-use insecticidal soap mixture over the petunias, being careful to cover both sides of the foliage and other areas that are plagued by pests. Monitor the plants for insects and reapply every five to seven days if needed.