Petunias are great for adding a touch of color to a patio or deck. They are grown as annual plants and make it easy for a gardener to change their color palette from year to year. Although Petunias are very easy to grow and have few maintenance challenges, there are some common pests that plague these sun lovers. All types of aphids (whitefly, greenfly, and blackfly), tobacco budworm, rose slugs, western flower thrip, and cyclamen leafminer can affect the appearance and growth of Petunias. The good news is that controlling these pests is easily done with minimal impact on people, pets, and the environment. Larger pests such as deer or rabbits may also decide to browse or nibble on Petunias, but there are easy measures to take to prevent them from destroying your seasonal containers.
Common Petunia Pests
Aphids come in many varieties that can show up at any time during the growing season. White, black, and greenfly all do the same damage to Petunias and can be controlled by the same methods. Aphids are classified as sap-sucking insects that have a larval stage and an adult stage. Both stages suck the sap from plant leaves and stems, eventually weakening the plant, stunting the growth of the foliage, and disfiguring the stems.
Large infestations will secrete a sticky ‘honeydew’ sap that collects on parts of the plant and can drip onto surrounding plants, patios, and furniture. The honeydew attracts ants that harvest and feed on it, which can result in a second infestation of ants in the area. When treated early, it is possible to avoid the honeydew stage and save the plants from collapse.
Treating Aphids on Plant Petunias
Treating a Petunia plant after it has been attacked by aphids is generally successful when the outbreak is caught early before more generations of the insects can be laid or hatch. Often the best treatment is a sharp jet of water from the hose during watering. A blast of water will both knock off adult aphids and drown larvae. Repeating this treatment every couple of days until there are no aphids requires no chemicals and is easy to do. The plants are tough and will not be bothered by a strong jet of water.
While you are treating the plants, the aphids may attract beneficial insects such as ladybirds, lacewings, and syrphid (hover) flies, which is a very good thing! All three will happily gobble up both the adult and larval stages of aphids. Keep in mind that if Petunias are completely free of pests, the beneficial insects will also leave to find another source of food. In order to have beneficial insects regularly in your garden, a balance needs to be maintained between predator and prey. Once this balance occurs, the effects of pests are often not noticeable and are kept to a minimum by the beneficial insects.
The last measure to take against aphids is still a sustainable and ‘organic’ method but does involve a chemical. Insecticidal soap sprays are available to keep aphid populations down. The soap spray is applied to the pest itself as well as the underside of all the plant leaves. Quickly, the soap in the spray eats away at the aphid's exoskeleton, eventually killing it. Repeated application will be needed for control.
After controlling the aphids, any badly damaged stems should be pruned to encourage new growth from the plant. If the infestation happens early in the summer, there is plenty of time to cut the whole plant back and wait for regrowth. Do not over-fertilize Petunias after an aphid infestation. Any excessive, new, juicy growth could bring the aphids back.
Preventing Aphids on Petunias
Preventing an infestation of aphids is easily achieved by following a few cultural guidelines. The number one rule for growing healthy and strong plants is to make sure they are growing in their preferred location with the correct type of soil, pH, and nutrients. Petunias require full to part sun, good drainage, and a neutral soil pH. Ample air circulation is an important element in keeping aphids at bay. Plants growing too close together will eventually weaken in vigor and are easily attacked by pests and fungal diseases. Growing a happy plant starts with buying the healthiest plant available. Plants that are ‘rescued’ from the bargain bin will need a period of quarantine from other plants until it has regained strength with no sign of pests or disease.
The tobacco budworm, Heliothis virescens, is a caterpillar of the moth by the same name. They are common in the east and southwestern parts of the United States. In colder areas, such as the Mid-Atlantic states, the budworm overwinters in greenhouses and other protected sites. These caterpillars feed on all parts of ornamental plants and are most likely to disfigure flowers in the bud stage, or seed pods in the early green stage. Damaged flower buds will not open, while affected foliage will have a ragged appearance on the edges or whole sections eaten away in random patterns. The caterpillars are yellow or light green with reddish-brown stripes on the sides of the body. The caterpillars are laid in the soil to overwinter by the moths and appear early in March or April. There are usually one or two generations a year. Newly planted Petunias may be affected by heavy feeding, although more mature plants usually recover well.
Treating Tobacco Budworm on Petunias
The caterpillars are easily plucked off affected plants and drowned in a bucket of water or squished. Remove any parts of the plant that are damaged to encourage regrowth. Petunias respond to pruning by growing bushy new growth that will quickly hide any damage.
Broad-spectrum insecticides rarely control populations of caterpillars and should not be used around beneficial insects such as bees, ladybirds, and lacewings. If there is an ongoing issue with tobacco budworm caterpillars, a soil drench made with beneficial nematodes may help to control future generations. Bt, Bacillus thuringiensis, is an organic product that is made from beneficial, predator nematodes. The powder can be diluted in a watering can and applied to the soil where budworms overwinter. Always follow the manufacturer's instructions on pesticide labels, even organic ones. Improper use could result in ineffective control or harm to the user.
Preventing Tobacco Budworm on Petunias
Good garden and greenhouse hygiene will help to control or prevent outbreaks of tobacco budworm. Using fresh, clean potting soil to repot plants every season will cut down on the chance that caterpillars will overwinter. Weed well around the areas where your Petunias will be growing. Overgrown or weedy areas are perfect for the budworm to overwinter.
Rose slugs, scientifically known as Endelomyia aethiops, are the larvae of sawfly and like to attack ornamental plants in particular. The damage is rarely life threatening to a perennial plant but can make an annual such as a Petunia weak and very unsightly. The leaves will have brown patches where everything but the lowest layer of leaf tissue has been eaten away. The whole plant can be defoliated if the larvae are left alone. The Rose Slug appears in April or May and is 3 ⁄ 4 of an inch long, and yellowish green with an orange head. The Rose Slug is given its name because the legs are very short and not readily visible giving the immature adult the look of a slug.
Treating Rose Slug on Petunias
The larvae can be controlled easily by mechanical methods. Pick any visible Rose Slugs off plants and dispose of them in a bucket of water. The larvae are also easily washed off stems and foliage with a strong jet of water from the hose. When they fall off the plant, they can be squished or disposed of.
Infestations that are recurring or not controllable with the above methods can be controlled easily with sustainable, organic chemical means. Always apply pesticides in the early evening to protect beneficial pollinators and insects that may be in the area. Chemical controls are still chemicals even if they are approved for organic gardening. Bonide Captain Jack Deadbug Brew is a ready-to-use (RTU) spray that will control larger outbreaks, or try Safer Brand BioNEEM. Treating an annual plant such as a Petunia with stronger chemicals may not be worth the potential damage to the environment or the gardener. If the damage occurs very late in the growing season, disposing of the plant and the soil it was potted in may be a much safer and more sustainable option.
Preventing Rose Slug on Petunias
Preventing Rose Slugs is easily accomplished by maintaining good gardening hygiene from season to season. Use clean or fresh pots and potting mix every year. Old potting mix can be used in the garden or composted if there have never been signs of Rose Slugs or sawflies. Dispose of any affected foliage or plant material in the trash. Home compost piles rarely reach the high temperatures needed to kill off pests and diseases. Catching outbreaks early is critical in helping annual plants to recover. Daily or weekly inspections of plants go a long way in catching problems while they are small. Picking off one or two Rose Slugs will keep future generations from being laid.
Western Flower Thrip
Western Flower thrips are a pest that occurs more often in a protected setting of the greenhouse, but it can become a problem in the home garden where it acts as a vector, passing viruses from one plant to another. While the adults are easily recognizable by their slender bodies and long narrow leaves, the nymphs are very tiny and almost translucent. There is more than one generation produced in a year with the adults preferring to lay their eggs in damaged and distorted foliage. The damage by the nymphs may be seen as stippled color on the leaves and striking color breaks in the flowers. Sometimes the damage is confused with spider mites and only inspection with a microscope will determine the true culprit. Even though most of the visible damage is cosmetic and usually not serious, thrips can pass viruses that are harmful to plants quite easily.
Treating Western Flower Thrips on Petunias
Thrips are rarely controlled by chemicals available to the home gardener. Using a broad-spectrum insecticide runs the risk of not only killing off the thrips, but any beneficial insects and pollinators that are in the area. Encouraging a balanced and varied habitat where lacewings, minute pirate bugs, and predatory thrips can dine on the nuisance thrips is the best way to control and prevent large outbreaks. At the plant nursery, buying plants that are healthy and undamaged will also help to keep thrips out of your garden.
Preventing Western Flower Thrips on Petunias
The best way to keep thrips from being a problem on your Petunias is to ensure that any nursery stock is clean and healthy. Discounted plants may seem like a great way to fill up your garden on the cheap, but sometimes those plants have more wrong with them than a bit of neglect. Buy plants that are virus resistant and do not show signs of unusual color breaks or streaked leaves. Grow your Petunias in the surroundings they prefer: full sun, well-draining soil, and nutritious soil.
Leafminers, Liriomyza ssp., will feed on a wide variety of ornamental plants. They do little permanent damage to plants and can be easily controlled. The larvae make randomly patterned tunnels in the leaves between the top and bottom layers. The tunnels have a translucent look to them. The adults lay eggs in these damaged leaves, producing new generations every two weeks during the growing season. The tunnels remove tissue that the plants use for photosynthesis, which weakens and kills the plants eventually.
Treating Leafminers on Petunias
There are no chemical controls that are effective against leafminers. The only way to control leafminers is to remove any leaves that are tunneled or appear stippled in color. Early discovery of the leafminers makes the difference between being able to save the Petunia and having to dispose of a plant that is too far gone. Daily inspection during watering is suggested.
Preventing Leafminers on Petunias
Keeping plants well watered without overwatering keeps them strong and healthy. Healthy plants are much less susceptible to pest attacks. Remove infested leaves quickly and dispose of them in the trash. Diseased or pest-infested plant material should not go into the home compost pile. Compost that is not heated above 150 degrees F for an extended period does not sufficiently kill off pathogens or weed seeds.
Author Robbin Small - Published 4-11-2023