Leslie Massey
If you're an Australian gardener, you may have asked yourself: What is eating my petunias? The answer is likely to be budworms, also known as tobacco hornworms. These worms are the larvae of a type of tobacco moth that tunnels into buds of flowers and then eats its way out, leaving telltale holes. They can be difficult to control because they spend so much of their lives inside the bud. Other culprits may include slugs and snails, which can be deterred by covering the soil around plants with wool.
| Characteristics | Values |
|---|---|
| Common Name | Budworm, Tobacco hornworm |
| Scientific Name | Heliothis virescens |
| Description | Green with black spots |
| Larval Stage | Worm |
| Adult Stage | Moth |
| Diet | Petunias, geraniums, calibrachoa, nicotiana flowers |
| Damage | Holes in flowers and leaves, excrement on leaves |
| Treatment | Bt spray, hand-picking, beer traps, copper tape, wool |
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What You'll Learn
Tobacco hornworm/tobacco moth caterpillar
The tobacco hornworm, or Manduca sexta, is a caterpillar that is known to feed on flowering tobacco or Nicotiana plants. They are hairless and green with reddish "horns" at their rear end, which gives them the name "hornworm". The tobacco hornworm can be distinguished from its close cousin, the tomato hornworm, by its seven white diagonal stripes with black borders, as opposed to the tomato hornworm's eight white V-shaped stripes. The tobacco hornworm's "horns" are also usually red, while the tomato hornworm's are black or blue-black.
The tobacco hornworm caterpillar is the larva of a type of tobacco moth. The adult moths, also known as "tobacco flies", drink nectar from honeysuckle and petunia flowers. The moths have a furry, brown-grey abdomen with six pairs of yellow or orange spots, and hairy wings with mottled patches of brown and black.
The tobacco hornworm is native to North America and has also been found in Australia. They hatch in late spring to early summer and develop through five instars to reach maturity. In warmer climates, two generations of caterpillars can coexist in a single summer. The caterpillars are often used as food for the young of certain wasp species. Female wasps lay eggs on the backs of the caterpillars, and the wasp larvae feed on the caterpillar's organs and tissues before pupating and emerging as adult wasps, which then kill the caterpillar.
If you suspect that your petunias are being eaten by tobacco hornworms, you can look for signs such as tiny holes in the leaves, flowers, and buds, as well as worm excrement on the leaves. You can also try filling a container with water and looking for the worms just above the excrement on the leaves. Once found, the worms can be plucked from the plant and dropped into the water to drown.
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Slugs and snails
To prevent slugs and snails from eating your petunias, you can use natural repellents such as coffee grounds, which slugs and snails dislike coming into contact with. You can also create difficult terrain with gravel and mulch, which they struggle to move across. Other natural barriers include copper strips, wire screens, and boards with grease on the undersides. You can also trap slugs and snails with cups of beer, milk, or yeast dissolved in sugar water.
Another effective way to control slugs and snails is by hand-picking them, either by digging around damaged plants or by using boards as slug traps. You can then dispose of them by dropping them in a bucket of saltwater or squashing them.
If you are open to using chemicals, metaldehyde is a widely used poison for killing slugs, although it is not very effective and can be harmful to other creatures, so it must be used with caution.
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Budworm
The adult moths are greenish-brown with light green or greenish-brown wings and four wavy, cream-colored bands. They are about 3/4 inch long with a wingspan of about 1 1/2 inches. The moths lay their eggs on blossoms, fruit, or shoot tips of plants. The very young larvae are yellowish or light yellow-green, but the color of older larvae varies.
To prevent budworm infestations, it is recommended to use new soil each year in pots or garden spaces where susceptible species are grown, as the pest pupates in overwintering soil. Alternatively, rotate your crops each year and plant something that budworms do not eat. Avoid using broad-spectrum pesticides on or around petunias, as these chemicals will damage the populations of beneficial budworm parasites and predators, such as tiny beneficial wasps, that help control budworm numbers.
To get rid of budworms, inspect your plants regularly and pick them off. The best time of day to do this is at dusk, as they are most active at that time. Carry a container of soapy water and throw the budworms into it. Then, you can compost the drowned caterpillars.
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Woolly bear caterpillars
The larvae of the Isabella tiger moth, also known as the banded woolly bear, woolly worm, or woolly bear caterpillar, can be a common pest for petunia plants. The Isabella tiger moth, or Pyrrharctia isabella, was first formally named by James Edward Smith in 1797. The thirteen-segment larvae are usually covered with brown hair in their mid-regions and black hair in their anterior and posterior areas. In direct sunlight, the brown hair appears bright reddish-brown. The Isabella tiger moth can be found in many cold and temperate regions, including throughout the west coast of Canada and the United States. The banded woolly bear larvae emerge from their eggs in the fall and overwinter in their caterpillar form, allowing most of their mass to freeze solid. They survive being frozen by producing a cryoprotectant in their tissues. In the spring, they thaw and feed on a wide range of plant species, including herbs, trees, and flowers such as petunias.
To prevent woolly bear caterpillars from damaging your petunias, it is important to inspect your plants regularly for signs of caterpillar activity, such as tiny holes in the leaves, flowers, and buds. You can also look for caterpillar excrement on the leaves, which is an indication of their presence. If you find woolly bear caterpillars on your petunias, you can remove them by hand or drown them in a container of water. It is important to dispose of them properly, as they can be harmful to your plants.
Additionally, you can try to eliminate as many of the first-generation caterpillars as possible to reduce the population. You can also try rotating your planting soil to starve the caterpillars and prevent them from feeding on your petunias.
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Beetles
The damage caused by beetles to petunias can vary depending on the species of beetle and the extent of the infestation. Common signs of beetle damage include chewing or scraping marks on the leaves, stems, or flowers, as well as holes of various sizes. In severe cases, beetles can completely defoliate petunia plants or cause the flowers and buds to drop prematurely.
To prevent and control beetle infestations on petunias, several methods can be employed:
- Insecticidal soaps or oils: These are natural, non-toxic products that can be sprayed onto the plant to repel or kill beetles.
- Neem oil: This natural pesticide is effective against a wide range of insects, including beetles. It works by disrupting their feeding and reproductive cycles.
- Row covers: Using lightweight row covers or garden fabric can create a physical barrier, preventing beetles from gaining access to the petunia plants.
- Trap crops: Certain plants, such as radishes or nasturtiums, can be used as trap crops to lure beetles away from petunias.
- Beneficial insects: Introducing natural predators of beetles, such as ladybugs or lacewings, can help control the beetle population in a sustainable manner.
- Cultural controls: Proper garden maintenance, such as removing plant debris and rotating crops, can deny beetles their preferred habitat and food sources.
It is important to correctly identify the specific type of beetle causing the damage, as different species may require tailored management strategies. Consulting with local gardening experts or extension offices can provide region-specific advice for dealing with beetle infestations in petunias.
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Frequently asked questions
The culprit could be budworms, also known as tobacco hornworms. These worms are the larval stage of the moth Heliothis virescens and are known to eat their way through petunia buds and flowers, leaving behind tiny holes.
Keep an eye out for tiny holes in the leaves, flowers, and buds of your petunias. You may also spot worm excrement, also known as frass or bug poop, on the foliage. The budworm itself can be challenging to spot as it blends well with the foliage.
You can use Bt (Bacillus thuringiensis), a naturally occurring soil bacterium that produces a crystal protein toxic to budworms. Alternatively, you can handpick the worms from your plants and drown them in a container of water.
Yes, slugs and snails are also known to feast on young petunia plants. You can use beer traps or wrap copper tape or wool around your pots to deter these pests.
