One year, when I was growing petunias, I noticed many irregularly shaped holes in the petals. I scoured the internet for what disease or pest could possible causing such a problem. It was only after closely looking at my petunias that I found a tobacco budworm sitting on the stem.
Not a worm, but rather, the caterpillar of a night-flying moth, Heliothis virescens is a common garden pest that feeds on the buds and blossoms of petunias, geraniums, and nicotiana. They eat into the flower bud, and when the flower opens, you will see round holes in the petals.
Tobacco budworms are also known as geranium or petunia budworms. In agriculture, they are a serious pest of cotton and tobacco, but in the home garden, the damage is simply cosmetic. The plant itself is not harmed – only the flowers are ruined.
While geraniums, nicotiana, and petunia family flowers are their favorites, they also dine on roses, snapdragons, verbena, and other flowers. Buds fail to open, or when they do, the flower petals are riddled with shot-holes and are chewed and ragged.
Because this pest hides during the day, the trouble it causes often goes unnoticed until the damage is severe. Recognizing damage early is your first line of defense. Here’s what you need to know.
Life Cycle
This pest overwinters in the soil as pupae and the adults emerge in spring. These moths have a 1-½ inch wingspan and are greenish-brown with wavy brown, tan, or cream-colored bands across their wings.
Eggs are round, white, or pale green, and are laid in the evening on foliage close to the ends of stems where flowers are forming. A female can lay 500 to 1,000 eggs in a 2 to 3 day period. After about a month, in early summer, the eggs hatch, and the tiny, yellow larvae burrow into the buds where they feed.
These caterpillars feed and grow until they are about 1-½” in length. They are usually brown, but can be green, purple, or red depending on what they’ve been eating. They all have stripes down the length of their bodies and are spiny (they feel rough to the touch). Once the caterpillars reach full size, they drop down into the soil and pupate.
Although overwintering pupae die if temperatures are below 20°F, adult moths are carried north by the wind. In southern states, there can be four to five generations per year. In northern states, there is one generation, but there can be two, especially if the previous winter was mild.
Identification
This pest is easily identified by the damage it does. Since they feed on flower buds from the inside out and only come out on the foliage at night, you usually won’t see them. When your geraniums open and you see holes in the petals and ragged, chewed-up edges, you know you have tobacco budworms.
Another sign of an infestation is a smaller number of blossoms. For instance, you may notice that your petunia plants have nice, healthy foliage, but only a few flowers. A third sign is the presence of little black specks about the size of a pinhead – those specks are caterpillar droppings (frass).
Prevention
Preventing these pests is next to impossible. But there are a few cultural practices you can use to keep the population low and lessen the damage budworms cause.
- Control host plants. These caterpillars will eat more than just your prized flowers. They are also found on a wide variety of weeds. If you keep the weeds under control, the adults will have fewer places to lay their eggs, which results in smaller populations.
- Use plant species that are not susceptible to these pests. They are not attracted to ivy geraniums, for example. But denying yourself the beauty of petunias, calibrachoas, roses, and other plants may not be practical or desirable.
- Till flower beds in spring to kill any overwintering pupae. As you are planting in the spring, destroy any of them that you come across.
- They can survive in greenhouses. When shopping for annuals in early summer, be on the lookout for small holes in the flowers and don’t buy those plants. You don’t want to take these pests home with you!
- Scout for eggs and holes in flower buds. Frequent inspection of your plants will allow you “nip it in the bud” – literally! If you are diligent, you can stop damage before it gets severe.
Control
Controlling them is difficult. Because they feed inside flower buds, gardeners don’t often notice them until a lot of damage is done. This is why scouting is so important.
Their feeding location offers them lots of protection – from predators and chemical controls. Like most insect pests, the eggs and pupae are invulnerable.
Mechanical Control
Handpick the caterpillars in the evening when they come out to explore. Use a flashlight to help find them and drop them into a bucket of soapy water to kill them.
Search for, remove, and destroy flower buds that have holes in them. Many times the caterpillar is still inside the bud.
If you are overwintering container gardens in a garage or other sheltered area, keep in mind that pupae can be in the soil, and the temperatures in these microclimates can be warm enough for the pupae to survive. Repot your plants in fresh soil before you overwinter them.
It’s a drastic measure, but shearing your plants back and removing all the buds when an infestation is first noticed can reduce the population significantly. If there is enough time left in the season for the plants to recover and rebloom, this control may be a good option. Be sure to fertilize! And destroy the trimmings, don’t compost them.
Biological Control
Since these insects are native in the United States, they have many natural predators. A healthy garden ecosystem will have many beneficial insects that will help keep the population down. Consider:
- Ladybugs, lacewings, and assassin bugs eat budworm eggs
- Birds will eat the caterpillars (if the caterpillars are out during the day)
- Parasitic wasps lay their eggs on the caterpillars, the wasp’s larval food source
- Spiders will eat caterpillars and moths
Chemical Control
Tobacco budworms are tough to control with chemical sprays for three reasons:
- The caterpillars are inside the flower buds and the spray does reach them.
- Over the years, they have developed resistance to many insecticides.
- Using insecticides when pollinators are active will kill the pollinators too.
A product containing Spinosad (a naturally occurring bacteria found in soil that is toxic to insects) can be applied after dusk when bees are not active. However, it needs to touch the caterpillars or be eaten by them for the product to work.
Bacillus thuringiensis (Bt) will also kill the caterpillars, but it needs to be eaten to work. So it’s often ineffective; the caterpillars just don’t eat enough of it. If you decide to use these products, follow the label directions carefully and spray all the surfaces of the plant – over, under, and all around – and spray until the point of runoff. You will probably need to spray more than once during the growing season.
Insecticidal soaps and insecticides containing pyrethroids are not effective against this pest. Homemade bug sprays won’t work either.
Of course, no one wants to grow geraniums only to have all the flowers eaten by caterpillars, and if you’re hosting a backyard party, you want your petunias in maximum bloom. But sometimes, trying to rid our gardens of one pest often invites others, or decreases the population of beneficial insects that can do the job for us. So keep scouting for tobacco budworms and other pests but keep your tolerance levels high and your pesticide use low.
