Preventing Budworm Damage: Tips For Controlling Budworms
By: Kristi Waterworth
Bedding plants like geraniums, petunias and nicotiana can create a riot of color when planted en masse, but gardeners aren’t the only ones drawn to these bright and prolific flowers. Feeding damage caused by budworm caterpillars is on the rise across the country, causing alarm and panic in the gardening community — so much so that some gardeners are refusing to grow the most frequent plant victims of budworm damage.
What are Budworms?
Budworms are moth caterpillars that chew their way into the tightly coiled buds of flowers and slowly eat them from the inside out. Budworm caterpillars start life as tiny larvae that measure less than 1/16 inch (1.5 ml.) long, but grow up to 2 inches (5 cm.) over the course of the summer. These larvae start out cream colored with brown heads and light colored stripes, but mature into colors ranging from green to rust to black. Identification should be simple — they’ll be the caterpillars eating your flowers from the inside out.
Budworms feed on all types of vegetative buds, but primarily focus on flower buds and maturing ovaries. Flower buds often fail to open, but those that do look ragged from all the petal chewing. As the summer progresses, the damage gets more severe. Fortunately, these pests only feed for about a month before dropping into the soil to pupate, giving your flowers a chance to recover. Two generations a year are common, with the second generation being much more damaging than the first.
How to Kill Budworms
Controlling budworms is all about timing. Since the larvae spend most of their time protected by the buds where they feed, treatment after hatching does little good to destroy populations. Instead, applying pesticides before hatching or to newly emerged caterpillars is the best solution.
Synthetic pesticides like permethrin, esfenvalerate, cyfluthrin, and bifenthrin require fewer applications because they last longer in the environment, but they can be dangerous to beneficial insects like bees, especially if part of your flower garden is already in bloom.
Bacillus thuringiensis (Bt) can be used safely against budworms, but timing is everything. Monitor your plants carefully for larval emergence and apply Bt as soon as the first few eggs start to hatch. Bt has a very short life when exposed to air, but it will target the caterpillars without damaging other insects.
Other, safer methods of control include checking buds for tiny holes and removing those that are infected in hopes of breaking the life cycle. Cold winters are believed to be devastating to pupating budworms, allowing potted plants to experience temperatures of 20 F. (-6 C.) and lower can reduce the next season’s budworm population.
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Budworm larvae are small caterpillars, often less than one inch long, and most commonly green with white or black markings. However, mature budworms can grow to more than 1.5 inches and there are also reddish and tan versions, as budworms can take on the color of the roses they are feeding on. Budworms can be difficult to spot because they are most active at night, protecting them from birds and beneficial insects and making them more difficult to control.
Because budworms can be inconspicuous, the rosarian's first clue of an infestation may be roses that fail to bloom. On closer inspection, eggs may be visible on the rose plant's leaves and shoots. In addition, budworms leave behind a black sticky residue that may resemble mouse excrement. Tiny round or oblong worm holes, one-eighth inch in leaves and flowers and one-sixteenth inch in buds, may be seen and the rose plant may cease flower production completely. Commonly, budworm-infested buds that open will have damaged petals. If not controlled, budworms can destroy several flowers per day, with infestations peaking in late summer.
BUDWORMS TARGET FRESH FLOWERS AND LEAVES ^
Budworms most like to feed on young flowering buds. They’ll ravish the flower as quick as it grows causing unsightly damage and though this damage usually isn’t enough to cause the plant to die, eventually the leaves and stems of the host plant will stop growing. If the plant stops producing new buds, the worms will move to the leaves of the host plant. Typical sign of damage will be brown spots, disappearing buds or leaves, black pellets (their feces) and the feeding worm.
Best Treatment for Budworms
To avoid confusion, the best way to treat these two species is to treat them as one and the same. Control of Budworms starts with keeping an eye on your plants, so that you can detect caterpillar damage early. For example you could watch for minor leaf damage in the veggie garden, or Budworm damage to buds and fruit while watering.
For this to work you will need to know what to look for (see Symptoms of Budworm Damage). If you find damage, you should have a closer look to see if you can find the culprits – i.e. caterpillars. To confirm that it is Budworm you will need to know what they look like (see Description).
Prevention is always better than cure, and you are more likely to be able to achieve that if you are out and about in your garden regularly. There are several things that you can do to prevent serious damage from Budworms (see How to Prevent Budworms Appearing).
Regular monitoring of your garden will ensure that a major infestation of Budworms doesn’t occur ‘overnight’. This is to be avoided, because Budworm damage can be devastating to plants.
If you want to use a spray, try Yates Nature’s Way Caterpillar Killer - Dipel. The product is based on naturally occurring bacteria (Bacillus thuringiensis), which act as a stomach poison against caterpillars. The product will only affect caterpillars, so your ‘good bugs’ are safe. Follow the label directions and it will work against Budworms in your garden.
How Do Geranium Budworms Damage Plants?
Geranium budworm caterpillars feed primarily at night. During the day they can be found hiding along plant stems and leaf undersides, but if you go out to the garden at night with a flashlight, it’s easy to spot them nibbling on the flowers of host plants.
When geranium budworms are present in your garden, you’ll first notice holes in flower buds or buds that fail to open or go missing entirely. When the caterpillars are small, the damage is small, too, but as they grow, they’re able to “deflower” more and more plants per night.
Tobacco (Geranium) Budworm – 5.581
Figure 1. Tobacco budworm feeding on petunia blossoms.
Figure 2. Tobacco budworm feeding on flower bud of nicotiana.
Figure 3. Tobacco budworm tunneling geranium bud.
Figure 4. Cluster of geranium buds damaged by tobacco budworm.
Figure 5. Eggs of tobacco budworm attached to geranium flower buds.
Figure 6. Adult tobacco budworm resting on zinnia flower.
Figure 7. Pupa of geranium budworm, exposed from the soil.
Figure 8. Tobacco budworm may be easily handpicked from small gardens. Tobacco budworm feeds mostly at night and handpicking is usually best done at dusk or dawn as these insects often hide around the base of the plant in day.
In some years, tobacco budworm (Chloridea virescens) can be a serious pest of many garden flowers grown in areas of Colorado. Geranium is a particularly common host, leading to a regional name used to describe it – “geranium budworm”. Petunia and nicotiana are other common hosts. Rose, snapdragons, verbena and many other flowers are occasionally damaged.
In Colorado, problems are most common in the Denver Metro area and Grand Junction. Nationwide, this insect is one of the most devastating insect pests of agriculture, particularly in cotton and tobacco. The caterpillars may closely resemble corn earworm (Helicoverpa zea), a related insect that is a common pest of sweet corn and fruiting vegetables (e.g., peppers, tomatoes).
The adult stage is a moth with a wingspan of about 1 1/2 inches. The wings are light green with gray or brown overtones and a few wavy, cream-colored bands. The moths are active in the early evening, and females lay eggs on buds or leaves.
Tobacco budworm caterpillars can be quite variable in overall color. Dark forms are common but red, green or light brown larvae also occur. Color differences may be related, in part, to the color of the flowers on which the insects are feeding.
The caterpillars become full-grown in about a month, burrow into the soil and transform to the pupal stage. Adults emerge to repeat the cycle, with two generations normally produced each year. At the end of the season, as day length and temperatures decline, the insects go into a state of suspended development (diapause), that they maintain through winter.
The insect is therefore poorly adapted to the harsh winters of Colorado. Tobacco budworm spends the winter as a pupa, below ground, usually 2 to 6 inches deep, within a packed earthen cell that the full-grown caterpillar produces. Overwintering pupae generally are killed if exposed to temperatures below 20 o F.
Survival through winter occurs best where there are warm soil microclimates, such as those found around the foundations of heated buildings. As a general rule, the number of overwintering tobacco budworms and the likelihood of problems are related to the severity of the previous winter.
Controlling Tobacco Budworm in Garden Plantings
To monitor for budworm and detect early stages of an infestation, periodically check buds and flowers for small holes and petal feeding injuries.
Use of plants that are not susceptible to tobacco budworm can avoid problems. Ivy-leaved geraniums (Pelargonium peltatum), which have smooth leaves, are much less frequently damaged than zonal types (Pelargonium x hortorum), which have sticky hairs on buds and leaves. Some variation in susceptibility to this insect has been observed among petunia cultivars.
In small plantings, the most practical control is hand picking the caterpillars. Some caterpillars may be on plants during the day, but most hide around the base of the plants in daytime and climb onto plants during dusk and early evening. An evening survey with flashlight should allow one to locate many of the caterpillars.
Maintaining potted plants in protected areas, such as garages, between seasons can allow tobacco budworm pupae to survive in the soil. If potted geraniums or other tender perennial host plants are kept between seasons, remove the soil to eliminate pupae and repot the plants before overwintering.
Insecticides. The tobacco budworm can be a fairly difficult insect to control with insecticides. Much feeding occurs within buds that cannot be reached by insecticide sprays. Also, since most feeding occurs at night, many caterpillars will be missed if sprays are applied during daytime. It is recommended that applications for tobacco budworm be made late in the day, preferably at dusk.
Insecticides that are most effective for control of tobacco budworm are products with some residual activity that can kill caterpillars for several hours or days. These include spinosad and certain pyrethroid insecticides (Table 1). Garden insecticides that will not be effective for tobacco budworm include pyrethrins, insecticidal soaps and the systemic insecticide imidacloprid.
Use of insecticides is complicated when the plants are in flower and also attractive to pollinating insects, particularly bees. No insecticides can be applied to any flowering plant during a time bees are visiting the plant.
Risks to bees is particularly high with some of the pyrethroid insecticides that can be hazardous to bees for a day or more after application. Pyrethroid insecticides should not be used on plants that have flowers that are attractive to bees.
There are two options for control of tobacco budworm on plants where there is a conflict with insecticide use and pollinators. One of these is to use spinosad and to apply it after dusk, when bees are no longer actively foraging. Spinosad can safely protect pollinating insects if sprays are allowed to dry and pollinators do not visit within 3 hours following the application.
The other alternative is Bacillus thuringiensis var. kurstaki (Bt). This is a microbial insecticide that is specific in its effects on insects, only killing caterpillars that eat it. Since Bt only works if a tobacco budworm caterpillar eats a part of the plant that is coated with Bt sprays or dusts, effectiveness for control can vary on different plants. On plants such as geranium, where the caterpillars drill into the buds and eat little of the outside surface, Bt will likely be ineffective. On plants where the caterpillars feed extensively on leaves or blossoms, such as petunia, Bt can be expected to be most effective. Since Bt insecticides breakdown rapidly (hours) in sunlight all applications should be made late in the day.
Table 1. Insecticides useful for control of tobacco budworm in flower gardens. Examples of trade names include products that are available either through retail or marketed to pest control professionals.