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Geranium Budworm? Try Rototilling |
By Whitney Cranshaw, Colorado State University Cooperative Extension, Entomology
If the pesky tobacco/geranium budworm (Helicoverpa virescens) creates havoc with your summer annuals, you might control it by rototilling your planting areas. Working the soil in the fall is preferable, but spring tilling may crush some of the budworm pupa. This pest usually arrives in your yard on greenhouse plants. Young larvae tunnel into small flower buds, while larger caterpillars eat flower petals, chewing the reproductive flower parts, and foliage.
Geranium budworm damage
Life History
The adult stage of the tobacco budworm is a moth with a wingspan of about
1 1/2 inches. 
The wings are very light, with brown overtones and
with a few wavy cream colored bands. Eggs are laid singly, during early evening. On
geranium, eggs are laid on buds; on other plants eggs are laid on leaves. The larvae that
emerge are marked with several stripes but can be quite variable in overall color. Dark
forms are common but red and green larvae may also be seen. The differences in color, in
part, are related to the color of the flowers on which the insects are feeding.
The caterpillars become full-grown in about a month, drop to the soil and pupate. Adults emerge to repeat the cycle with two generations normally produced each year. At the end of the season, determined by declining day length and temperatures, the insects go into a state of suspended development they maintain through winter.
The tobacco budworm spends the winter in Colorado as a pupa. The pupal stage occurs below ground, buried 4 to 6 inches deep, and is formed within a packed earthen cell the full-grown caterpillar produces. Overwintering pupae are generally killed if exposed to temperatures below 200F and the insect is therefore poorly adapted to the harsh winters of Colorado. However, warm soil microclimates, such as are found around foundations of heated buildings, can allow many to survive. Furthermore, as a general rule, the number of overwintering tobacco budworm, and the likelihood of problems, is related to the severity of the previous winter.
Control
To monitor for budworm and detect early stages of an infestation, check buds and flowers for small holes. In small plantings, repeatedly hand picking the caterpillars should be considered as it is often the most practical control. Tobacco budworm larvae are most active during dusk and are best discovered at this time. During day light hours they often hide around the base of the plant.
The tobacco budworm is a difficult insect to control with insecticides. Most commonly available garden insecticides (e.g., Sevin) are not effective against this insect due to insecticide resistance. Synthetic pyrethrins (pyrethoid) insecticides (e.g., Talstar, Scimitar) have been most effective against tobacco budworm. However, excepting the few permethrin insecticide products distributed (e.g., Ford's Intercept, Bug Stop Concenetrate) such products are generally not currently marketed for general use through nurseries and other outlets. The more widely available and shorter persisting, natural pyrethrins (pyrethrum) have not proved effective in CSU trials.
Insecticides containing Bacillus thuringiensis/Bt (Thuricide, Dipel, etc.) are effective biological controls when used on some plants. However the insect must eat Bt in order to be effective. On plants such as geranium, where the caterpillars drill into the buds and eat little of the outside surface, Bt is not effective. On petunia, where caterpillars eat a great deal of the blossom, Bt can provide a marginal amount of control.
Developing varieties of bedding plants resistant to tobacco budworm may provide a long term means of managing tobacco budworm. For example some variation in susceptibility to this insect has been observed. Salmon, red and pink cultivars tend to be most severely damaged; yellow and white cultivars are among the least damaged. Also, ivy geraniums (Pelargonium peltatum) is much less frequently damaged than are "standard" (Pelargonium x hortorum) types.
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 the soil should be removed and plants repotted before overwintering.
For more information and photos see CSU fact sheet 5.581
Photographs courtesy of Curt Swift (damage), Whitney Cranshaw (adult moth), and Larisa Vidmar (budworm).
