Small in size and big in color, geraniums (Pelargonium spp.) are cheery flowers that are generally low-maintenance and trouble-free. Like other garden plants, geraniums sometimes fall prey to insect pests that mar their beauty and lead to a decline in overall health. Telltale symptoms of insect damage include holes in leaves, flowers and buds, failure to flower, distorted leaves or a sticky black residue. Although insects may feast on them, deer usually leave geraniums alone.
Geraniums are sometimes plagued by the larvae of moths, sometimes called worms or caterpillars. Tobacco budworms, also known as geranium budworms, feed on geranium buds, preventing them from blooming. Budworms may also eat flower petals. Other foliage-feeding larvae include cabbage loopers, variegated cutworms, beet armyworms, obliquebanded leafrollers and omnivorous leaftiers. Other garden pests that may feed on geraniums include whiteflies, small flying white or green insects; aphids, crawling, sap sucking insects; mealybugs, downy, cottony insects; weevils, dark-colored bugs; spider mites, tiny, reddish, spiderlike insects; thrips, thin, winged insects; and cottony cushion scale, soft, lumpy masses.
Pick off larger insects, such as weevils and caterpillars, by hand and squish them or drown them in a bucket of soapy water. The most effective way to combat tobacco budworms is to hand pick them, advises Colorado State University Extension. Tobacco budworms are easiest to find at dusk, when they are the most active. Look for yellowish green, pinkish or maroon caterpillars speckled with tiny black spines. If you're overwintering geraniums indoors, repot in spring with new potting soil, as the larvae can overwinter in the soil. Remove nearby weeds, which can harbor caterpillars and thrips. A direct stream of water may help dislodge colonies of sap-sucking pests, including mealybugs and aphids.
An insecticide containing spinosad may help control caterpillars and thrips, though because it's toxic to bees and other beneficial insects, use it only on geraniums that are not flowering. Thoroughly mix 16 ounces of spinosad in 1 pint of water, pour the solution into a garden sprayer and spray the plant evenly on both sides of the leaves until the mixture runs off the leaves. Store any leftover insecticide in a secure spot away from children and pets. Spinosad is combustible: do not store it near heat or open flame. Do not pour the mixture down the drain, as this product is toxic to aquatic invertebrates. For sap-sucking pests, such as spider mites, scale insects, whiteflies and aphids, thoroughly mix 2 to 4 tablespoons of neem oil in 1 gallon of water, pour it into a garden sprayer and spray the entire geranium until the plant is wet. Spray every seven to 14 days, until pest infestations have disappeared. Do not apply while bees are actively visiting the plant. Wear eye protection, gloves and long sleeves when using neem oil. For any insecticide, always follow the safety instructions on the label.
Preventing Pest Problems
Healthy, stress-free plants planted in the right conditions are less likely to succumb to pest infestations. Plant your geraniums in a bright, sunny spot with well-draining soil. Spider mites tend to attack dry, water-stressed plants, so water regularly to keep the soil moist, but not waterlogged or flooded. Avoid using insecticides in your garden, as it kills the insect predators that otherwise naturally take care of scale insects and other pest infestations. Fertilize weekly with 2 teaspoons of a 20-20-20 water-soluble fertilizer mixed in 1 gallon of water. Geraniums grow outdoors year-round in U.S. Department of Agriculture plant hardiness zones 9 through 11, but are usually treated as annuals.