The biggest threat to sunflowers in the home garden is birds, who love the seeds and may damage the flowers with their scavenging. Common insect pests include aphids and other leaf sucking insects, as well as flea beetles, sunflower beetles and thistle caterpillars. Most of these pests can be controlled with insecticidal soaps, biological controls or through the introduction of predatory insects.
Palestriped Flea Beetle
The Palestriped flea beetle is only about 1/8 inch long, and shiny black with two white stripes on the back. Palestriped flea beetles jump and move very quickly. They chew tiny holes in the leaves, creating a lacy effect. Flea beetles do the most damage to young sunflowers, and insecticidal soaps are ineffective against their hard bodies. Till the soil in late winter to kill any overwintering larvae and lay floating row covers over the soil immediately after planting to prevent flea beetles from invading young sunflowers. Remove the floating row covers when the sunflowers stand 12 inches high or when temperatures climb above 23.9 degrees C. Pesticides are also effective against flea beetles.
Sunflower beetles resemble Colorado potato beetles, but do not feed on potatoes. They are 1/4 to 1/2 inch long as adults. They have reddish-brown heads and cream-coloured wings with dark markings. Both larva and adults eat the leaves and can cause extensive damage. Adult sunflower beetles eat only the margins of the leaves, while the larva eat buckshot holes throughout the leaves. Handpick sunflower beetles and drop them in a bucket of soapy water or spray them with pesticide. Predatory insects, such as lacewings, ladybirds and stink bugs destroy both the eggs and larva.
Thistle caterpillars are the larva of painted lady butterflies. These caterpillars grow to 1 1/4 inches long, are black or brown with a gold stripe down each side and have a spines. Thistle caterpillars munch the leaves, quickly defoliating sunflowers in large quantities. They may leave fine webs or black fecal pellets on the leaves. Pull off the caterpillars by hand and destroy any eggs or chrysalids on the undersides of leaves, or spray leaves with bacillus thuringiensis. This naturally occurring bacterial disease organism destroys the stomach lining of caterpillars, causing death by starvation.
Several weevils, maggots and moths lay their eggs on the seed heads, causing extensive damage to the sunflower seeds. Tilling the plot in the fall or winter kills overwintering insects. Plant sunflowers later in the season if you've had a problem with maggots or weevils and rotate where you grow sunflowers so they don't grow in the same space each year. Pesticides will control these insects, but try other methods first if you plan to eat the sunflower seeds.