The colorful overlapping petals of annual cosmos (Cosmos bipinnatus) can bring a splash of variety and color to borders and gardens. The bright blooms come in many colors and attract a number of beneficial insects that prey on other insects. In many cases, these beneficial insects can help reduce potential damage from insect pests that feed on these flowers. Only a few pests bother the cosmos itself.
Aphids, small wingless insects, feed off the leaves of cosmos. In small numbers, aphids cause little damage but a severe infestation can cause stunted shoots or yellowed leaves with a curled or deformed shape. Aphid infestations on your cosmos can leave behind a shiny, sticky residue known as honeydew. The honeydew may attract ants. Large numbers of ants on your cosmos indicate an aphid problem. The best defense against aphids is to prune damaged and infested stems before large numbers of aphids begin to spread over the cosmos. You generally don't need to use chemicals to control aphids.
Japanese beetles are a more serious threat to cosmos. These small beetles have a brown head with a metallic green shell that covers their wings. Japanese beetles feed on the leaves of plants, beginning with the outermost leaves. As Japanese beetles feed, they produce a pheromone that attracts other adult Japanese beetles. The adult form of this pest appears during the summer for a period of weeks. In most cases, manually removing these pests after they appear will prevent severe infestations and limit damage to your cosmos.
Thin or distorted leaves with a papery texture are signs of damage from thrips. New leaves can become discolored and may roll in on themselves, while established leaves often develop a mottling of light-colored patches as a result of thrips damage. These miniscule insects measure less than 1/20 inch, making them hard to see. Pesticides generally don't work for thrips. The best way to prevent thrips is to limit the conditions that encourage them to spread. Thrips are attracted to succulent new growth that forms after heavy applications of nitrogen-based fertilizers. You can prune out infested stems to control small infestations and you can spray the foliage of your cosmos with water to knock thrips off your plants.
In most cases, cosmos do not suffer from severe infestations of harmful pests. If your cosmos become infested, you can usually remove the pests and the damaged parts of the plant to prevent a more severe infestation that could damage your plants. Aphid infestations of cosmos often become more severe when ants begin protecting aphids from their natural predators while they harvest the honeydew the aphids leave behind. If your cosmos are infested with aphids, and you see a lot of ants, you can reduce the number of ants by using ant traps placed around the base of your cosmos. Once the population of ants is reduced, the natural predators that cosmos flowers attract will help reduce the number of aphids on your flowers.