What Pests Do Cosmos Get?

Aphids

• Two types of aphids bother cosmos: a foliage eater and a root dweller. Aphids have specialized structures in their mouths that allow them to suck the juices from a plant. While the foliage-munchers can sometimes be knocked off a plant with a strong blast of water, control of the root dwellers -- the western aster root aphid -- requires an insecticide soil drench. You’ll know the aster has root aphids if it is yellow, growth is stunted and there are no other pests on the aboveground portions of the plant.

Thrips

• Thrips is a small, yellow or light to dark brown, winged insect that feeds on the cosmos’ flowers. It is rarely found on the plant’s foliage. Damage includes wilting, a loss of color and premature drop. Thrips have an 11- to 13-day life cycle that requires an air temperature at or above 80 degrees Fahrenheit during that time. Eggs are laid in the flower and hatch within 80 hours. Use a neem oil spray to control thrips. The product must be applied as soon as you notice an infestation.

Spider Mites

• Spider mite infestations are easy to diagnose -- they leave a spider-like webbing on the cosmos, especially in the areas where the leaves join the stems. Spider mites damage the cosmos by scraping the foliage and then sucking out the juices. The leaves take on a gray cast and drop from the plant. A large spider mite infestation may kill the plant. Control spider mites with a miticide that has bifenthrin as the active ingredient. These miticides don’t destroy the eggs so you’ll need to reapply every 10 days to get the infestation under control.

Japanese Beetle

• The Japanese beetle is especially fond of cosmos and feeds on the leaves as well as the flowers. They have a tendency to invite their friends with a special pheromone, so where there is one, there are most likely others. Suspect Japanese beetles if the cosmos’ foliage begins to take on a lacey appearance. This is known as skeletonizing and if you don’t control the pests, they will skeletonize the entire plant. Luckily, Japanese beetles are large and easy to see. Take a bucket of soapy water to the garden, early in the morning, and knock the Japanese beetles into it.