Though some chrysanthemums produce an insecticide called pyrethrum, there are many other ornamental varieties that make little or none of this substance. This leaves the plant open to insect infestation, especially if it is grown in an area where broad-spectrum insecticides have killed all natural predators of chrysanthemum pests. Homemade and commercially prepared insecticides can help protect these flowers against unwanted visitors.
Chrysanthemums in the Garden
These late-blooming perennials tend to resist disease and mortality from insects when planted in well-drained, rich soil with plenty of sun. If the garden has been planted to attract predatory or parasitic insects to control the insects that commonly attack plants, chances are you'll have no problems keeping the chrysanthemums healthy. If not, there are several pests that find chrysanthemums tasty.
Aphids and spider mites do the most damage to chrysanthemums. Aphids gather at nodes and around buds. Mites hide on the underside of the leaves, spinning protective webs. At first, they are noticeable only for the yellow pin-pricks on the leaf surface. Both these insects suck the vital moisture and nutrients from the plant, leaving it vulnerable to drought and disease. Caterpillars chew on leaves and buds to store up energy before cocooning, but leave the plant alive. Leaf miners make little pale green tunnels in the leaves, and only extreme infestations affect plant health.
First, decide if the insect is a true threat. Spider mites and aphids will only multiply until the cold weather hits, so your chrysanthemums may need your help. Drench the plant in a quart of water mixed with a teaspoon of dishsoap. If the weather is consistently under 80 degrees Fahrenheit, you may add a teaspoon of vegetable oil to this mixture for long-term control. Parasitic wasps (Diglyphus intermedius) have proven to keep leaf miners in check. Powdered applications of the selective bacteria Bacillus thuringiensis will control caterpillars.
Keep in mind that many chemical insecticides are broad-spectrum, meaning they'll kill your praying mantis as quickly as they will spider mites. This means you'll have to keep re-applying, since there's no line of defense left in case a new infestation crops up. Fort Valley State University Extension Service recommends applications of Diazinon or Malathion for aphids and mites (two teaspoons of either per gallon of water), and Sevin dust for caterpillars.
The insecticide that chrysanthemums are well-known for producing (pyrethrum or, when mixed with other agents, pyrethrins) actually comes primarily from chrysanthemum cinerariaefolium, or Dalmatian flower. This plant is far more daisy-like in appearance than the more popular varieties of chrysanthemums. The insecticide produced in this plant resides exclusively in the buds. It is also a broad-spectrum insecticide; it is not recommended for gardens cultivating predatory insects.