Fall's color palette would be sorely lacking without chrysanthemums' (Chrysanthemum morifolium) bronze, purple, orange or electric-yellow flowers to mirror the brilliant leaves overhead. When ants invade these late-blooming perennial beauties suitable for U.S. Department of Agriculture plant hardiness zones 5 through 9, they aren't admiring the flowers. They've come to collect sweet goo deposited by sap-feeding insects. Botanical justice dictates that a chrysanthemum-based pesticide spray kills both the ants and the insects.
Brown or black chrysanthemum aphids join pink or green aphids of other species in attacking mums' tender growth to drain sap through sharp, hollow mouthparts. Look for colonies of them lining new shoots, leaf backs, flowers and buds. Heavy aphid populations stunt and distort the plants, but even small infestations make a serious mess by excreting sticky, clear waste called honeydew. Honeydew attracts both ants and airborne sooty mold spores. Sooty mold disfigures the plants with mats of greasy black fungus.
Whiteflies are occasional, honeydew-excreting chrysanthemum pests. The adult flies, typically measuring 1/16 inch long, swarm from the plant's leaves when disturbed. Their light-green nymphs attach to and drain sap from the backs of the leaves. The infested foliage may yellow, wither and drop prematurely.
Ants and Honeydew
Ants prize carbohydrate-rich honeydew so highly that they kill aphids' and whiteflies' natural predators, such as lacewings or ladybugs, to protect their supply. Ants go to great lengths to protect aphids. Ants herd aphids away from areas where there predators are lurking, and they store aphid eggs in their nests over the winter to protect them from cold. This relationship stacks the odds in favor of the sap thieves, but chrysanthemums get their revenge thanks to one of their own.
At first glance, the Dalmatian chrysanthemum (Chrysanthemum cinerariafolium, Tanacetum cinerariafolium) seems like any other open-faced, sunny-centered white daisy. The flowers of this unassuming perennial hardy in USDA zones 3 through 7, however, yield pyrethrum when dried and crushed. Pyrethrum contains pyrethrins, common ingredients in mosquito repellents. Pyrethrin sprays also kill ants, aphids and whiteflies, making them one-stop solutions for your infested mums.
Using Pyrethrin Spray
For effective ant, aphid and whitefly control, treat your mums with ready-to-use pyrethrin spray at the first sign of infestation. On a calm afternoon or evening with the temperature between 50 and 75 degrees Fahrenheit, apply the pesticide until it drips from the leaves. Because sunlight starts degrading pyrethrins within 24 hours, you may need follow-up applications every four to eight days -- or according to the label's specified frequency -- until the pests are gone. Always wear protective clothing, shoes and socks when handling any insecticide.
As broad-spectrum insecticides, pyrethrins are as toxic to aphid and whitefly predators as they are to the pests themselves. Unfortunately, the broad-spectrum insecticides also kill bees, butterflies and other beneficial insects. Spraying the plants in the late evening minimizes the risk to to pollinators.