Water lilies are more than a car park for frogs. The plants decorate the surface of ponds and other water features with broad, interesting leaves, and delicate blooms of white, yellow, red and pink that attract butterflies and dragonflies. Some varieties have lightly fragrant flowers. Water lilies may be grown in U.S. Department of Agriculture hardiness zones 3 to 11. Some varieties only thrive in warmer zones. The plants are durable in their planting zones and free from disease. Some insects, like aphids, occasionally attack water lilies, but treatments are available to restore the plants' health and vigour.
If your water lily's leaves are turning yellow or becoming deformed, check the leaves for small, black, pear-shaped dots. These are probably black aphids. Aphids are often called plant lice. Black aphids suck on the juices of new leaves, which may curl from the injury. Look for a protrusion on the rear end of the insect that looks like a car's tailpipe to be sure what you have is truly a black aphid problem. Aphids are most prevalent in spring and summer.
Aquatic Leaf Beetles
Leaf beetles are a metallic green, blue, black or copper colour, and have long antennae. The insects chew holes in the leaves. If you see brown speckles on the lily's leaves, larvae are probably making air holes for themselves.
Spider mites may decide to set up house on the relatively flat surface of water lilies. These tiny spiders are virtually invisible, but their miniature, white webs atop the foliage are not. In addition to the webbing, yellow stippling on the leaves is an indicator that spider mites are in residence. Spider mites suck sap out of leaf cells. Spider mites may abandon the leaves for the flower buds once they appear.
China Mark Moths
This moth has white patches on brown-wings and is about 1 inch long. Look for cocoon-like rolls made out of leaf pieces tied together with silk. These structures, which give the insects the nickname "bagman" and "sandwich men," house the larvae of the China mark moth. Larvae eat the leaf margins and may chew on the water lily's stems. You may find clusters of eggs on the underside of the leaves or on the circumference of holes adult beetles chewed in the leaves.
Use a strong spray from a garden hose to wash away black aphids, spider mites, beetles and their larvae and eggs. Because aphids may come from eggs deposited on nearby plum or cherry trees, treat those trees with dormant oil to kill overwintering eggs. Apply the oil in early spring, before the eggs hatch. Follow all package instructions. Remove and destroy any larval cocoons of China mark moths you find floating in the water or attached to the lily's leaves.
Be sure to check the underside of the leaves. Pick off larvae by hand whenever you spot them. Adult beetles spend the winter hiding in nearby plant debris, so keep all dead foliage away from and out of the pond or water garden.
If washing and wiping away the pests don't work, take the plants out of the water and place them in a separate container. Use an insecticide or insecticidal soap to kill the insects, then rinse the plants, and put them back into the pond or water feature.
Do not use insecticides containing pyrethrum, permethrin or other pyrethroids if your water feature includes fish.