Water Garden Pests and Water Lily Diseases


Unlike most specimens of the plant kingdom, pests or diseases seldom afflict water lilies. Water gardeners have it easy compared to gardeners who grow roses, veggies, or lawns. Plus, most water lily troubles are superficial and do little permanent harm. Often they may be prevented and controlled with a watchful eye and careful maintenance.

Here are the most common, from big to small. Some may not be a problem at water garden businesses, but might affect your customers. Remember that the key to keep pests from becoming problematic is to regularly monitor the condition of all your aquatics. Then before anything can become a concern, you can nip it in the bud.

Terrestrial Animals

Dogs don’t eat water lilies. However canines cause problems when they go for a dip, blissfully overturning pots. While some dogs can be trained to stay out of the pond, breeds like Labradors have an innate love for water. In those cases some owners solve the problem by giving their lab its own kiddie pool. A harsher solution is the Fido Shock, which delivers a small electrical charge through a wire fence.

The electrical fence can also deter raccoons, who regularly knock over aquatic plant containers. Keep pots away from the pond edges since the raccoons won’t go into deeper water. These critters are very persistent and outwit most devices designed to scare them away. The most effective deterrents are the Fido Shock and the Scarecrow, a sprinkler activated by motion sensor. Move the Scarecrow periodically to increase effectiveness. It also helps to place two at right angles. If deterrents don’t work, check with your animal control departments. Often they’ll provide traps then remove the captured animal. To bait the trap use the unlikely, but extremely successful, delicacy – Twinkies.

Aquatic Fauna

Turtles will eat anything slower than they are and that includes water lilies. Symptoms are lily pads that appear to have been cut with a knife or scissors. The best solution is to relocate the turtle to a more appropriate pond.

Some koi will snack on lilies and root around in pots while others don’t. Until someone figures out why this happens, take precautions to reduce koi damage. Cover the soil in containers with gravel and then with stones–bigger than the largest koi’s mouth. (Some ponders say lava rock is uncomfortable in a koi’s mouth and they’ll avoid it.) Place lilies very close to the surface (3-6?). This prevents koi from grazing in the pot and also gives new leaves a chance to grow.

Butterfly koi usually make better water garden pondmates, as do goldfish and koi raised from babies. Whatever you do, don’t add a single pot of plants into a pond that has been sterile of vegetation. The new diversion will soon become lily salad. Another strategy to protect water lilies is to buy or make a cage around the plants. As a last resort, create an adjacent but separate pond area for the lilies.

Ramshorn and Japanese Trapdoor snails don’t usually harm aquatics since they feed on decaying plant material. However Pond and Apple Snails do feast upon lily pads and other fresh vegetation. A technique to get rid of snails without altering your water chemistry is to place a lettuce leaf or zucchini slice in the pond. Leave it overnight, then remove it and destroy the snails it has attracted. Repeat as needed. Adding snail-eating fish, like the Clown Loach, is another biological control. Potassium permanganate and other specialized chemicals can be used but the biological controls work best in backyard ponds.




The key to controlling aphids is to keep them from ever becoming a problem. As soon as you notice the little buggers, squash them by hand. They usually appear on new growth or older yellowing leaves and may start reproducing in terrestrial plants near the ponds. Although many books recommend washing aphids off leaves so the fish can eat them, this only works for light infestations. You can overflow the pond, spraying hard to flood them out. Repeat every day or two until aphids are under control.

Light oil sprays will suffocate the aphids and are not harmful to fish or plants. Sprays should be repeated every 10 days to be most effective. Mix two parts vegetable oil to eight parts water and a dash of dishwashing detergent. Treat in the evening and rinse off the oil the next morning. A Volck oil spray (5 Tbsp to 1 gal water) also works. Spraying trees and vegetation around the pond as soon as any aphids are detected is the quickest way to prevent an infestation in the pond.

Other environmentally safe controls are available. Diatomaceous earth is a microscopic abrasive that kills the aphids. It can be dusted on the leaves or mixed with water and sprayed. Again, flush the pond of extra residue so it doesn’t harm other pond inhabitants. Blade Runner, Aphid-X, and Herbal Aphid Spray are all made from natural ingredients. A weak solution (1.5%) of insecticidal soap left on for less than an hour also works well.

A very low-tech aphid control strategy is to drown the aphids. This can be done by submerging the plants overnight or by putting some newspaper on top of the leaves and leaving it there for several hours.


Leaf mining midges chew wavy lines in the lily pads. These very small larvae can be handpicked, the leaves can be removed, or the water can be treated with Mosquito Dunks (which contain Bt).

China Mark Moth


This small nondescript brown moth is the water lily’s major pest and is also called the Sandwich Man. It is nocturnal and lays eggs on the underside of floating leaves. After hatching, the larva cuts leaf pieces to make protective sandwiches. They affect water lilies, although the larva also burrows into any floating leaves or debris. They have a two week cycle, so keep a close check for them throughout the growing season.

The mechanical control method, better known as squishing, works well to control an initial outbreak – fish just love the worms. Bacillus thuringiensis (Bt), a natural bacteria, can be used as a spray. Once ingested, it kills the larva but won’t hurt people, pets, or fish. It is the active ingredient in Dipel, Insecticidal Soaps, and Thuricide. As with many sprays, it is best applied at the end of the day (see sidebar). If there is a severe infestation, the best remedy is to remove all affected foliage close to the crown of the plant and destroy it.


Many years ago several varieties of hardy lilies were susceptible to crown rot, a fungal disease. The leaves on affected plants would curl and turn yellow, and buds would rot below the surface. The plant would soon die since the rhizome had rotten away, leaving a stinky mess. Treatment was to thoroughly soak the tuber in a fungicide. However, since the disease is highly contagious the best option was to remove the plant and completely destroy it. Luckily the incidence of this and other fungal diseases has decreased as less susceptible hybrids have been developed.

Natural or Chemical Treatments


Be cautious of all pesticides and always use the least harmful treatment first. If mechanical control (aka squishing by hand) doesn’t work, then try the appropriate insecticidal soaps, sprays, or dusts. These rely upon natural bacteria that target specific organisms, diatomaceous earth, or other natural derivatives. Unlike pesticides, they are usually not harmful to other insects and pond inhabitants. (Unfortunately those based on pyrethrum and rotenone are toxic to frogs and fish.) Numerous environmentally friendly treatments are now available, such as Blade Runner, Herbal Aphid Spray, Dipel, and several Insecticidal Soaps.

If biological controls are unsuccessful and you must resort to a pesticide, follow some simple precautions. Check what the label says about use with fish, pets, and other wildlife. Many products may be safe on terrestrial plants but should never be used in or around the pond. Whenever possible, remove the plant and treat it outside of the pond. After it has been treated, rinse it off and return it to the water garden. Some chemicals might require water changes after treatment if applied in the pond.

Most pesticides and biological controls are best applied at the end of the day. There is less breeze to blow spray to surrounding areas or plants; there is less chance the spray will burn or damage the plant; there is less opportunity for UV to degrade the effective ingredients; and absorption of the active ingredients into the plant’s system is usually higher.

An Ounce of Prevention Provides a Big ROI*

• Before adding any new plants to your pond, closely examine them for signs of pests, especially if you’ve had infestations from a particular grower. Look at both sides of the leaves and in the crevices where stems and leaves overlap. Remove anything suspicious. You can also soak the plant to kill unwanted pests and parasites. Plants can be soaked in a potassium permanganate solution (4-6 Tbsp in 12-13 gal water) for 1-2 hours.

• Regularly fertilize plants and divide them when overcrowded. Since insects and diseases attack sickly and stressed plants, thriving plants usually do not have pests.

• Trim off all old and damaged foliage from lilies and marginals. This weaker growth is often where insects thrive. Removing dying leaves eliminates their food source, helps rejuvenate the plant, and reduces material that falls on the bottom to decay. A carefully trimmed lily displays beautifully and sells quickly, plus it’s usually disease and pest-free.

• When adding water to your ponds, be sure to spray all the plant foliage, especially if it hasn’t rained lately. This cleans the plants plus washes off pests before they can become a problem.

(Okay you should know what ROI means but just in case – Return on Investment)

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