Waterlily Pests and Diseases: A Ten-Year Update

trimmed waterlilies

Well-fertilized and trimmed waterlilies look and sell better, like the N. ‘Tina.’ The one on the right is stressed and unhealthy. Fertilizing the plant will make it less stressed and less susceptible to insect attack.

Author’s Note | We never guessed that our 2009 article on waterlily diseases and pests would be such a hit with readers for so long. As the author, I’m especially gratified that it has helped many Nymphaea growers. The article also prompted numerous online questions about individual waterlily problems.

Several queries revealed a common denominator — poor waterlily growing conditions. Unfortunately, unhealthy conditions dramatically raise the odds for a plant to get attacked by pests or diseases. So these updated sidebars reinforce ways that waterlily growers can keep their plants healthier, stronger and more resistant to assault.

Unlike most specimens of the plant kingdom, pests or diseases seldom afflict waterlilies. Water gardeners have it easy compared to gardeners who grow roses, veggies or lawns. Plus, most waterlily 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 waterlilies. 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, which are known to 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 a 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 and 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 waterlilies. 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.

Other environmentally safe controls include Diatomaceous earth, a microscopic abrasive that kills aphids. It can be dusted on the leaves or mixed with water and sprayed.

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 to 6 inches). 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 waterlilies 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, and 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.

Insects

APHIDS. 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.

Monitor regularly for China Mark Moth larva. Kill them by squishing for tasty fish treats, or treat with safe BT spray.

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 include Diatomaceous earth, a microscopic abrasive that kills 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 made from natural ingredients. A 1.5-percent solution 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.

MIDGES. 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.

China Mark Moth. This small, nondescript brown moth is the waterlily’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 waterlilies, 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. 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.

Diseases

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.

SIDEBAR | Natural or Chemical Treatments?

Be cautious of all pesticides and always use the least harmful treatment first. If mechanical control (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 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 to 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.

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