Water Lily Pests and Diseases – Pest Control and Leaf Problems

The chemical control of pests and diseases afflicting water lilies is made more difficult by the susceptibility of the other occupants of the water, particularly fish. If pest damage reaches serious proportions, the handpicking of infected areas is recommended.

Fortunately, water lilies are not prone to a wide range of pests and diseases causing serious damage, and when they are present, the fish themselves control many of the insect pests by devouring the larvae.

Many successful and well-established water gardens have never had a chemical spray near them for years, as the vigour of the plant and the balanced ecological environment in which they live to provide natural resistance to pest and disease.

As with most plants, if feeding patterns and the conditions of growth are as natural as possible, the level of infection or damage will not get out of hand. In extreme cases, where chemical measures of control have to be resorted to, try to remove the plants from the water first, and treat them by immersing in a bucket of dilute pesticide or fungicide.

After being immersed for a few hours, thoroughly rinse the whole plant before returning it to the pool. While the plants are out of the water, don’t waste the opportunity to remove any damaged or badly diseased tissue or leaves, particularly if waterlily crown rot is suspected.

Water Lily Pests and Control

Water Lily Aphids (Rhopalosiphum nymphaeae)

Although several aphids or greenfly attack water plants, this small black species can be particularly troublesome. It is not only disfiguring but also smothers the leaves in large colonies.

As aphids suck sap to feed, a heavy infestation inevitably reduces the efficiency of the leaf and results in a loss of vigour. In common with many other forms of aphids, the overwintering eggs are laid on trees; in this case, plums and cherries are the hosts.

In the spring the eggs hatch into winged females, which fly to water and give birth asexually to further generations of wingless offspring. The colder autumn days signal a change, and winged forms are born. The cycle is completed when eggs are laid away from the lily on the nearby trees.

How do you get rid of aphids?

Winter spraying with ovicides (egg-killing sprays) of nearby dormant members of the Prunus (plum) family helps, as does the physical removal of the early colonies from the leaves in the spring.

If no livestock is present in the pool, contact washes of pyrethrum-based insecticides or nicotine soap should be given early in the season if there is any infestation. Alternatively, a light spray on the waterlily foliage with a dilute proprietary oil solution will smother the pests but not harm fish or other inhabitants of the pool.

If fish are present in your water lily pond, carefully read the instructions of any proposed spray. Spraying the leaves forcibly with a fine jet of freshwater dislodges vast quantities of aphids, which are readily devoured by any fish present.

Water Lily Beetle (Galerucella nymphaeae)

These beetles are by no means as widespread as aphids, but once a colony is established on a collection of lilies it becomes increasingly difficult to eradicate. Small dark brown beetles, about twice the size of a ladybird, overwinter on poolside vegetation and visit the waterlilies in June.

What is eating my water lily?

Eggs are laid in groups on the leaf surface, and after a week or so small, dark brown larvae, which resemble small slugs, emerge and feed voraciously on the foliage.

The surface layers of cells on the leaves are stripped by the larvae, and this causes rapid rotting off, and gives a tatty appearance to, the leaves. Several generations occur in one season, and the aerial foliage of surrounding aquatic vegetation provides the ideal home for the larvae to pupate.

How do I get rid of water lily beetles?

Jetting the foliage forcibly with fresh water in summer dislodges the larvae, which have little chance of survival in the water if fish like golden orfe are present. In addition, cutting back surrounding marginal vegetation in the winter reduces the overwintering hosts.

An alternative method is to spray all water lilies in the infested pool with a solution containing powdered Bacillus thuringiensis bacteria (available through specialist aquatic suppliers), which parasitize the larval caterpillars and kill them. The bacteria are completely non-toxic and harm only the larvae.

Brown China Mark Moth (Nymphula nymphaeata)

The symptoms of the mark moth damage are holes in the foliage, particularly around the leaf margins. The holes are caused by the larvae of the insignificant and small brown adult moth, which has a 1 inch (2.5 cm) spread of orange-brown wings patterned with white. It lays clusters of eggs close to the edge of the leaves, and the eggs soon hatch into small white larvae.

The insect does the most damage in the larval stage when it bites off pieces of leaf to make a protective case for the silk cocoon it occupies in a small, floating home. From such a home, it can drift around the edges of the waterlily leaves and have the odd meal from the edges of the foliage until it pupates.

Eggs are laid on the undersides of the leaves, and the new breed of caterpillars feed there until they, too, make cases out of pieces of the lily foliage for their cocoons.

The closely related china mark moth (Nymphula stagnata) also damages the plant when the small caterpillars eat into the leaf stems. The damage done by both species is normally not too serious, particularly if fish are present.

Control is by the removal or netting of the floating homes seen, and, if the damage is very severe, the removal of all the adult leaves. This temporary defoliation enables the young leaves to grow away with less risk of infection, or use B. thuringiensis as above.

Leaf Mining Midge (Cricotopus ornatus)

A severely disfiguring pest, the leaf mining midge lays eggs on the surface of the leaves. The tiny, slender and almost transparent larvae then proceed to tunnel in a random fashion in the surface tissue.

If the infestation is heavy, the leaves can be completely skeletonized, which could be serious on young plants of the pygmy varieties. Leathery, thick-leaved varieties seem more resistant to attack.

Control is by removing damaged leaves as soon as the tunneling marks are noticed. If this does not check the infection, use B. thuringiensis as above or remove the plant and dunk it into a dilute insecticide, rinsing it thoroughly before returning it to the pool.

Caddis Flies (Trichoptera)

A pest that is a nuisance only to the waterlily grower who excludes fish from the pool, caddis fly larvae consume just about everything growing in the water.

The larvae, however, are aquatic and favorite food of fish, particularly goldfish, which prevent the larvae from reaching any significant numbers. The adult flies resemble brownish moths but differ from moths in having a sparse covering of small hairs on the wings instead of scales.

There are nearly 200 species of caddis fly, which are mostly nocturnal and range in size from 0.25-0.5 inches (6-12 mm). The flies lay their eggs during the evening, embedding them in long cylindrical tubes of protective jelly on submerged or aerial foliage, sometimes with parts of the tube dangling in the water.

When the larvae hatch from the eggs, they spin a web of silk around themselves and disguise or protect these shelters with pieces of broken plant, old snail shells, or specks of sand and gravel. As it gets bigger, the larva keeps adding to its case and never fully emerges from it. B. thuringiensis, used as described above, is good control.

Snails

It seems, at first sight, confusing to include snails as pests when they are frequently to be seen for sale alongside aquatic plants. They are sold because of their scavenging role in a pool, where they help to clean up algae, decaying plant material, surplus fish food, and dead or drowned animal matter such as worms and fish.

There are, however, many schools of thought on their overall value and the policy of deliberately introducing them into a pond.

There seems little doubt that one species of snail, the ramshorn (Planorbis corneus), is safe to introduce and will perform the true scavenging role without causing any damage to water lilies and other aquatic vegetation.

As its name suggests, the shell resembles the circular horn associated with a type of sheep, although a flattened Catherine wheel is the best description I have seen used.

The shell is carried vertically as the snail meanders around the pool, cleaning up the algae like a little vacuum cleaner. The black Japanese snail, Viviparis malleatus, is another excellent pool scavenger that will clean away decay but spare aquatic plants.

Other snails, however, may not confine their diet to algae. Although the common pond snail or freshwater whelk (Limnaea stagnalis) is a very efficient and busy scavenger, it can become a pest if there is insufficient alternative food and it turns its attention to young waterlily leaves and the foliage of submerged oxygenating plants.

It can be identified by its pointed shell, rather like a spiral dunce’s hat, on which the spiraling is in the opposite direction to nearly all the other snail shells. It is only small, no larger than the tip of a little finger, which makes it efficient in reaching between foliage in search of algae that other larger snails find difficult to reach.

The pointed shell is only 1-2 inches (2.5-5 cm) high, but it protects the potential delicacy from large fish. Like the ramshorn, it sticks its eggs on the undersides of waterlily leaves in long strings of jelly, from which the young snails emerge in three or four weeks.

The ramshorn’s egg jelly tends to be in flat pads, while the common snail jelly is in longer strands. In a new, small ornamental pool, where plants are just establishing themselves, it would be unwise to introduce the common snail; the chances of their natural introduction are high anyway.

In large pools, lakes, or wildlife pools with masses of vegetation there is less problem. If they do become a problem, float cabbage or lettuce leaves on the surface of the water for 24 hours. The snail population will be attracted to the bait, which can be easily removed together with the snails.

Mosquito Larvae

Although much rarer now, mosquito larvae can be a problem for young plants as they appear in early summer when they attack the leaves and buds. Unless the larvae are checked, the leaves will turn yellow and the buds will fail to mature.

Again, if fish are present there should be no problem; introduce golden carp, golden rudd, golden orfe, or even native minnow.

Diseases

Leaf Spots

Several fungi attack waterlily leaves and, if allowed to develop unchecked in warm wet weather, they can cause the pads to die. All types start as dark patches on the leaves, one form causing the margins of the leaves to dry and turn up.

In the early stages of attack spray a mild solution of Bordeaux mixture on the pads every other day for a week. If spraying is not possible, remove the infected leaves quickly and burn them.

Water Lily Crown Rot

A serious fungus disease, particularly to a grower of waterlily collections, waterlily crown rot is a species of Phytopthora, a disabling and widespread disease.

Why are my water lilies rotting?

In the case of the water lily, the fungus causes the crown and stem base to blacken and rot. It spreads like wildfire through a collection, and, as the infection is below water level, a wary eye must be kept for loss of vigour and early yellowing of the foliage.

Further examination of the crown and stems will reveal rotting black, jelly-like tissue and a vile, unmistakable smell, even when roots appear healthy. Routine dipping in fungicides for prevention is practised in commercial nurseries, but in the small pond, the only remedy is to remove and burn the infected plant and to replace it and its soil.

Seek professional aid for collections suffering from crown rot. A Japanese system of growing water lilies involves growing the plants on a ridge and furrow system, similar to potatoes in fields, during which they are repeatedly flooded and drained.

Imported rootstocks from this system initially show good vigour, but later they are more susceptible to crown rot when submerged. The most susceptible varieties are ‘Laydekeri Fulgens’ ‘Marliacea Ignea’, ‘Ellisiana’ and ‘Rose Arey’.

Water Discoloration

Although not fungal infections, the following three conditions affect the pleasure that can be gained from a waterlily pool: green water, dark or milky water, and blanket weed.

If the water turns green, check that the surface and submerged planting adequately deprives the offending algae of mineral salts. As a temporary measure, algicides may be used, but the water will probably turn green again in time if the planting balance is incorrect.

To control dark or milky water remove rotting vegetation from the base of the pool. If there is so much weed that it blankets the pool’s surface, remove it with a net or twirling on a forked stick or rake.

Algicides have been specifically developed to control blanket weed, but some products damage dwarf and weak-growing waterlily varieties.