Aquatic Plant Pests and Diseases

Aquatic Plant Pests and Diseases

Several pests and diseases afflict aquatic plants, but unlike elsewhere in the garden, these cannot be sprayed with chemicals, for they would kill the fish. Therefore, in the water garden, more than anywhere else, the gardener needs a comprehensive understanding of the life cycles of the various pests and diseases so that they can be attacked at the most appropriate time. In addition, garden hygiene must be of the highest standard.



The most troublesome pest of waterlilies and other succulent aquatic plants is the waterlily aphid. In warm humid weather during summer, it reproduces at an alarming rate, smothering the plants and causing widespread disfigurement of both flowers and foliage.

Understanding the aphid’s life cycle is the only way by which effective control can be implemented. During early autumn, eggs from the late summer brood of aphids will be laid on the branches and in fissures of the bark of plum and cherry trees. These will overwinter, hatching in the spring, when winged female adults will fly to the host plants. Here they live happily, reproducing asexually and giving birth to live wingless females, which continue the process every few days. As autumn approaches, a winged generation of adults unites sexually, the females flying to the winter host trees to deposit their eggs.

From this, it will be apparent that only in their overwintering stage are they vulnerable to attack by the gardener. During the winter, when the trees are completely dormant, spray with a tar oil winter wash. This is specifically manufactured as an ovicide for destroying the overwintering eggs of insect pests on fruiting and decorative trees. A winter wash will effectively break the life cycle of the aphid, and any reinfection will have to come from afar.

During the summer, if waterlily aphids return, the only way of coping with them is to spray infested plants with clear water from a hosepipe, which should knock the creatures into the water, where many will be devoured by fish. Although, inevitably, some will crawl across the surface tension of the water and back onto the plants, with luck, the majority will be eaten.



An extremely destructive pest, the waterlily beetle can spread with impunity in warm areas. Where there are severe winters, however, its progress will be impeded. It is a very specific pest, only attacking waterlilies, but where it does take hold, it becomes a major problem.

The small, dark brown beetles and shiny black larvae are usually found on waterlily pads, where they strip away the epidermal layer of tissue, leaving the slimy tattered remains to decay. Opening blossoms and buds are also attacked. Severe infestations will leave little except flower stalks and skeletonized leaves.

The adult beetle hibernates in pondside vegetation during the winter, emerging in late spring and early summer, just as the waterlily pads reach the surface of the water. They migrate to the waterlilies, depositing clusters of eggs on the leaf surfaces. After a week, curious black larvae with distinctive yellow bellies emerge and feed on the foliage until pupation takes place, either on the waterlily pads or in surrounding vegetation. Under warm conditions, there may be three or four broods in a season.

Apart from ensuring good pond hygiene, little can be done to control this pest, except to spray the plants with clear water to knock the larvae into the pool where, hopefully, the fish will feed on them. Clearing up the marginal vegetation thoroughly in the autumn, to remove any hiding places where the beetles can overwinter, is the most useful means of exercising some control. However, once the pest is established, constant vigilance will be required. If necessary, they may have to be picked off by hand.



Aquatic Plant Pests and Diseases - brown china mark moth The larvae of this insignificant looking, brown patterned moth is one of the most destructive pests of aquatic plants, especially waterlilies. Not only does it cut and shred the floating foliage of aquatic plants, but it also makes a shelter for itself, prior to pupation, by sticking down two pieces of leaf within which it weaves a greyish silky cocoon. Damaged plants look very unsightly, their chewed leaves crumbling at the edges and surrounded by pieces of floating and rapidly decaying foliage.

The brown china mark moth lays its eggs on the undersides of floating leaves during the summer. They hatch and the caterpillars burrow into the undersides of the juicy foliage, later making oval cases out of pieces of leaf. They continue to feed and grow, passing the winter before pupating. Small infestations can be picked off by hand, but do not forget to net off any floating leaf debris, as this may have cocoons attached. In severe cases, it is worth defoliating the plants completely, removing all the leaves and allowing fresh submerged foliage to emerge unscathed.

The beautiful china mark moth is a species that behaves in a very similar manner, except that the caterpillars burrow into the stems of aquatic plants in the early stages of their life. Here they hibernate, eventually emerging to make leaf cases and their silky cocoons. Again, picking them off by hand is the only reasonable option.



Most of the caddis flies produce larvae that feed on the foliage of aquatic plants, many of them being totally submerged in their larval stage. The flies lay their eggs during the summer in long strings, which are often wrapped around aquatic plants in the water. When the larvae have hatched, they start to spin their silken cases, to which are attached small stones and plant debris. The larvae live in these hollow cases while they devour the foliage of aquatic plants. When large populations are present, they can be quite destructive, but the presence of fish helps to exercise some control over them. Otherwise, the only option is to pick them off by hand.



A most irritating pest that can be seen in many garden pools, the false leaf mining midge seems to be particularly troublesome with small waterlilies cultivated in tubs and containers. The minute larvae of this pest eat a narrow tracery of lines over the surface of floating foliage. This eventually turns brown and rots. There is no control, except regularly spraying affected plants with clear water to dislodge the pests. However, in severe cases, it is a good idea to remove all the floating foliage, together with the pest, to give the replacement growth a fresh start.



While the ramshorn or planorbis snail is a valuable consumer of algae, which rarely if ever devours desirable garden plants, this is not true of many other snails. Species like the greater pond snail, or freshwater whelk, and the fountain bladder snail can be extremely destructive, damaging aquatic plants in the same manner as garden snails attack land plants. Aquatics with floating foliage are particularly vulnerable to being grazed upon, and severe infestations can cause complete defoliation. For the most part, snails with tall pointed shells, which produce eggs in cylinders of jelly, are the most destructive; flat rounded snails, which lay their eggs in flat pads of jelly, are generally quite innocuous. Unfortunately, gardeners sometimes mistake the cylinders of jelly for fish spawn, especially when they arrive attached to plants. In this case, a population will quickly take hold in a pool.

Small tubs and planters can be rid of snails by mixing a proprietary aquatic snail killer into the water, but this would be impractical for a garden pool. Therefore, physical methods must be relied upon. Picking off by hand is a tiresome occupation, but it can be made easier by floating fresh lettuce leaves on the surface of the water overnight By the morning, there is likely to be a considerable congregation of snails beneath them, which can be collected and destroyed.



There are two species of leaf spot disease that damage waterlily leaves. One causes dark patches to appear on the surface of the leaves, which eventually rot through; the other tends to start at the outer edges of the leaves, causing them to turn brown and crumble. Both are debilitating and disfiguring, but they are not very serious problems. Their incidence will vary considerably from year to year, depending upon the prevailing conditions.

Removing damaged foliage, and discarding it safely elsewhere in the garden, will do much to prevent their spread. In decorative pools where there are no fish, a copper fungicide, like Bordeaux mixture, can be utilized effectively.



There are two distinctive forms of waterlily crown rot, which is believed to be caused by at least two different organisms. One has been known variously over the years as waterlily crown or root rot. It is believed to be a phytophthora, a relative of potato blight, and is specific to waterlilies and their allies. It has a preference for attacking yellow-flowered cultivars like Nymphaea ‘Marliacea Chromotella’, although it does attack others on occasion. Apparently healthy plants suddenly develop a blackening of the leaves and flower stems, which become soft and rotten, the rootstocks turning evil smelling and gelatinous.

All affected plants should be removed and destroyed, the pool being cleansed and sterilized by swirling a muslin bag filled with copper sulphate crystals through the water. All fish must be removed before any such treatment begins, and not returned until the pool has been emptied, swilled out and refilled with fresh water.

The more recent strain of crown rot disease is believed to be caused by a number of pathogens, which have not yet been fully identified. This disease is believed to have been introduced from the Far East on imported waterlily plants. It attacks all cultivars with impunity, causing the crowns to decompose rapidly into an unpleasant brown mass. At present, there is no cure, so it is vital to remove and destroy any infected plants. Then the pool should be cleaned thoroughly and sterilized with a solution of sodium hypochlorite before being flushed out with fresh water. Once clear water has been run back in, new plants can be safely introduced.


19. March 2011 by admin
Categories: Gardening Ideas, Pest and Disease Control, Water Gardening/Water Features | Tags: , , | Comments Off on Aquatic Plant Pests and Diseases


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