Waterlily beetle (Galerucella nymphaeae)
Plants affected: Waterlilies (Nymphaea
and Nuphar species and cultivars)
Symptoms: Circular or elongated slots are eaten in the leaves where both adult beetles and larvae can be seen during the summer. The flowers are also damaged by the adult beetles.
Biology: This pest has two generations during the summer. Adult beetles overwinter in sheltered places and emerge to lay clusters of eggs on the upper surface of lily leaves in May-June. The larvae are up to 9mm long when fully grown and are black with a pale yellow undersurface. They pupate on top of the lily pads. The beetles are yellowish brown and 6-8mm long. The second generation of larvae is present during August to early September.
Control: Control is difficult as pesticides cannot be used on plants growing in or adjacent to ponds because of the danger to fish, frogs, toads and other pond wildlife. Hand removal of larvae, pupae and adults is feasible on small ponds. On large ponds the damage often has to be tolerated. The beetles can be dislodged from the foliage by a jet of water and some of the floating larvae and beetles may be eaten by fish.
Waterlily aphid (Rhopalosiphum nymphaeae)
Plants affected: Waterlilies (Nymphaea and Nuphar species and cultivars) and various pond margin plants.
Symptoms: Dense colonies of greenish brown aphids form on the upper surface of lily leaves and on the flower buds. White cast aphid skins also litter the leaves. Heavy infestations spoil the appearance of the flowers.
Biology: Waterlily aphid overwinters as eggs on blackthorn and other Prunus species. The eggs hatch in the spring and the aphids feed on the foliage until early summer when winged forms of the aphid develop. These migrate to waterlilies and other water-side plants where they establish summer colonies. In the autumn there is a return migration of winged aphids to the winter host plants.
Control: Control is difficult as pesticides cannot be used on plants growing in or adjacent to ponds because of the danger to fish, frogs, toads and other pond wildlife. In small ponds aphids can be wiped off the leaves and flower buds to keep their numbers below the level where damage occurs. For larger ponds, directing a strong jet of water at the lily pads will dislodge many of the aphids and expose them to predation by fish.
Leaf-mining midges (Cricotopus species)
Plants
affected: Waterlilies and other pond plants
with foliage that floats on the water surface.
Symptoms: The leaves gradually decay and are eaten away, especially around the leaf margins. Slender white maggots, up to 6mm long, feed within the leaves at the edges of the damaged areas.
Biology: These leaf-mining insects belong to a group of flies known as chironomid midges, most of which are harmless to pond plants. The leaf-mining midges seem to be a particular problem in newly established ponds or those that have been cleaned out and refilled. In this situation, vulnerable plants can struggle to get established. Damage is rarely seen in established pools where this pest is probably controlled by fish and predatory pond insects.
Control: Control is difficult as pesticides cannot be used on plants growing in or adjacent to ponds because of the danger to fish, frogs, toads and other pond wildlife. Remove badly damaged leaves from the pond to get rid of some of the larvae. In time the situation will correct itself as the pond matures. If plants have failed to get established, delay replacing plants with surface-floating leaves for a year.