These beetles and their larvae feed solely on water lilies (Nymphaea) and are active in the late spring and summer months.
The symptoms are seen as circular or elongated slots in the leaves, to a point where the whole leaf may disintegrate, which in turn leads to a general weakening of the plant.
The adult beetles can cause significant damage to the flowers.
The yellowish brown adult beetles are approximately 6mm (¼”) long, and overwinter in sheltered places.
They emerge to lay clusters of eggs on the upper surface of lily leaves in May/June, and then again in late summer - early autumn.
There are normally two generations of larvae, the second generation appearing in August / September.
The larvae are black with a pale yellow undersurface and grow up to 9mm (3/8”) long.
They pupate and feed on the top of lily pads.
Control;
Control with pesticides can be difficult, particularly if there is fish in the pond.
The most feasible method of control is; to remove the beetles, and or the larvae, by hand in small ponds, or a jet of water directed at the foliage on larger ponds.
If using the latter method any fish in the pond will soon eradicate those than have been washed into the pond.
If there are only a few plants in the pond then perhaps one can sink the leaves under the surface of the water for a few hours and again let the fish get to work.