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Water Lily Beetle- image from Donsgarden.com

Today’s GQT comes from Adele Inwood from the lsle of Wight:

‘I have a new pool in my garden and I want to know how to deal with the main pests and diseases of pool plants please.’

Adele, the worst pest of pool plants is the Water Lily beetle- it’s larvae are like small slugs, dark on top and pale underneath. They feed on the leaves of water lilies.

The small brown beetles hibernate in the hollow stems of other aquatic plants, which should therefore be cut down in the autumn and burnt. You can control the larvae by laying a double thickness of newspaper over all the foliage from the first appearance of the pest (indicated by holes chewed through the foliage). If this is done in the evening and the papers removed in the morning, and the process is repeated at weekly intervals for at least four weeks, you should find that the beetle larvae will have been eaten by other water life.

Remove the worst damaged leaves. This method of control is also good for the reddish-black aphids which can seriously damage the leaves. Hosing off the aphids and beetles is also effective – but be careful not to add too much new water to the pool.

You might also see a thick green scum appearing on the surface of the pool. If the pool hasn’t been filled for a matter of only a few days or weeks, remove the worst of the scum with a fine mesh net. The scum – really an algae – appears after you change the water, before it settles down again. The presence of foliage on the surface will help to speed up the process by preventing light from getting to all of the water, stopping the formation of the algal ‘bloom’. So perhaps look at trying to cover more of your water surface with plants – about a third coverage is a good target.

Pond algae can be reduced by increasing leaf cover on the water surface

Pond algae can be reduced by increasing leaf cover on the water surface

Further information:

Water Lily Pests

Old School Gardener

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