Unfortunately the Aphid is at the moment harming many Leylandii in the UK. It is one of those things that because of warmer and wetter weather in the UK over the last few years. Aphid's should be killed off in cold winters. unfortunately the winters in the UK over the last few years have been warmer than usual and this has caused the growth in population of the insects.

There are 4 ways that will cure this problem

1) Colder Winters will kill off the Aphid

2) Insecticides that will be developed over the next few months will kill the Aphid's. Current Insecticides will kill the Aphids, however people often don't know what they are looking for and won't notice the damage.

3) There will be a rise in the predators that feed off the Aphid's. There are certain bird species that can feed off Aphid's and it is predicted that these will grow in population because of the numbers of Aphids available to be consumed. Caterpillars are also a good predator of the Aphid, so don't think of it as a permanent problem that will make the Leylandii extinct. Nature always keeps a balance of these problems, but a cycle may take a few years before a balance is returned.

4) Different strains of leylandii that have the ability to repel the Aphid.

The damage caused by these Aphid's is not always fatal to the leylandii, in fact hardly ever if left alone by the owner. the Aphids will die long before the tree does. It often looks a lot worse than it is and most Leylandii can take a one hell of a beating before dying off. It usually takes anywhere from 10 to 18 months to repair itself, so don't be over concerned.

Tips, Spray insecticide along the conifer, lop them in the summer, plenty of water to prevent them from drying out and don't over trim in the Autumn. Try not to trim at all, if the Aphid does attach then it will feed on the cutting you were going to cut off anyway, thus doing no harm.