Cypress aphid (greenfly) is occasionally found on Leylandii in late winter or early spring (January to April). It over-winters on Leylandii and can cause a problem especially on hedges that have been trimmed back very hard in the autumn or plants that are under stress for some other reason, e.g. plants where the soil is too dry or too wet. The damage is not apparent until the late spring when brown patches appear in the hedge, by this time the aphids have long gone and it is too late to treat the hedge.
The colder winters of 2010 and 2011 will have reduced the number of aphids that have survived and significantly reduced the problem but it is worth checking your Leylandii hedge in January, February and March for large greeny/brown 'greenfly' on the young shoots. These are usually found 2 or 3 inches (5-8cm) within the hedge on the bark of the shoots. They are well camouflaged so can be difficult to spot as they are the same colour as the bark. They align themselves on the shoots in rows.
If the aphid is present, spray with an insecticide such as Scotts Bug Clear Ultra or Bayer Garden's Ultimate Bug Killer. These are systemic insecticides that will kill the aphids on contact and remain in the plant to protect against new attacks for approximately six weeks.
Cut out the brown patches as they will not re-shoot. Encourage other green branches to grow back over the brown patches to cover them up.
Note: Brown patches can also be caused by trimming a Leylandii hedge too many times a year. A Leylandii hedge only needs trimming once a year. If it is trimmed more frequently, the hedge doesn't have enough of a chance to re-grow and recover from each trimming and this weakens the hedge making it more susceptible to attack from other problems as well as Cypress Aphid. Trimming frequently also creates a lot of "thatch" or dead material that can act like a sponge holding water on the foliage. This can results in diseases like Botrytis and Pestalotiopsis which can also cause brown patches. If you brush or rake the hedge after trimming, it removes a lot of this "thatch" and improves the air flow around the foliage making it much less likely for problems to occur.