Charlie Pridham writes


Spraying against aphids will probably make things worse as leylandi hedges
are major overwintering sites for ladybirds who will soon sort out any
aphids problems. browning of hedges is more likely due to wrongly
timed/badly done trimming or too wet/ too dry conditions.

Interesting article in latest issue of Garden by a lady who used to be
involved in research which used aphids to transfer viruses from plant to
plant thus requiring a good supply of aphids. When aphid populations
were waning, their approach was to spray once with an insecticide. This
killed all aphids on leaf surfaces but not those in nooks and crannies.
It also killed all predators (who roamed widely over leaf surfaces and
therefore picked up the insecticide). Resultant rebound from breeding of
the hidden-away aphids resulted in bumper crop of aphids within two
weeks.
--
Kay