our guide books tell us that for taking care of lilacs a systemic
insecticide must be used.

Which guide book? And taking care of what? Lilac borer?
A systemic insecticide is an insecticide which is absorbed into the plant and
makes it poisonous when the insect eats it. Its main advantage is that you
don't have to hit the insects to kill them. This is in contrast to contact
insecticides, which have to land on the insects to kill them. Each has
advantages. Systemic insecticides are surer and more effective, and need to be
used less often. On the other hand, they tend to be more toxic. The most common
systemic insecticide sold for home use, Orthenex, is relatively harmless, but
only sold for outdoor use. Contact insecticides, like pyrethrins, are safer and
approved for indoor use, but are less effective.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)