Aphids will suck the sap from sycamore.
Heavy infestations deposit honeydew on lower leaves and objects beneath the tree.
Use malathion or Orthene. Lace bugs feed on the undersides of the leaves causing a stippled appearance. The insects leave black flecks on the lower leaf surface. Use Orthene, rotenone or malathion when the insects are first noticed then repeat in 10 days. Spraying weeks later may be necessary if the insects return.