Experts and BHG readers answer.
Sticky Leaves on a Ninebark Diablo
The stickiness is probably being caused by the insects' droppings. You might try spraying your ninebark with a product called Captain Jack's Deadbug Brew; it has a low toxicity to animals and beneficial insects, but should help clear up the pesty insects. Be sure to follow the instructions on the packaging carefully.
---Justin, Senior Garden Editor, BHG.com