Oh! I just found out that a book I own is now online in an updated version:
Field Guide to Common Insect Pests of Urban Trees in the Northeast
Since it’s the time of year to look for these things, here’s the page on oystershell scales, which I have in abundance on my lilacs. Grrr.
Now is a great time to treat scales with dormant oil sprays. From that Extension publication:
“Proper timing is critical for success when using oils. Dormant oils should be applied in late March or April before leaves or flowers show signs of breaking dormancy; that is, before “bud break.” A common mistake is to apply ‘dormant’ oil sprays too early (on the first warm day in February or March) before insects are actively respiring and susceptible to the oil’s suffocating effects. Wait until as close to bud break as possible before applying oil sprays. “