Experts and BHG readers answer.
European pine sawflies attack mugo (Pinus mugo), Scotch (P. sylvestris), red (P. resinosa), and Austrian pine (P. nigra) in early spring. The adults look like flies, but the larvae resemble caterpillars. Larvae feed on needles and can strip 1-year-old needles right down to the branch. The grayish-green larvae feed in a group, and freeze into position when disturbed. Because these are not true caterpillars, Bt (Bacillus thuringiensis) will not control them. However, you can handpick them off plants and drop them in soapy water to kill them. Or spray with insecticidal soap, horticultural oil, or an insecticide labeled for use on trees.