Two insect growth regulators, a mixture containing about 25% Cecropia juvenile hormone with its geometrical isomers (AY-22342-3: Ayerst Research Laboratories, Saint-Laurent, Quebec) and a preparation containing 75% of the aromatic terpenoid ether, 4-ethylphenyl-6,7-epoxygeranyl ether (R-20458-B: Stauffer Chemical Co., Mountain View, California), were tested under field conditions for ovicidal activity against eggs of Neodiprionsertifer (Geoff.). Single egg clusters on Scots pine (Pinussylvestris L.) and whole red pine (P. resinosa Ait.) trees were sprayed to the drip point with formulations containing 0.1, 1, or 10 g/litre of insect growth regulator. AY-22342-3 reduced egg hatch at 10 g/litre on both tree species but was more effective in the red pine trial. R-20458-B did not reduce egg hatch significantly. Results with AY-22342-3 in the laboratory were about twice as effective as field trials. The application of fertilizers (urea, KNO3, or NH4NO3) accompanying a spray of AY-22342-3 on whole red pine trees does not enhance larval mortality. For trees receiving relatively intensive care, notably ornamentals and Christmas trees, the ovicidal activity of insect growth regulators might be useful as a means of preventing sawfly defoliation.