The acute toxicities of an extract obtained from a plant within the Piperaceae family and related synthetic analogues were tested against four common Canadian forest pest insects. The acute toxicity of the extract from black pepper, Piper nigrum L., was assessed after 1, 24, and 72 h by the percent larval mortality. The 24 h LC50 estimates for the P. nigrum extract were (in order of decreasing sensitivity) 0.012% for the introduced pine sawfly, Diprion similis (Hartig) (Hymenoptera: Diprionidae), 0.053% for the forest tent caterpillar, Malacosoma disstria Hubner (Lepidoptera: Lasiocampidae), 0.282% for the gypsy moth, Lymantria dispar (L.) (Lepidoptera: Lymantriidae), and 0.998% for the spruce budworm, Choristoneura fumiferana (Clemens) (Lepidoptera: Torticidae). There was no significant increase in mortality after 72 h. Seventy percent of L. dispar larvae dropped off or moved from branches within 1 h of application of 0.2% P. nigrum extract, indicating that these compounds have a repellent effect. Pipercide and nor-pipercide were more toxic to L. dispar and M. disstria larvae than piperolein A and a P. sarmentosum Roxb. amide 72 h after either oral or topical administration of these compounds. Toxic effects of piperamides were more pronounced by oral ingestion. Ninety percent mortality of L. dispar larvae occurred following an oral dose of 5 µg pipercide in diet, whereas mortality was only 40% following topical treatment at 5 µg pipercide/insect. Whole Piper extracts might be useful for the control of sawflies and tent caterpillars in small-scale applications, based on the demonstrated efficacy and reduced risk potential.