Alpha-cypermethrin is widely used in Australian forestry to control defoliating beetle pests but no information exists on its efficacy against beetle pests or impacts on non-target arthropods. To address this deficiency I quantified the short-term impacts of single commercial applications of the insecticide alpha-cypermethrin on populations of both defoliating beetle pests and beneficial arthropods over 12 months in two southwestern Australian eucalypt plantations. A single application of alpha-cypermethrin (24 g a.i./ha) in October or November reduced beetle pest populations to near zero levels. Beetle recovery began immediately after spraying but high populations of eucalyptus weevil and chrysomelid beetles, and high defoliation levels did not eventuate until 10'12 months later. Despite the success of alpha-cypermethrin in controlling defoliating beetle pests, the broad-spectrum nature of the insecticide led to nearly complete mortality of all beneficial arthropods immediately after spraying. Spiders were the main group of beneficial arthropods present after spraying probably because some individuals occupy a concealed and protected habitat. Populations of Anaphes nitens (Girault), the important eucalyptus weevil egg parasitoid, also partly survived the spray because they were protected within weevil egg masses. Post-spray recovery of both beetle pests and beneficial arthropods occurred gradually in the following 12 months, presumably through immigration from outside the plantation.