AbstractAn outbreak of western spruce bud worm, Choristoneura occidentalis Freeman, in the Blue Mountains of eastern Oregon, USA, was treated with helicopter-applied carbaryl. Effects on forest ants were determined from pre- and post-spray samples taken from replicated 0.75-ha sprayed and unsprayed plots. Arboreally foraging ants were sampled with sticky drop traps placed under trees. Ground-foraging ants were sampled from food-baited boards. During 8 weeks of sampling, 13 225 ants from seven genera and 19 species were collected. Seven species were known budworm predators. After spraying, ant species diversity decreased in sprayed plots. Post-spray foraging decreased in all plots, but the decrease was more rapid and pronounced in sprayed plots. Among ground-foragers, budworm predators were clearly affected by spraying. Arboreal foragers, nearly all budworm predators, showed a significantly lower foraging rate in sprayed plots. Varying effects on different species were noted. Overall ant foraging remained depressed for at least 6 weeks after spraying, long enough for budworm development to be completed. Reduced ant predation on sparse budworm populations after spraying may contribute to budworm resurgence.
