Ant predators of the Western spruce budworm (Choristoneura occidentalis Freeman) : species composition and patterns of occurrence in eastern Oregon and western Montana Public Deposited

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  • The identity and distribution of ant species preying on the western spruce budworm (Choristoneura occidentalis Freeman) was investigated in eastern Oregon and western Montana. A sticky trap was developed to index ant foraging activity in the canopy by intercepting ants falling from the tree as they traveled between canopy and ground. For all common ant species, trap catches were highly correlated with the average number of ants moving up the tree bole. Thus, traps indexed both species composition and density of ants foraging in the canopy. Ant species preying on pupae of the budworm were investigated on 2 sites in eastern Oregon in 1981, and 4 sites in eastern Oregon and 6 sites in western Montana in 1982. Predation was observed by checking pupae stocked on trees actively foraged by ants. The following ant species and numbers were collected as they fed on budworm pupae: Camponotus modoc Wheeler, 160; C. laevigatus (Smith), 53; C. vicinus Mayr, 48; Formica obscuripes Forel, 174; F. podzolica Francoeur, 43; F. accreta Francoeur, 32; F. neorufibarbis Emery 12; F. lasioides Emery, 1; F. sp. (microgyna group), 1. Camponotus species dominated the Oregon collections, and F. obscuripes dominated the collections in Montana. The distribution of canopy foraging ant species was studied on 24 sites in Montana and 24 sites in Oregon. Sites were selected along a moisture-temperature gradient indexed by vegetational composition (habitat type). Ant foraging activity was measured using sticky traps, and several site and stand variables were correlated with the abundance of ants collected at each site. Nine species of Formica and Camponotus were collected in Montana and 11 species in Oregon. Four species were common in both states--C. modoc, C. vicinus, F. neorufibarbis, and F. podzolica. In addition, C. herculeanus (L.) was common in Montana and F. accreta was common in Oregon. The distribution of these species varied among habitat types and between states and the abundance of each was correlated with a different set of environmental variables. At least one ant species predaceous on the western spruce budworm was common in each habitat type in either state. Across almost all habitat types, each common species was more abundant in stands where average canopy coverage was less than 90%. The distribution of ant foragers in the canopy on each plot was highly aggregated in all species in either state.
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