Pattern of Attack on Spruce Budworm Egg Masses by Trichogramma minutum (Hymenoptera: Trichogrammatidae) Released in Forest Stands

Female Trichogramma minutum Riley released from a central point in a white spruce, Picea glauca (Moench) Voss, plantation attacked eggs of the spruce budworm, Choristoneura fumiferana (Clemens), locally around the point of release. Dispersion parameters showed an aggregated pattern of attack—mean ( $$\bar m$$ ), 1.6 parasitized egg masses per station; mean index of crowding ( m^* ), 20.5; Lloyd's index of patchiness (IP), 7.3. Parasitoids dispersed a total distance of 18.5 m over 5 d, with a mean horizontal distance (d) of 4.3 ± 0.2 m and an average dispersal rate of 7.2 cm/h. Movement by T. minutum within trees was not affected by wind; however, dispersal between trees during the first 2 d after release tended to be downwind. Average vertical distance of dispersal within trees from a point 0.25 m above ground was 2.4 m (±0.05 m). When T. minutum was released at this height in 1984, parasitism of egg masses was significantly higher at 3.25 m above ground than at 2.25 or 1.25 m above ground. The reverse was true when parasitoids were released at ground level. Greater parasitoid movement was seen on jack pine, Pinus banksiana Lamb., than on white spruce. Dispersal by female T. minutum was influenced by stand complexity and volume.