Site Susceptibility, Population Development and Dispersal of the Pine Beauty Moth in a Lodgepole Pine Forest in Northern Scotland

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Abstract
(1) The population behaviour of the pine beauty moth (Panolis flammea D and S (Lepidoptera: Noctuidae)), in a lodgepole pine (Pinus contorta Douglas) forest (North Dalchork) in northern Scotland was analysed from 1977 to 1984 to see whether part of the forest was intrinsically more susceptible than the rest to attack (as a result of, for example, a greater degree of host-plant susceptibility). Population growth was not significantly greater in the small part of the forest defoliated by larvae in 1984, refuting this hypothesis. Instead, the 1984 outbreak centered on the area where pine beauty moth numbers had been greatest 1 year after an insecticide control operation in 1979. (2) Large and significant differences were observed in population development in different years. (3) Significant differences in adult emergence (1.5-181.5 m-2) and egg laying (156-5496 eggs per tree) were observed at ten sampling locations throughout the forest in 1985. The number of eggs laid per tree was negatively and significantly correlated with the degree of defoliation caused by pine beauty moth larvae during the previous year. (4) The number of eggs laid per female per site (an estimate of fecundity) ranged from 18 (at the defoliated site) to 163. The fecundity predicted (from a previously derived model) from March-May temperatures was 58 eggs per female. This implies that the moths in the defoliated area were less fecund than the others, or laid a large proportion of their eggs outside the defoliated area. More than twice as many eggs as expected were found at three sites to the north-west and south-east of the defoliated area. However, densities, there, and throughout most of the rest of the forest, were already much greater than the economic threshold, as a result of local population development.