Monitoring of Phenology and Population Trends of Lepidopteran Cone Pests in South Swedish Spruce Seed Orchards

Regeneration of Norway spruce, Picea abies, forests is to a large extent depending on the production of seeds from high quality plus trees in orchards. However, the seed yield is variable and unpredictable due to weather, pest insects and diseases. Thus, more knowledge is needed to improve planning of harvest and mitigation of pests and diseases to secure future forest regeneration. In this study, we used pheromone traps and cone inventory to document the phenology and population trends of three important lepidopteran seed and cone consumers, viz. the spruce seed moth, Cydia strobilella, the spruce coneworm, Dioryctria abietella and the cloaked pug, Eupithecia abietaria, in six south Swedish orchards over 6 years. The flight period of C. strobilella was concentrated in May, whereas D. abietella had an extended flight period from May to October, and E. abietaria mainly flew in June and early July. Population densities measured by trap catch and cone density fluctuated between years and orchards, despite these were located within 74 km of each other. For C. strobilella, but not for D. abietella, there was a significant relationship between the number of males captured per trap and the number of larvae per ha or the proportion of infested cones in the subsequent generation. No significant relationships occurred between larval density and trap catch the following year (within generation) for any of the species. Notably, the trap catches were high also after years almost void of breeding material, that is, cones. Our results indicate that we still lack information on several parameters essential for reliable predictions of population trends, such as prolonged diapause, winter survival and the presence and abundance of natural enemies.