Cost-Plus-Loss Evaluation of Double Sampling Strategies for Insect Defoliator Mitigation Decisions

Abstract Forest damage may increase in frequency, extent and severity as a consequence of climate change and global trade. To mitigate the negative consequences of forest damage caused by insects and fungi, pest management strategies need to be established. An important component of such strategies is the assessment of pest population densities to predict subsequent potential damage during outbreaks. In this study we evaluated different inventory strategies for assessing the population density of Scots pine ( Pinus sylvestris ) insect defoliators through cost-plus-loss analysis where the cost of the inventory is added to the expected losses, mainly from incorrect decisions due to inaccurate information. We assessed what inventory strategy minimized the expected cost-plus-loss for differentspatial distribution of pupae populations, forest stand ages and forest areas affected. The results showed that double sampling was a more efficient sampling strategy than a single-phase survey. With the double sampling strategy, a second phase sample was selected if the first phase results were inconclusive regarding whether pest mitigation efforts should be implemented. The average cost-plus-loss varied widely over all simulated defoliator pupae populations and stand ages. For the double sampling strategy, the average cost-plus-loss was between ~ 18.7 thousand SEK and 5.6 million SEK, while the alternative cost-plus-loss ranged between ~ 18.7 thousand SEK and 9.7 million SEK. The results underscored the importance of accurate information for decision-making in defoliator control, and that a double sampling strategy is better than a single sampling scheme for most scenarios evaluated. We found that the least inventory efforts should be used to evaluate defoliator populations in stands close to a scheduled harvest, and more efforts should be allocated to younger standsdue to the higher expected loss from incorrect decisions. In younger stands, salvage logging due to defoliation induced mortality resulted in higher costs.