Animals and Money: Common Pine Sawfly - Mass Outbreak Could Cause half a million Euros in Damage

Mass outbreaks of forest insect pests are an increasing problem¹, particularly for the forestry industry. Germany`s federal state of Brandenburg (BBG), one of the most densely forested states, comprises over 1.1 million ha of forest. This corresponds to 37% of the total area of the state, where the forestry and wood industry are the largest economic sector and entail 10% of all enterprises of the state ². Here, the most important and most abundant tree species is the pine tree, comprising 735,000 ha, or 67%, of the forested area ².
In 2012, the forestry and wood sector generated 3.2 billion euros in revenue ². However, the economy and forest protection are frequently challenged by a multitude of plant-eating insects, among them the common pine sawfly (CPS), Diprion pini L ¹. Monitoring and control of CPS are particularly challenging to forest protection due to the variable, highly adaptable and mostly unpredictable lifestyle of the CPS, and population dynamics with comparably strong dependency on climatic factors ^1,3. The annual climate determines whether CPS follow a univoltine or bivoltine life cycle. Importantly, these life cycle patterns are a key aspect in the occurring forest damage. ^1,4. Throughout the common univoltine cycle, only one generation of larvae occurs and preferably feeds on pine needles from the previous year¹. This mostly remains unnoticed by forest protectionists ¹. Extended warm and dry periods from April to May induce the bivoltine cycle, bearing potential for mass outbreaks. Here, first-generation larvae finish their development faster, and second-generation larvae appear within the same year, primarily feeding on needles from the current year¹. This can pose a severe threat to plant stands¹. This often comes as “surprising” to forest monitoring ¹. BBG pine forests have been affected several times in the past by CPS mass propagations. Recent infestations in 2005, 2009, and 2016 inflicted heavy damages¹. Throughout the latest mass outbreak in 2016, researchers have monitored a study area of 23,000 ha within the south of BBG. Identified were 1,300 ha with over 90% defoliation, which knowingly increases tree mortality significantly¹. Assuming that such an infestation definitely leads to the death of trees and thus to direct loss of income, the economic damage for this area alone can be estimated at 428,000 €. Costs for the entirety of BBG will not be estimated here due to unknown variables and thus even more error-prone calculations. Nonetheless, costs to the state's economy which greatly exceed the estimate above must be considered. Furthermore, it must be assumed that the economic costs caused by CPS will increase in the future, as there is a continuous trend of an increase in the area affected and the intensity of defoliation⁴. While the cost assessment also hinges on regional parameters such as soil properties, anthropogenic forces have a crucial impact in this context as well ^1,5. Climatic conditions promoting mass propagations include periods of heat and drought, which are expected to enhance due to human-made climate change ^4,5. Considering this an increasing future problem, the impact of CPS on pine stands and the Brandenburg economy should be continually monitored.

Issue 5 (PDF)

Natalia Wulff