In this post Anna-Sara Liman discusses her recent paper ‘Predator refuges for conservation biological control in an intermediately disturbed system: the rise and fall of a simple solution

The first so called willow “energy forests” were planted in Sweden (and the UK) in the early nineties and represent among the first steps towards development of a bioenergy sector and a future bioeconomy. Willows are fast-growing trees with outstanding abilities to transform sunshine, nutrients and water into biomass. This resource is latterly utilized in the Swedish district heating system to generate heat and electricity.

The first reported outbreaks of leaf beetles in energy forests came only a few years after the first cuttings were planted and posed a significant threat to the economic side of biomass production. Conservation biological control, i.e. to promote abundance of naturally occurring predators, was the one option at hand.

The main predators of leaf beetle egg and larvae are mainly plant but also flower bugs. The plant bugs are small, green or brown insects that feed on plant sap as well as various insect preys. These omnivorous insects have a fascinating and close association with their host plants and e.g. insert their eggs into willow stem tissue to overwinter and thereby synchronize their development with the phenology of the tree.

The willow “energy forests” are harvested in February on frozen soils, every four to five years. Cutting the stems at this point, however, has devastating consequences for the biological control of leaf beetles – since the removed willow stems contain the entire local population of predatory plant bugs.

Salixplantering pΠSLU Ultuna.
Willow ”energy forests”. Photo credit ©Viktor Wrange, SLU.

A simple and intuitive solution to overcome this problem started to emerge and this is where the present study comes in. We predicted that by retaining a part of the willow stand we would preserve a source population of overwintering predatory plant bugs that could recolonize the re-growing parts of the stand. The positive effect of retaining refuges in reducing winter mortality should be limited to the predatory plant bugs since the leaf beetles leave the willow stems for the winter.

What we did not anticipate was that leaf beetles returning from their overwintering sites would aggregate in the refuges – eventually transforming the refuges from a “source” of predators to a “sink”. Retaining predator refuges thus increase rather than decrease the risk of leaf beetle outbreaks in willow short rotation coppice. Coppicing during the dormant season modifies the phenology of the shoots so that leaves on younger shoots unfold later and attain smaller sizes, than leaves on older shoots. We believe that the leaf beetles (and thereby also their predators) aggregated in refuges simply because the older stems provided a better phenological match.

The lesson learned – management for conservation biological control requires detailed ecological understanding of the interacting species and simple intuitive solutions does not always hold all the way.