Disturbances such as harvesting often interfere with the ecological processes that lead to the biological control of insect pests. For willows, grown as short‐rotation coppice crops harvested every third to fifth year, it has been suggested that high plant quality in the resprouting shoots after harvesting may explain observed high densities of herbivorous insects, especially leaf beetles (Coleoptera: Chrysomelidae), in the plantations. In this study, we show that generalist predators may be important as regulators of leaf beetle populations. All three leaf beetle species, which were studied for five years in 12 plantations, showed a negative correlation between the population growth rate from spring to fall and the abundance of the most common generalist predator Orthotylus marginalis (Heteroptera: Miridae). For the most abundant leaf beetle, Phratora vulgatissima, we also found a significant positive correlation between its population growth rate and egg survival, indicating an overall effect of predation on herbivore population growth. Harvesting, which takes place during the winter, had a negative effect on the abundance of leaf beetles and predators. However, the first year after harvesting, all three leaf beetle species regained this loss with a very high population growth rate. A reason for the better ability of the herbivores to recover from the disturbance may be that they, unlike the predators, mainly overwinter outside the plantations. All three leaf beetles peaked in density three years after harvesting whereas the density of generalist natural enemies increased or leveled off during the five‐year period after harvesting. We conclude that predation by generalist predators is potentially important for population control of leaf beetles in willow coppice, but that the intermediate disturbance regime of around five years between harvests, appears to be too short to avoid disruption of biological control. Alternatives for more efficient biological control in short‐rotation coppice systems may be a longer period between harvests that enables the predators to fully respond numerically, to leave natural enemies refuges at harvest, or to harvest adjacent plantations asynchronously.

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Abstract1 The effect of defoliation by larvae of the leaf beetle Phratora vulgatissima on current‐year stem wood production of resprouting Salix viminalis was investigated for two years. Adjacent subplots with varying levels of defoliation within one large willow plantation in south Sweden were studied in the two years.2 High defoliation levels reduced stem wood production by an average of 32 and 39% in the two years, respectively.3 Medium defoliation levels reduced stem wood production by 16% in one year. In the other year, the stem wood production of medium‐defoliated stools did not differ significantly from stools exposed to low defoliation, i.e. there was full compensatory growth.4 The main difference between the year with compensatory growth and the one without was that overall productivity was higher in the year with compensation. This finding forms the basis for a mechanistic model by which compensation could be accomplished. We propose that the major contribution to full compensation comes from an increased growth among intermediate‐sized shoots of medium‐defoliated stools relative to the corresponding shoots in stools exposed to low defoliation.

1 Leaf beetles are major pests in willow plantations used as short-rotation coppice for biomass production. The beetles overwinter mainly outside the plantations. An understanding of the factors affecting adult leaf beetles seeking overwintering sites may provide information that could be valuable when developing methods to control the beetles. 2 We conducted a field experiment in a willow plantation with a high abundance of the leaf beetle, Phratora vulgatissima. We positioned overwintering constructions (OWCs) made out of bundles of reed (Phragmites australis) outside the plantation. 3 Leaf beetles preferred (i) to fly towards silhouettes; (ii) OWCs positioned 1.5 m above the ground compared with ones 0.5 m above the ground; (iii) OWCs orientated vertically compared with OWCs orientated horizontally; and (iv) reeds of a diameter of approximately 5 mm. 4 Leaf beetle size was not correlated with reed diameter. 5 It is concluded that the leaf beetle P. vulgatissima selects an overwintering site based on factors at both larger and smaller scales. The possibilities for using OWCs made of reed bundles as tools to monitor and control are discussed.

1 Willows are frequently attacked and defoliated by adult leaf beetles (Phratora vulgatissima L.) early in the season and the plants are then attacked again when new larvae emerge. The native willow Salix cinerea has previously been shown to respond to adult grazing by producing new leaves with an increased trichome density. Subsequent larval feeding was reduced on new leaves. This type of induced plant response may reduce insect damage and could potentially be utilized for plant protection in agricultural systems. 2 Here, we investigated if the willow species most commonly used for biomass production in short rotation coppice, Salix viminalis, also responds to adult beetle grazing by increasing trichome density. Larval performance and feeding behaviour on plants previously exposed to adult beetles was compared with that on undefoliated control plants in a greenhouse. 3 We found an overall decrease in trichome density within all the plants (i.e. trichome density was lower on new leaves compared to that for older basal leaves on S. viminalis). However, leaves of beetle defoliated plants had a higher trichome density compared to control plants. Larval growth and feeding was not affected by this difference between treatments. Larvae appeared to remove trichomes when feeding on S. viminalis, a behaviour that might explain the lack of difference between treatments.