Weeds within willow short-rotation coppices alter the arthropod community and improve biological control of the blue willow beetle

Outbreaks of the blue willow beetle Phratora vulgatissima (L.) (Coleoptera: Chrysomeli- dae) threaten the yield of willow plantations that rely on biological control by natural enemies. Here we show that weed presence increases herbivore and predator/parasitoid diversity on willow shoots and causes increased predation on P. vulgatissima eggs compared to shoots within plots without weeds. We argue that, in addition to higher abundance, the community shift in favour of egg consumers and additive predator effects causes the higher predation pressure. This increase (*35 %) was apparent despite more alternative prey. Neither the willow genotype on which the predator-prey interaction occurred nor the clutch size affected egg predation levels. Our results support root's enemy hypothesis and suggest that intensive weed control might counteract biological control in willow plantations. However, at least during the establishing phase of the plantation, increased biocontrol probably does not compensate for yield losses caused by competing weeds.