Intraguild predation of weevils by livestock reduces acorn pests in oak silvopastoral systems

Abstract

Iberian oak savannahs are traditional silvopastoral systems in which acorns constitute a key food source for livestock. Acorn feeding insects provoke significant economic losses; however, the high natural value of Iberian oak savannahs precludes any chemical treatment. This paper shows a novel way of biological pest control based on promoting livestock predation on these insects. Female Curculio elephas (Coleoptera: Curculionidae) oviposit into developing acorns, which are prematurely abscised before reaching their potential size. Larvae still spend 20 days feeding on the cotyledons after acorn drop, being during this period very vulnerable to predation by livestock. We experimentally assessed that cows, pigs and sheep ate sound and infested acorns in the same proportion, so that infestation rates were lower in those Iberian oak savannahs in which livestock density was higher. An effective biological control of Curculio elephas weevils should involve an early predation of infested acorns by livestock, preferentially within 10 days after falling. Doing this, most larvae will be predated before completing their development and, in addition, the nutritional value of infested acorns will still be high (cotyledons not yet depleted by weevils). We encourage landowners to increase livestock densities during September–October, when the infested acorn dropping peaks. These increased densities should rotate over the farm and be maintained at the same plot for a maximum of 3 years in a row. Otherwise, a prolonged and concentrated livestock predation on sound acorns and seedlings would hamper long-term oak regeneration.