Livestock as a biological pest control: Experimental validation for oak savannas

Abstract

Ungulates consume plants and seeds together with the insects that feed on them, which have consequences at the ecosystem level. We carried out an experiment to assess the effects of intraguild predation by livestock on the main acorn pest (Curculio spp. weevils) in Iberian oak Quercus spp. dehesas, a widespread traditional agroecosystem. Acorns are a key food source for free-range livestock (especially pigs), but, if properly managed, livestock could become a pest control agent.

In three dehesa farms, we replicated eight experimental trios, each including one tree from which livestock was excluded, another from which all dropped acorns were removed manually (simulating intensified predation by livestock), and a control one in which livestock was allowed at standard densities. Removal of marked infested acorns by livestock, adult and larval weevil numbers, and acorn production/infestation rates were recorded at all trees. Livestock predated most weevil larvae within the prematurely dropped infested acorns before larvae had time to finish development. Hence, in those trees subjected to intensified predation, the local number of larvae decreased. Consequently, adult weevil abundance in the following year was lower than in trees within the exclosures of livestock.

The consequence of the decrease in the number of adults was a reduction in the rates of acorn infestation in control oaks and intensified predation compared to those excluded from livestock (8% and 20%, respectively). C. elephas mobility is low, especially when trees are isolated, and acorn infestation is greatly dependent on the number of weevils emerging beneath the canopy of each oak. Therefore, a locally focused elimination of infested acorns may succeed in reducing the negative impacts of the pest.

Synthesis and applications: We suggest intensifying livestock predation on prematurely dropped-infested acorns by allowing livestock foraging from October 1st onward. Pigs are usually released free range in early November, once weevil larvae have completed their development and escaped predation. The proposed management would increase the availability of healthy acorns, thus increasing the farm capacity and the economic profit by up to 20%, tens of millions of euros of additional profits when translated into the prices of the Iberian pork market.