Impact assessment of Leptoglossus occidentalis in Pinus pinea: Integrating population density and seed loss

Abstract

The production of the highly valued seeds of stone pine has decreased all over the Mediterranean region during the last two decades. This phenomenon coincided with the establishment in Europe of the invasive species Western Conifer Seed Bug, Leptoglossus occidentalis (Hemiptera, Coreidae). The putative impact of L. occidentalis on pine seeds has raised the need for responses among the scientific community. Still, the subject is at some extent controversial and other hypothesis has been raised, such as the influence of climate in flowering, pollination and cone survival. Until now, we lack an effective measure of the impact of this bug on seed yield. Here we integrate results from seed loss on mature cones with the monitoring of the bug population at site level during five consecutive years. The seed loss was assessed by comparing naturally exposed cones (N) with those protected by an insect exclusion bag (E). Both type of cones were analysed on fertilized plus irrigated trees (fertirrigated plots) and on control ones.Observed seed loss per cone was closely related with the population dynamics of L. occidentalis. We found a strong positive correlation between seed loss per cone and average insect density of current Nt and previous year Nt-1 (R2 = 0.98). For similar insect density, cones from fertirrigated plots presented a higher seed loss than control ones.Additionally, the insect population dynamics was framed with climate data for the 5 years period and a climatic theoretical explanation is hypothesized.