The devastating impact of an invasive ambrosia beetle on the riparian habitats of the Tijuana River Valley, California

Background. The Tijuana River Valley is the first natural habitat in California to be substantially invaded by the Kuroshio Shot Hole Borer (KSHB, Euwallacea sp.), an invasive ambrosia beetle native to Southeast Asia. This paper documents the distribution of the KSHB in the riparian vegetation in the valley and assesses the damage done to the vegetation as of early 2016, approximately six months after the beetle was first observed in the valley. Methods. I divided the riparian habitats in the valley into 29 survey units such that the vegetation within each was relatively homogenous in terms of species composition, age and density. From a random point within each unit, I examined approximately 60 individuals of the dominant plant species for evidence of KSHB infection and evidence of major damage such as limb breakage. In the 22 forested units, I examined the dominant arroyo and black willows (Salix lasiolepis and S. gooddingii), and in the seven scrub units, I examined mule fat (Baccharis salicifolia). In addition, I recorded evidence of infection in other common species as they were encountered. Results. Evidence of KSHB infection was found in 25 of the 29 units. In the forest units, infection rates ranged from 0 to 100% and were high (>60%) in 16 of the units. In the scrub units, infection rates ranged from 0 to 33%. Infection rates were significantly negatively correlated with the dryness of a unit; drier units had lower infection rates. Evidence of physical damage was found in 24 units, and dense stands of willows were reduced to broken trunks in several areas. Overall, I estimated that more than 280,000 (70%) of the willows in the valley were infected, and more than 140,000 had suffered major limb damage. Of the 23 species examined, 14 showed evidence of beetle attack. The four species with the highest rates of infection were native riparian trees in the Salicaeae family. The three species considered to be the worst invasive plants in the valley, Ricinus communis, Tamarix ramosissima and Arundo donax, had low rates of infection. Discussion. The KSHB has substantially infected and damaged the dominant native trees in the riparian forests of the Tijuana River Valley, and this has led to a drastic alteration in the structure of the canopy of the forests. The loss of canopy is likely to promote the growth and spread of invasive plant species that were relatively inconspicuous in the forests prior to the beetle attack. The beetle’s impacts are a novel disturbance, and it remains to be seen whether the affected forests can recover. If resource managers are to control the spread of the beetle, they will need to develop an effective early detection and rapid response program. Because infection rates in the valley varied along a gradient of dryness, resource managers trying to detect the KSHB in other areas should thoroughly search trees near surface water.