The symptoms are characterized by the presence of holes, discolorations and deep cracks on the bark of branches or trunk. If the bark is cut out and removed near these holes, an extensive tunneling of the wood directly beneath the bark can be observed. The decay of the cortex is denoted by the presence of reddish stains or cankers on the places where the galleries are found. Affected branches gradually loose vigor and wither, and trees are weakened. Usually the feeding galleries are not harmful for the healthy trees but they can kill branches already weakened by fungi or other stresses. It is not a major pest of olive tree but can nevertheless cause significant damage.
The symptoms observed on olive trees are caused by the beetle Hylesinus toranio. The larvae of these insects are xylophagous, meaning that they feed from the sapwood below the bark. Adults are of a dull black or dark-green color, with two orange antennae and rows of yellow leathery hairs on the back. Females usually choose weakened trees, pierce a hole through the bark and bore a tunnel into the sapwood. The eggs are laid along this mother gallery. After hatching, the larvae bore shorter and narrower galleries underneath the bark, starting from the mother tunnel and nearly perpendicular to it. The damage to the vascular tissues of the tree blocks the transport of nutrient and water, leading to the withering of branches and fruit drop. Infestation weakens the trees even more, leading to the decline of the orchard. Alternative hosts are oak, ash, beech, walnut and pine trees.
Parasitoid wasps of the species Cheiropachus quadrum, Raphitelus maculatus or Eurytoma morio could be introduce in the orchards as part of an integrated approach to reduce populations of olive borer.
Always consider an integrated approach with preventive measures together with biological treatments if available. The organophosphate insecticide dimethoate can be sprayed locally when adults are still in the wood. Care should be taken since the treatment can also affect beneficial insects.