The list of invasive insects from Southeast Asia continues to grow. In Catalonia, one of the recent additions is the species with the scientific name Xylotrechus chinensis, known as the mulberry borer wasp beetle. This strikingly colored beetle (yellow and black) could have arrived in Catalonia a decade ago, but its existence has gone largely unnoticed in general, except in the almost twenty municipalities of Vallès Occidental and Oriental, Baix Llobregat and the metropolitan area of Barcelona in which has caused significant damage to the mulberry trees in streets and gardens (public and private).
This wasp beetle (it is named for its colors but does not bite humans) belongs to the cerambycid family and is native to East Asia (China, Korea, Japan, and Taiwan). The insect could have been accidentally introduced into Catalonia in 2012, probably inside wooden pallets from China.
The damage is caused by the larvae of this insect that bore (borer) and feed on the living external part of the trunks and branches (the phloem) of the tree, in such a way that they interrupt the flow of sap, and can reach cause the death of the trees.
The most obvious symptoms of infestation are the exit holes of the adults (the beetles) which can be seen on the trunks and branches of mulberry trees.
The infested bark of the tree also eventually dies, and then dries up and flakes off, exposing the hard part, or xylem.
Due to its boring habits, it is a pest that is difficult to control. The plant protection services of the Generalitat indicate that the recommended control measures are the following:
- Physical control: involves the severe pruning of the affected branches, and, where appropriate, the felling and destruction of the affected trees.
- Phytosanitary treatments: only those products that are authorized by the Registry of Phytosanitary Products of the Ministry of Agriculture, Fisheries and Food can be used,
-Preventive treatment: it is recommended to spray with authorized pyrethroids on the trunk and main branches to prevent females from laying eggs.
- Phytosanitary treatments by endotherapy (injection into the trunk), to fight against the larvae that make galleries in the phloem. It is recommended to apply a treatment, although there is still no certainty that these treatments are completely efficient.
- Use of natural enemies (biological control): in the research phase. The native wasp Stephanus serrator could be a probable parasitoid of Xylotrechus chinensis larvae.
Víctor Sarto, an entomologist specializing in plant pests and a researcher at the Institute of Environmental Science and Technology of the Autonomous University of Barcelona, recalls that at the end of June 2014, a gardener who was in charge of a private garden located in Cerdanyola del Vallès (Barcelona) , "observed round holes 5-6 mm in diameter in the trunk of a mulberry tree and managed to photograph a wasp-like beetle as a possible culprit, as it walked and flew over and around it."
The photograph was sent to the Plant Health Service of the Catalan Generalitat, and finally reached Víctor Sarto. Although the image had low resolution and was somewhat blurry, Sarto concluded that it was "a cerambycid from the Clytini tribe, which includes species of beetles that mimic wasps, both in their black and yellow colors and in their movements."
"Neighbors of the nearby town of Barberà del Vallès assured that they had already seen it on their mulberry trees in June/July 2013. This led to the conclusion that the colonization and establishment of this beetle in the Catalan region of Vallès Occidental must have occurred during the year 2012 or earlier, calculates Víctor Sarto.
In the last two years, the plague has spread to several municipalities in the Vallès Oriental, with damage that in some cases has forced the respective councils to cut down the affected mulberry trees and replace them with other trees.
