At the end of the 1990s, damage produced in the bark of cork-oaks was detected. It affected, with variable intensity, 30% of the trees. Previously, at the beginning of the 1980s, damage had already been observed during the extraction of bark. It was produced by an unidentified ant species. In a cork-oak grove of the Guilleries, in Sant Hilari Sacalm, during the spring 2001, while carrying out a particular cork management procedure ("ratllat del suro"), that involves making a couple of long vertical slits, considerable damage was observed in the cork. This prompted some attempts at control, based on: a) elimination of the older, non exploited cork ("bornizo") and, b) immediate application of insecticide. No satisfactory results were obtained.

A broad description of the problem was presented in a leaflet (Espadaler and Rojo, 2002) , published through Forest Ownership Centre with the aim of involving landowners in the detection of the presence of this new pest on their properties. As a result, three further sites (Agullana, La Vajol and Santa Coloma de Farners) were discovered.

SYMPTOMS AND DAMAGE

Although in European forests Lasius brunneus usually nests in trees, it had never before been related to damage in cork-oak. In the field of forestry management, people working in cork extraction are well acquainted with the acrobat ant Crematogaster scutellaris, although this species has never been considered a cork-exploitation problem. Nevertheless, Lasius brunneus has been considered a domestic pest in the United Kingdom, entering houses in search of food (Green and Kane, 1958; Kane and Tyler, 1958).
In the case of cork-oak, the bark is directly damaged by the construction of nest galleries. These cavities generate difficulties during the cork extraction process because of adhered portions of cork, of a somewhat rounded shape, remaining. This surface does not come off easily during the peeling out process. They correspond to the space occupied by the ants. After peeling the cork, the stratum generatrix begins to produce cork again and the previously infested area maintains a remarkably distinct aspect and remains as a reliable, visible external marker of a previous infestation by Lasius brunneus in a given tree. However, these patches of old cork do not imply that the ant colony is still present in the tree. Instead, the presence of sawdust iin the vertical slits of bark or in natural cracks, is a proof of present nesting of Lasius brunneus. Sawdust acts as a protection for this light-avoiding ant species. If sawdust is present, workers may be found in the tree as well as in the ground, around the base of the tree.

Ant activity degrades cork, diminishing its market value, besides making the extraction of cork difficult because insufficient or faulty detachment of the bark.
 

Aspect of the cork degraded by the ant.

On summary, at the time of detecting if a tree is infested, two external, physic markers, are useful, in addition of the observed presence during the extraction of bark:

a) Previous Infestación: a tree formerly infested will have several spots, of thin and dense cork, different in aspect from the newly produced cork.

b) Present Infestación: a presently infested tree is detected by the presence of sawdust, usually deposited within the vertical slits.  That sawdust is not to confuse with the excrements of Diplopoda (millipeds), bigger in size and of more defined shape.

The intensity of the attack of Lasius brunneus is variable, with values that oscillate between 12,5% of trees at Agullana to 56,5% at tLa Vajol. This variability not only occurs between localities, but also within a locality. At the four localities considered together, the broad average of present attack is 27.7%.

Studied localities and degree of infestación (present and past; % infested trees ) by Lasius brunneus. For each locality several plots, located in the same or different private properties were surveyed.

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