X. Bernal, V. i Rojo, M. 2006a.)
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.
broad description of the problem was presented in a leaflet (Espadaler
and Rojo, 2002)
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.
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
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
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
% present Infestación (range)
% last Infestación (range)
Trees (studied plots)
Altitude (m) (range)
Agullana (Alt Empordà)
La Vajol (Alt Empordà)
Sant Hilari (La Selva)
Santa Coloma (La Selva)
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.