New aspects of Platypus cylindrus Fab. (Coleoptera : Platypodidae) life history on cork oak stands in Portugal

Platypus cylindrus is an Ambrosia beetle known to attack mainly dead andweakened trees. However, the new outbreak in cork oak stands in Portugal
since the 1980's, has led to new hypotheses about the insect - host
relationships. Although its biology has been studied since the beginning
of the 20(th) century, it is not sufficiently known to enable a
quantitative analysis of P. cylindrus population dynamics. Recent
studies carried out in Portugal have developed new approaches to the
understanding of host selection and colonisation mechanisms, insect
bio-ecology and insect-fungus symbiosis. Host selection by P. cylindrus
mainly depends on factors associated not only with the stand and tree
characteristics (weakness, dimensions) but also with exploitation
methods (decorking intensity and period). On the other hand, the
colonization process appears to be very structured on the tree surface
with a quasi-systematic vertical gradient of density and high rates of
attack on the same tree suggesting the presence of aggregation
pheromones. The insect life cycle takes place inside the tree over a
long period of time and the temporal variability of egg laying results
in the coexistence in the galleries of P. cylindrus descendants at
different stages. of development. Space management strategies can e
responsible for this behaviour. Because of this out of phase life cycle,
the emergence period is long (from spring to autumn), and may extend to
a second generation during the spring of the following year. The new
observed symbiotic associations involving P. cylindrus can be partially
understood as an evolution, suggesting that more favourable conditions
have been developed. Recent results confirm that P. cylindrus is
associated with a complex of six Ambrosia fungi. The presence of
pathogenic fungi in the insect galleries suggests that this symbiosis
can contribute to the weakness of the host providing the ideal
conditions for the insect establishment.