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 Portugalsince the 1980's, has led to new hypotheses about the insect - hostrelationships. Although its biology has been studied since the beginningof the 20(th) century, it is not sufficiently known to enable aquantitative analysis of P. cylindrus population dynamics. Recentstudies carried out in Portugal have developed new approaches to theunderstanding of host selection and colonisation mechanisms, insectbio-ecology and insect-fungus symbiosis. Host selection by P. cylindrusmainly depends on factors associated not only with the stand and treecharacteristics (weakness, dimensions) but also with exploitationmethods (decorking intensity and period). On the other hand, thecolonization process appears to be very structured on the tree surfacewith a quasi-systematic vertical gradient of density and high rates ofattack on the same tree suggesting the presence of aggregationpheromones. The insect life cycle takes place inside the tree over along period of time and the temporal variability of egg laying resultsin the coexistence in the galleries of P. cylindrus descendants atdifferent stages. of development. Space management strategies can eresponsible for this behaviour. Because of this out of phase life cycle,the emergence period is long (from spring to autumn), and may extend toa second generation during the spring of the following year. The newobserved symbiotic associations involving P. cylindrus can be partiallyunderstood as an evolution, suggesting that more favourable conditionshave been developed. Recent results confirm that P. cylindrus isassociated with a complex of six Ambrosia fungi. The presence ofpathogenic fungi in the insect galleries suggests that this symbiosiscan contribute to the weakness of the host providing the idealconditions for the insect establishment.