Influence of plant quality on pine sawfly population dynamics

The contribution of plant quality to the population dynamics of herbivorous insects has been an issue of much controversy. Many studies have documente d how variable plant quality differentially influences the survival and fec undity of insect individuals. Whether or not such effects can be translated to the level of insect populations is, however. not clear. In order to tes t this hypothesis one needs to combine processes at both the level of the i ndividual and the population This is difficult with an empirical approach, but could be achieved by means of modeling given that appropriate data exis t for both levels of organization. In this paper Ne report on a model devel oped to analyze whether altered Scots pine (Pinus sylvestris) quality can c ontribute to the build-up of populations of the European pine sawfly (Neodi prion sertifer). Experimental data on responses of sawfly larvae to variabl e plant quality, i.e. needle concentrations of resin acids, were used to pa rameterize the model. Larval survival and sawfly fecundity are reduced at h igh resin acid concentrations. However, high resin acid concentrations are, at the same time, beneficial because larval defense against predators is e nhanced. in the model, data on individual responses were combined with Lite rature data at the population level, a type III functional response related to cocoon predation was presumed to be the density-dependent process regul ating sawfly populations. The analysis showed that the risk for an outbreak is high when needle resin acid concentration (,) or larval predation press ure (p) is low. When r or p is high there is no risk. By analyzing differen t scenarios it was found that small changes in r and p can result in the sa wfly population moving from low to high outbreak risk. Changes of the same, or larger, magnitude in r have been observed in empirical studies. The rol e of tritrophic interactions was also considered. This was done by removing the positive effects of resin acids on larval performance in the model. It was found that the anti-predator defense of N. sertifer makes it prone to outbreak under wider combinations of r and p than an insect without the def ense. We conclude that small changes in a density-independent factor, such as needle chemistry, can have significant effects on herbivore population d ynamics because increased fecundity and survival caused by needle quality m ay allow the population to escape the control of density-dependent factors, such as cocoon predation.