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 documented how variable plant quality differentially influences the survival and fecundity of insect individuals. Whether or not such effects can be translated to the level of insect populations is, however. not clear. In order to test this hypothesis one needs to combine processes at both the level of the individual and the population This is difficult with an empirical approach, but could be achieved by means of modeling given that appropriate data exist for both levels of organization. In this paper Ne report on a model developed to analyze whether altered Scots pine (Pinus sylvestris) quality can contribute to the build-up of populations of the European pine sawfly (Neodiprion sertifer). Experimental data on responses of sawfly larvae to variable plant quality, i.e. needle concentrations of resin acids, were used to parameterize the model. Larval survival and sawfly fecundity are reduced at high resin acid concentrations. However, high resin acid concentrations are, at the same time, beneficial because larval defense against predators is enhanced. in the model, data on individual responses were combined with Literature data at the population level, a type III functional response related to cocoon predation was presumed to be the density-dependent process regulating sawfly populations. The analysis showed that the risk for an outbreak is high when needle resin acid concentration (,) or larval predation pressure (p) is low. When r or p is high there is no risk. By analyzing different scenarios it was found that small changes in r and p can result in the sawfly population moving from low to high outbreak risk. Changes of the same, or larger, magnitude in r have been observed in empirical studies. The role 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 defense. We conclude that small changes in a density-independent factor, such as needle chemistry, can have significant effects on herbivore population dynamics because increased fecundity and survival caused by needle quality may allow the population to escape the control of density-dependent factors, such as cocoon predation.