Estimating population dynamics parameters of cabbage pests in temperate mixed apple tree-cabbage plots compared to control vegetable plots

Abstract

Mixed fruit tree-vegetable systems, a particular type of agroforestry, simultaneously combine the production of vegetables at the ground level with that of fruits in tree canopies. Crop diversification generally promotes pest natural enemies but its impact on pest abundance is more ambivalent, possibly because the presence of trees also directly impacts pest reproduction, survival and movements. The purpose of this study was thus to estimate population dynamics parameters of vegetable pests (predation, immigration and survival) and compare them between mixed fruit tree-vegetable plots and vegetable control plots. For this purpose, seven pest stages were monitored in an experiment with exclusion cages set up in mixed fruit tree-vegetable plots, combining apple and cabbage, and two control plots grown with cabbage only, over one growing season. The data were then fed into a mechanistic-statistical model to estimate these parameters. Most significant results concerned the aphid Myzus persicae that had higher intrinsic growth rate, less immigration and suffered less predation in mixed fruit tree-vegetable plots. Egg laying also appeared larger for Lepidoptera species in these plots but egg survival was lower. Predation of caterpillars was also lower. Lastly predation of the aphid Brevicoryne brassicae, in contrast to M. persicae, was higher in mixed fruit tree-vegetable plots. Either no difference was noted between plot types for other parameters or the pest biology made it difficult to relate the model parameters to population dynamics process. We thus provide evidence that reproduction or survival of some pests may increase in mixed fruit tree-vegetable plots and their predation level may be similar or lower than in control plots which could explain the diversity of results concerning the effects of agroforestry on pest control. We also found no strong evidence that trees were a barrier to immigration in our simple mixed fruit tree-vegetable plots.