Abstract
Most orchards in the Netherlands are run under a regime of integrated pest management (IPM) and only few are Organically Farmed (OF) orchards. Control measures both in Organic as in IPM orchards are only taken if numbers of harmful insects exceed thresholds of economic damage and thus the objective is
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to assess whether great tits can reduce damage and thereby allow higher threshold numbers for control measures in the presence of great tits.
Whether great tits contribute to biological control of caterpillars and thereby reduce damage by caterpillars in apple orchards and under which circumstances damage reduction can be expected is investigated with three complementary approaches i) experimental work in the experimental orchard "de Schuilenburg", ii) monitoring in regular Integrated Pest Management and Organically Farmed orchards and iii) theoretical modelling.
In chapter 2 I show that in the experimental orchard in plots with high caterpillar densities great tits can reduce damage. However, if great tits are to serve as natural control agents they also have to be effective under regular farming regimes of IPM or OF orchards. Chapter 4 shows a similar effect for commercial regimes. In several IPM and OF orchards caterpillar densities and damage were assessed in areas with and without breeding pairs of great tits to answer this part of the question. However, these observations cannot explain the variation in reduction of caterpillars damage by great tits between orchards and years. Therefore the field data were compared with a theoretical model that was developed.
Ecological theories on foraging behaviour gave us tools to make the model to predict the foraging behaviour of great tits in orchards. How do great tit forage for different densities of caterpillars and thereby reduce the number of caterpillars and damage in the orchard. The trade-off between travel- and search time is the most important factor to determine the place where birds search for food. We carried out an experiment to determine the missing relation between search time and caterpillar density in a semi-natural environment (chapter 3). When density decreases due to exploitation by other predators, encounter rate decreases much more than expected from the decrease in density alone. This effect of depletion was stronger at higher current prey densities.
The model predicted that the number of breeding pairs and the hatching date of the chicks mainly influenced the amount of damage reduction. I investigated how these same factors influenced the total number of caterpillars removed from apple orchards (chapter 5). On average 3 pairs of breeding great tits occur per ha in Integrated Pest management and Organic Farming orchards, and the total number of caterpillars removed was 23% of the caterpillars present in the orchard and could run up to 49% when all foraging takes place inside the orchard.
The findings in this study imply that the caterpillar densities at which pesticides are applied can be raised when great tits are breeding in orchards. Field data and the model show that for equal numbers of caterpillars the resulting damage in the presence of great tits is lower and thereby for a fixed damage threshold more caterpillars can be allowed.
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