Great tits can reduce caterpillar damage in apple orchards.
The potential contribution of vertebrate predators to biological control in orchards has been largely overlooked to date. A few studies have shown that birds reduce numbers of pests, but data are scarce on the effects on the pattern or timing of damage. Consequently, the practical value of birds as biological control agents remains unclear. This study considered whether great tits Parus major can reduce caterpillar numbers and fruit damage by caterpillars, and increase biological yield, in an experimental orchard of apple trees (Malus domestica [M. pumila]) with high caterpillar numbers in the Netherlands, during 2000. The outcome would depend on the coincidence of the period during which great tits forage and the period during which caterpillars cause damage. In the first experiment nets were put over trees at different times of the growing season, thus creating different periods during which great tits had access to the trees. In the second experiment caterpillars were removed from trees at different times in the growing season. In both experiments, the resultant caterpillar damage to apples was assessed in the autumn. The longer the period of foraging by great tits, from the start of egg incubation until fledging of young, the less the overall pest damage to fruit. Damage caused by caterpillars was greater the later they were removed, from the young apple stage onwards. The effect of great tits on caterpillar damage to apples was small (percentage damage was reduced from 13.8% to 11.2%) but significant (P<0.05), and the yield of fruit increased significantly (from 4.7 to 7.8 kg apples per tree, P<0.05). The only cost to the producer was that of erecting nest boxes (approximately 2 ha-1) to encourage great tits to breed in the orchard. Depending on the great tits' numeric response to insect densities, their relative impact may be greater at lower densities more typical of commercial orchards and, if so, the presence of breeding great tits may allow economic thresholds for other controls to be reduced. Furthermore, the contribution of natural predators to biological control of insect pests may be especially useful in orchards and in the future when a further reduction of pesticide use may be enforced.