Prey density and rates of predation by tits (Parus spp.) on larvae of codling moth (Cydia pomonella) under bark.
Abstract
In a cider-apple orchard near Bristol, England, in late January 1972 (Experiment 1) and December 1972 (Experiment 2), fully grown larvae of Cydia pomonella (L.) were allowed to select cocooning sites under the bark of apple logs about 2 m long at a density of 1-30 larvae/log. The logs were then tied into large trees. Within the first 18 or 19 days, blue tits (Parus caeruleus), and sometimes also great tits (P. major), reduced the larvae on logs with 4 or more larvae initially to a mean number of 1.8/log, this being spatially density-dependent mortality. Tits destroyed the remaining larvae at intervals through winter and spring. In Experiment 2 the mortality appeared density-dependent through time, a smaller percentage of the larvae present being taken in successive periods as numbers declined. This was not established for Experiment 1, but the percentage mortality declined along with the numbers of tits seen frequenting the orchard. In an aviary, blue and great tits initially explored a log with C. pomonella larvae under the bark and, on finding and eating a larvae, searched intensively, usually beginning near the site of the catch (area-restricted searching), and continued intensive searching until they found no more. This mode of hunting was probably responsible for the rapid early removal of larvae from logs in the orchard. Exploratory (i.e. less intensive) searching was seen in the orchard as well as in the aviary; repetition of this, when larvae had become too sparse for area-restricted searching to succeed, could account for the slow loss of larvae through the winter. The observations suggest that predation by birds may regulate the population of C. pomonella through successive years in the unsprayed orchard investigated.