Effect of apple-orchard structure on interception of Rhagoletis pomonella (Diptera : Tephritidae) flies by odor-baited traps

We released marked mature male flies of the apple maggot, Rhagoletis pomone lla (Walsh), inside and outside of square blocks of apple trees, Molus dome stica Borkh. (Rosaceae), to test the effect of orchard structure on the int erception of flies by odor-baited red-sphere traps. The blocks were compose d of large trees planted at low density, medium-sized trees planted at medi um density, or small trees planted at high density. The experiment was carr ied out in six commercial apple orchards during summer in 1997 and 1998. Re leased flies moving into blocks were intercepted in large proportions by ba ited perimeter traps, thus preventing fly penetration into sphere-protected blocks. In 1997, proportions of intercepted flies were higher for traps in blocks of small and medium-sized trees than for traps in blocks of large t rees. In 1998, a year with an unusually low fruit load, interception remain ed high, but there were no differences in proportions of flies intercepted by perimeter traps among blocks of trees of different sizes. Overall we con clude that the tendency of apple growers in Massachusetts to replace large trees planted at low density with small or medium-sized trees planted at hi gh or medium density will not adversely affect behavioral control programs for apple maggot flies. In fact, this practice may enhance the effectivenes s of these programs through increased interception of immigrant adults.