Abstract
When different size sticky-coated red spheres were positioned within the foliage canopy of sour cherry or apple trees, more Rhagoletis pomonella (Walsh) flies were always captured on 7.5-cm-diam. ones than on 3.4 or 1.5 cm ones. Early in the season, 7.5 cm spheres attracted fewer or about as many R. pomonella as 23 cm spheres, but later on, 7.5 cm spheres always attracted more. R. fausta (Osten Sacken) and R. cingulata (Loew) flies were always more attracted to 7.5 cm spheres than to any other size tested. However, both these species showed a proportionately greater attraction to smaller spheres than did R. pomonella. The phenomena of super-normal stimuli and learning are proposed as possible explanations of certain of these findings.

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