Response of Apple Maggot Flies1 to Rectangles of Different Colors and Shades

In experiments conducted in a large, white laboratory flight chamber, significantly more adult Rhagoletis pomonella (Walsh) arrived on 15×20-cm rectangles painted Lemon Yellow enamel than on similar rectangles painted light green, blue, light orange, red, white, 6 shades of gray, or black enamel, or covered with aluminum foil or constructed of clear Plexiglas®. When interpreted in the light of the respective spectral reflectance curves, these data are clear evidence that (1) the strong response of the flies to yellow is primarily one to the hue of yellow and not primarily one to the intensity of light reflected from yellow, (2) the response to yellow is one of positive attraction and is not merely the result of repulsion by other hues or shades, and (3) radiation in the ultraviolet portion of the spectrum is not attractive. In experiments conducted in apple trees, significantly more apple maggot flies were captured on sticky-coated 15×20-cm rectangles painted daylight fluorescent Saturn Yellow than on similar rectangles painted daylight fluorescent Lightning Yellow, Arc Yellow, Signal Green, or Fire Orange, or painted Lemon Yellow enamel or covered with aluminum foil. It is proposed that yellow color, particularly Saturn Yellow, is so attractive to the flies because it constitutes a “supernormal” foliage-type stimulus eliciting food-seeking and/or host plant-seeking behavior. When compared with stickycoated 7.5-cm-diam red spheres, sticky-coated 15×20-cm rectangles of Saturn Yellow captured a comparable number of apple maggot flies during the 1st 2 weeks of the fly season but fewer and fewer flies in comparison with the spheres as the season progressed. However, under circumstances where the positioning of the devices in the trees might be less than optimum, a medium or large surface of Saturn Yellow combined with a simple ammonia-type compound could prove more effective than a red sphere as a monitoring device for the flies.