Cardboard traps painted 48 chromatic or achromatic hues were used to study the attractiveness of colors toDelia antiqua(Meigen) within an onion field. White, the standard chosen for these studies, was always among the most attractive colors tested. With respect to hue, violet and blue were as attractive as white, and significantly more attractive than saturated hues with peak wavelengths between 480 and 700 nm. Yellow was not an attractive hue. Any saturated or unsaturated color that reflected wavelengths between 350 and 480 nm at an intensity greater than or equal to that of violet was as attractive as white. It is suggested that attractiveness of colors toD. antiquain the field is determined by color hue or by saturation, and that the magnitude of a response elicited by hue or saturation is determined by the intensity of attractive key wavelengths. Brightness itself is not a determinant of attraction. White was determined to be the best color for use in a monitoring program forD. antiqua.