Spectral sensitivity and color discrimination of Euxesta eluta and Chaetopsis massyla (Diptera: Ulidiidae)

Cornsilk flies, Euxesta eluta and Chaetopsis massyla (Diptera: Ulidiidae), are serious economic pests affecting sweet corn production in Florida. As a basis for development of enhanced trapping strategies, the sensory and behavioral basis for response to color was examined. Using electroretinograms, spectral sensitivity curves for both species revealed broad curves with a peak in the UV (350 nm) and the green region (500-550 nm) of the spectrum. Curves for males and females of each species were relatively similar in shape. Using pigment templates, the measured curve for E. eluta was matched by pigment combinations with maximum sensitivity at 350, 430, 500 and 560 nm in a ratio of 25:21:25:29. Similarly, the curve for C. massyla was best matched by pigments with maximum sensitivity at 350, 430, 500 and 560 nm in a 28:18:27:27 ratio. Laboratory behavioral assays were conducted to evaluate if attraction occurred in response to the color or the brightness of a target. Attraction responses were evaluated in paired tests to blue, green and yellow cards paired with gray cards of matching brightness. Despite relatively similar visual pigments, the two species differed greatly in their behavioral attraction. Euxesta eluta was only attracted to yellow cards at low or high brightness levels, to mid-level bright blue cards and not attracted to green cards at any brightness level. In contrast, C. massyla was highly attracted to yellow and green cards compared to gray cards of the same brightness but avoided blue cards. These differences in response to color and brightness are important for interpretation of surveillance results as well as development visual traps targeted for these species.