Western flower thrips, Frankliniella occidentalis (Pergande), and onion thrips, Thrips tabaci Lindeman, are economic pest insects of head lettuce, Lactuca sativa L, and dehydrator onions, Allium cepa L, in the Imperial Valley, California. Colored sticky traps were evaluated as potential detection and monitoring devices of the two thrip species over a two-month period in 2006. Results showed that blue sticky card traps consistently captured more adult thrips of both species compared to yellow sticky card traps. Numbers of thrips captured on blue sticky traps were positively correlated to numbers captured on yellow sticky traps. Adult thrips were captured in much higher numbers on sticky card traps than recovered from whole plant samples. Significant negative correlations between F. occidentalis numbers on plant samples and colored sticky traps suggest that adults left lettuce plants and were attracted to nearby traps In addition, the numbers of thrips caught on traps relative to estimated plant populations support the hypothesis that mass trapping was significant in the treated area. Spectroradiometric and RGB (red, green, blue) digital image color analysis of the traps in sunlight showed the blue sticky cards reflected considerably more light in the 400–500 nm range (R = 49, G = 187, B = 255) than the yellow sticky cards that reflected more light in the 550–700 nm range (R = 227, G = 234, B = 67). Because blue sticky card traps captured many times more adult thrips than recovered from whole plant samples, blue traps may be more sensitive in detecting early presence of thrips in lettuce and onion fields.