Associative learning of color and firmness of oviposition substrates in Drosophila suzukii

Drosophila suzukii Matsumura (Diptera: Drosophilidae), also known as the spotted-wing drosophila, is a global pest fly. The degree of infestation of D. suzukii varies greatly among host plants, possibly depending on the physical properties of the fruits. Because some of the physical properties of fruits can only be judged by touching them, D. suzukii may choose to oviposit based on the physical properties of fruits and remote sensory information such as visual cues. Here, we conducted associative learning experiments with artificial substrates consisting of water, cornmeal, fructose, and agarose with different colors and firmness. We assessed associative learning by conditioning flies with combinations of two colors (purple or green) and degrees of firmness (firm or soft), and comparing their choice between the conditioned color and the other color afterward. We demonstrated that D. suzukii had an associative learning ability but it was limited to the association between green and soft. Drosophila suzukii conditioned with green and soft substrate preferred green as the first choice in the test phase. The proximate cause of this asymmetrical associative learning ability is completely unknown, but the pattern of learning coincides with D. suzukii's preference for immature fruits on the tree in nature, possibly to avoid competition with other fruit flies. The associative learning ability of D. suzukii was found to be limited to a certain combination of color and firmness. It suggests that the asymmetrical association of the natural fruit color and physical properties played a role in the evolution of color-specific associative learning ability of D. suzukii.