Leaf surface preference of Pieris rapae larvae on Brassica rapa under different light conditions

The leaf surface preference of first instar Pieris rapae larvae on Brassica rapa was studied under different light conditions. When exposed to continuous overhead fluorescent white light, first instar larvae that hatched from eggs laid on the upper adaxial or lower abaxial leaf surface preferentially moved to or remained on the lower surface. This preference was observed on upright and on inverted leaves. In contrast, when the eggs laid on the adaxial or abaxial leaf surface hatched in darkness, the first instar larvae distributed equally between the two surfaces. When exposed to blue LED lights, the larvae that hatched from eggs laid on the adaxial surface of upright leaves migrated to the abaxial surface. However, under red LED lights, the newly hatched larvae distributed equally between the two surfaces. Taken together, these results suggest that light is the primary cue for directing the movement of first instar larvae, and they can perceive and respond to blue wavelengths. This light avoidance behavior may have evolved due to the protection shaded leaf surfaces provide against predators, parasitoids, and adverse weather.