Matching Colors

Female stink bugs lay eggs of different colors on different surfaces.

A female stink bug (P. maculiventris) and, below, the different colored eggs it lays, depending on the surface color.

Leslie Abram

The common North American stink bug, Podisus maculiventris—so named, and eschewed, because of the pungent odor it discharges when threatened—has a less maligned attribute. In the first known example in any animal, stink bug females have the ability to modify from one moment to the next the color of the eggs they are laying. They select lighter, yellow hues when laying their clutches on the shaded undersides of leaves, or opt for dark brown pigments when depositing eggs on sunlit leaf tops. 

Paul K. Abram, a Ph.D. candidate in entomology at the University of Montreal, made this discovery quite by chance. He was studying the beneficial use of parasitic wasps—which lay their eggs in stink bug eggs—as a way to control stink bug populations that destroy commercial crops. He was surprised to find some stink bugs under study had laid dusky-colored eggs on the black squares and pale-colored eggs on the white squares of a crossword puzzle lining their cage.

An experiment with soybean plants confirmed the preference for darker eggs on leaf tops and lighter eggs underneath. Abram and colleagues then showed that the pigment in the sun-exposed eggs on leaf tops worked as an effective sunscreen against damaging ultraviolet radiation. But why not opt for under-leaf nurseries, which also shelter against wind, rain, overheating, and desiccation? The color option may have evolved as a hedge against risk. Insect predators, such as ants and ladybugs, more commonly hunt on leaf undersides than on tops. The production of egg-coloring pigment, however, is probably costly for stink bug mothers. “We think that stink bugs may choose a laying site,” Abram said, “depending on the real-time balance between predation risk and pigment limitation.” 

How stink bugs control egg color isn’t clear. “A change in lighting conditions during laying can change resulting egg color within minutes,” according to Abram. Stranger still, the egg pigment is not melanin but appears to have similar properties. Future research will focus on “identifying the chemical composition and structure of this potentially novel pigment.” (Current Biology)