In a study at Virginia Commonwealth University and the University of Richmond, researchers found that the harlequin bug (Murgantia histrionica), a pest of cabbage and other cruciferous crops in the southern United States, can adapt its pigmentation based on external environmental conditions—namely, temperature—during its nymphal stages. Such adaptability illustrates the species’ potential to invade new regions and environments. (Photo credit: Jennifer Olson)

By Andrew Porterfield

Most, if not all, insects live in geographic ranges that cross over climatically variable regions, and many areas that have distinct seasons. For an animal that regulates its temperature via its outside environment (i.e., an “ectotherm”), that presents a challenge: how to keep physiologically active when your sole source of temperature regulation keeps changing. This challenge has become especially acute as climate change further alters the variables of an insect’s niche.

Andrew Porterfield

For insects, body temperature affects all physiological functions, and regulating body temperature has proven an evolutionary advantage, allowing adaption to these different environments. One way insects do this is thermal melanism, wherein individuals with darker colors (more melanin) have the advantage in cooler temperatures, heating up faster than lighter-colored individuals (this coloration is, of course, the opposite of the pattern in endotherms, i.e., creatures that generate their own body heat).

But how melanism is encouraged during developmental stages is not entirely known. Understanding how different factors, like changes in temperature and length of daylight, play a part can not only help us understand how thermal melanism works but may also assist in determining the degree of pest infestation in changing climates.

Using the common harlequin bug (Murgantia histrionica), a scourge of cabbage and other cruciferous crops in the southern United States, a research team led by Jennifer Olson of Virginia Commonwealth University and the University of Richmond found that insects can, during development, exhibit a physical plasticity to climate, permanently increasing melanin in individual responses to temperature change. The study, believed to be the first in Harlequin bugs, also found that length of day had less of an infect on this plasticity. The study was published this month in the Journal of Insect Science.

The researchers collected 200-300 individual bugs from collard green plants (Brassica olaraea) in Virginia as the parental generation. Eggs were collected and newly hatched nymphs used in the experiment, in which the nymphs were raised in four chambers:

• Longer days/warm temperatures (15 hours of light, 30 degrees Celsius)
• Longer days/cold temperatures (15 hours of light, 20 degrees C)
• Shorter days/warm temperatures (10 hours of light, 30 degrees C)
• Shorter days/cold temperatures (10 hours of light, 20 degrees C)

Pigments were analyzed for their responses to these conditions in 457 adults. Overall, adult melanism was most strongly influenced by temperatures during rearing. The largest effects were from cold chambers, where individuals had a 33 percent larger ratio of black to yellow coloration compared to those from warm chambers. Day length did interact with temperature, so that short-day, cold-weather adults had four percent more melanization than long-day, cold-weather adults; long-day, warm-chamber adults were seven percent darker. Warm-chamber insects were larger than cold-chamber insects. Developmental time of day had no effect on body size or the interaction effects (the four and seven percent changes).

“Our results suggest that the temperature this species is exposed to during their juvenile development influences how much melanin will develop in their adult cuticles,” Olson says. “Nymphs that are exposed to consistently colder temperatures will have a darker pigmentation as adults.”

Researchers at Virginia Commonwealth University and the University of Richmond conducted a lab experiment with harlequin bugs (Murgantia histrionica) to examine how temperature during their nymphal stages influenced pigmentation. They found colder temperatures led to darker pigmentation, an adaptation that would allow the insects to better maintain body heat in cooler climes. (Photo credit: Lily Thompson)

In a study at Virginia Commonwealth University and the University of Richmond, researchers examined how harlequin bugs (Murgantia histrionica), a pest of cabbage and other cruciferous crops in the southern United States, can adapt their pigmentation based on external environmental conditions. Nymphal-stage bugs were fed collard green leaves and kept in chambers with varying levels of light and temperature through the course of the experiment. (Photo credit: University of Richmond)

Once a nymph has developed into an adult, this coloration can’t change, regardless of ambient temperatures. One unanswered question, Olson notes, is “when, exactly is that level of melanization determined? Is it only the temperature they are exposed to at a certain point during their development that determines their adult pigmentation, or is the environment of all instars equally important?”

The genetics of melanization in insects is well known. A certain group of melanin compounds is the source of brown and black pigments, while another group is the source of red and orange pigments. Several genes in insects play roles in melanization and shell hardening, which occur at the same time during molting. But what’s not as well-known is how enzymes and other proteins are regulated to produce certain color shades or patterns, including the often brilliant patterns in seen in Harlequin bugs.

Knowing how this plasticity works in response to changing climates may help determine the possible ranges of these common pests, even as environmental conditions change in farm fields and elsewhere. “Phenotypic plasticity, particularly in passive thermal regulation, is a key trait of successful pest species, and can impact the duration and extend of damage to crops, and seasonal limits on activity,” the authors write.

Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.