Stink Bug Saliva: A Potent Mix to Bypass Plant Defenses
By Ed Ricciuti
Not long after insects appeared about 400 million years ago, some of them began eating plants, which in turn developed defenses against becoming food, triggering an evolutionary arms race. It’s a course without a finish line as, over time, plants evolve measures to protect against predation and insects come up with countermeasures that enable them to maintain their food supply. And so the cycle goes.
Morphology, such as the shape of insect mouthparts and plant spines, plays a role, but the main weapons in the conflict between insect and plant are biochemical, complex, and far from understood by science. One step toward understanding the complexities of chemical warfare in the insect-plant world is described in a new report about research on voracious, plant-eating stink bugs (Pentatomidae), published in July in the Annals of the Entomological Society of America.
In the study, researchers at the United States Department of Agriculture (USDA) and Washington State University (WSU) identified an astounding 677 proteins from the salivary glands of only five species of stink bugs that could be involved in suppression of plant defenses against predation. Past research on other insects suggests many of these proteins could suppress plant alarm systems that trigger defenses and could even deactivate the chemical molecules that make those defenses work. Plants defend themselves with weapons such as chemicals that kill or trap attackers, attract as allies natural enemies of herbivores, and changing insect behavior, and insects like stink bugs have intricate strategies in opposition.
It may sound esoteric but the effort to understand the chemistry involved in the give-and-take between insect and plant has exceedingly practical implications. Why focus on stink bugs, though?
“Stink bugs encompass many species which are major pests of agricultural commodities including soybean, cotton, wheat, and tree fruits,” says Adrian T. Marshall, Ph.D., lead author of the study and a postdoctoral research associate at the USDA Agricultural Research Service and previously at WSU.
The research was conducted by analyzing ribonucleic acid (RNA) of stink bug salivary glands for the coded instructions necessary to manufacture various proteins. RNA not only carries the code for making proteins but also the amino acids that are the building blocks of these all-important, highly complex molecules.
Despite identifying several hundred proteins, untangling what they all do is tall task yet to be cleared. “We want to make clear that we can only guess their role based on previous literature,” Marshall says.
The study was sparked by the fact that stink bug damage is difficult to identify for growers and packing warehouses. “Data from the study can help future research start to piece out the individual functions of stink bug saliva in feeding,” Marshall says. “We hope this can be used to build tools for specifically identifying stink bug feeding damage through their excreted salivary enzymes.”
Salivary proteins of many insects have various functions. The ability of some proteins to predigest plant food is well documented in several types of insect, including stink bugs. When feeding, they pierce a plant with tubular mouthparts, called a stylet, and through it inject saliva containing enzymes that liquefy and predigest tissue, which is then sucked in.
During feeding, these insects produce salivary secretions, some of which solidify around the stylet and are believed to aid feeding and suppress plant defenses. “Some interesting trends we saw is that [our analysis] included proteins for different types of stink bug feeding activities,” says Marshall.
Some of the proteins Marshall and colleagues describe seem to aid macerating and slurping in plant tissue. Others fight off microorganisms harmful to insects. Still others disrupt plant alarm systems that signal the presence of alien chemical molecules and activate defenses. Certain proteins, for example, deactivate the calcium ion Ca2+, which triggers the deposition of a compound called callose. It repairs wounds in the phloem when pierced by an herbivore’s stylus, curbing feeding.
The researchers say they hope this foundational work will spur future studies on stink bug biology and management.
“Beginning to examine and understand the ways they feed and interact with plant defenses can open new avenues for developing damage-identification tools and implementing control methods,” Marshall says. “The work can also begin laying the groundwork towards understanding other non-stink bug Hemipteran insect interactions with plants.”
Annals of the Entomological Society of America
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.