It is winter here in Montana, and after a frigid walk with my dog Beau, I am grateful for a warm cup of chamomile tea to warm my insides. Wisps of fragrant vapor curl off the liquid surface; a woody, citrus scent tickles my nose.

The familiar aroma of my favorite chamomile tea is the result of many different compounds, but one in particular: (E)-β-farnesene (EBF). While the name might conjure up images of a high school chemical storage room, it is a natural compound produced by many plants including hops, ginger, and peppermint.

Our associations with EBF may be comforting and cozy, but it is anything but for a little insect called an aphid. Aphids, too, produce EBF…when they die. As their little insect bodies expire, EBF disperses into the air, sending a distress signal to other aphids: “Flee! Disperse! It is not safe here!”. The presence of EBF warns other aphids of predators and inhospitable environments that might endanger their well-being. Once an aphid senses EMF in an area, they steer clear.

Novel as it seems, chemicals as messengers like EBF are so ubiquitous in nature, scientists have given them a name: semiochemicals. These chemicals convey information from one organism to another. Plants, too, produce semiochemicals, many of which play important roles in plant defense. Though often undiscernible to our nose, plants are constantly engaged in chemical warfare against herbivores. Plants of the mustard family—think broccoli, kale, and radish—produce a special class of compounds called glucosinolates. These semiochemicals are toxic to many herbivores and serve as repellents to insects. Another example comes from the forest: pinene, a semiochemical produced by pine trees, deters beetles from nesting beneath their bark. Pinene is what gives forests their characteristic fresh pine scent.

Given the communicative nature of semiochemicals, they have long been recognized as potential tools for pest management in agriculture. Some chemicals—such as EBF—have the potential to serve as insect repellents, minimizing pest damage to crops. But how to produce and release such a chemical in an agricultural setting? Why not engineer the plants to do it themselves?! That is exactly what scientist Toby Bruce and his collogues attempted to do at Enland’s Rothamsted Research Center in 2015. The scientists created a strain of wheat that repels pest insects by producing insect-deterring semiochemicals.

Both the cereal you ate for breakfast and the bread you enjoyed as part of your sandwich for lunch are made from wheat. In 2016, the world production of wheat was 749 million tons, making it the second-most produced cereal after corn. Human’s love of wheat cannot be understated, and we are not the only ones—wheat is equally enjoyed by an array of insect critters that feed upon the growing plant. Aphids, sawflies, and worms all enjoy feasting upon succulent, sugary green wheat grass.

Unfortunately, if aphids eat our grain, we cannot. Since the creation of DTT in 1939, farmers have managed pests with an increasingly complex cocktail of insecticides and chemicals. The ubiquitous use of such pesticides has resulted in unforeseen environmental catastrophes. The ecological reality of pesticide use has pushed scientists and farmers to work towards more sustainable forms of pest control.

In an attempt to leverage naturally occurring semiochemicals as pesticides, Bruce and his lab genetically engineered wheat plants with the ability to synthesize chemicals that deter aphid pests. They knew that EBF (the compound in chamomile) is a natural aphid semiochemical that causes aphids to flee. Wheat, however, produces very low levels of EBF. For this reason, they turned to a plant known to produce a lot of natural EBF: peppermint.

Bruce and his collogues isolated the EBF gene from peppermint and inserted it into a common variety of wheat. The transformed wheat produced large bouquets of EBF, which tricked the aphids into responding the way they normally do: “Flee!” and “Disperse!”. Through this aphid-death mimicry, the transformed wheat plants were visited less by aphids in comparison to wheat plants without the EBF gene.

Despite the positive results in the lab, this clever technique did not translate to the field. After planting transformed wheat (with the EBF gene) in the field, the scientists found no difference in aphid attraction; aphids spent just as much time on the transformed wheat as they did on the normal wheat. Why the difference?

The short answer is—we do not know! It might be the rate of release; EBF released by dying aphids likely occurs in short bursts. EBF-transformed wheat constantly releases high amounts of EBF. It is possible that aphids were able to sense this difference in the EBF production. More research is needed to better understand the factors influencing EBF production in transformed wheat and the subtleties of how EBF influences aphid behavior. The better we understand the complex nature of these semiochemicals, the better we will be able to mimic them in the lab and the field.

However, understanding the ramifications of engineered semiochemicals the ecosystem might be even more complex. EBF repels aphids, but it also attracts natural predators to the aphid. Increasing the amount of EBF produced by wheat might increase its attraction to other, unintended insects. What other chemical conversations might (E)-β-farnesne influence and what are the ecological consequences of altering these conversations?

I hope these questions lead you back to the inevitable connectedness of nature. All living things—us included—are part of an intricate network that is often controlled by chemicals invisible to the human eye. Despite their hidden nature, these chemicals can have strong impacts on behavior and interactions between species.  So next time you drink your tea or enjoy a big bowl of pasta, keep in mind the many ways in which chemicals are controlling our world.

Reference

1.     Bruce, T.J.A. et al. The first crop plant genetically engineered to release an insect pheromone for defence. Sci. Rep. 5, 11183; doi: 10.1038/srep11183 (2015).