Plants and insects have a complicated relationship. On the one hand, herbivores, including leaf-munching caterpillars (Lepidoptera) and sap-sucking bugs such as treehoppers (Membracidae), injure and sometimes kill the plants they snack on. On the other hand, plants use scents and colors to attract pollinators that can help make new plants, as well as parasitic insects that can infect and kill harmful herbivores. To make things even more complicated, a single plant must often deal with many harmful and beneficial insects at the same time, and compartmentalizing these relationships is all but impossible. To explore how plants navigate these stressful social situations, Riley Anderson, Andrew Hennessy, Kiran Kowalski and Michael Singer of Wesleyan University, USA, teamed up with André Kessler of Cornell University, USA, and Robert Bagchi of the University of Connecticut, USA, to investigate how combined attacks from caterpillars and treehoppers affected a white oak tree's (Quercus alba) capacity to recruit parasitic insects to its defense.
The scientists traveled to four forested sites in central Connecticut and looked for sap-sucking bugs on the branches of white oak trees. A suitable branch generally had one to three treehoppers drinking its sap and no caterpillars eating its leaves. The researchers then removed the treehoppers from half of the branches and added two to four caterpillars from the local caterpillar population to all the branches to see if any parasites would come to infect and kill the leaf-eaters. About a week later, the scientists returned to collect each caterpillar and bring it back to the lab, where they monitored whether or not a parasite burst out of its body. They found parasites in 19.3% of the caterpillars from the debugged branches, but none in the caterpillars sharing real estate with treehoppers, suggesting that white oak trees struggle to recruit parasitic defenders to rid them of pestilent caterpillars when bugs are draining them dry.
The researchers saw two possible explanations for their observations. The treehoppers may simply have interfered with the ability of the trees to make scents that attract parasites. Alternatively, knowing that the treehoppers attract friendly ants to fend off harmful insects with the goo they exude as they drink sugary sap, the scientists wondered whether the treehoppers’ ant bodyguards had killed off the parasites attracted to the trees’ scents. However, when they tested this theory, the same percentage of caterpillars was parasitized regardless of whether ants were around; but they were never parasitized when the branch was infested with treehoppers. The ant bodyguard theory could not be true.
The scientists then returned to the first idea: that the sap-sucking treehopper bugs kept trees from making parasite-attracting odors. Once again, the researchers found white oak tree branches with treehoppers, removed the sap-suckers from half the branches and added caterpillars to all the branches to see if the trees started making odors in response to caterpillar bites. Then they returned 5 days later, using airtight plastic bags to collect odors from the branches, rather than collecting the caterpillars themselves. When they tested the gases in each bag, they confirmed that branches infested with treehoppers were not producing the odors that the other branches were using to attract parasites.
Sap-sucking bugs had an outsized impact on the white oak trees from which they drank, leaving the plants unable to attract parasites using odors for protection from leaf-munching caterpillars. Farmers and foresters moving away from pesticides towards a more holistic approach to pest management may need to look out for key insects such as planthoppers that could keep a plant from defending itself.
