Wherever the stink bug Bagrada hilaris invades, it does not go well for the local cauliflower or broccoli. The pest, native to Africa and Asia, feasts on seedlings of those plants and others in the group known as crucifers, causing extensive crop damage. Now, a study reveals that a previously unknown compound called brassicadiene attracts the bugs, a discovery that could help farmers make their crops less vulnerable (Org. Lett. 2020, DOI: 10.1021/acs.orglett.0c00707).
Stefano Colazza and colleagues at the University of Palermo have been studying the bug since 2005, when it began devastating caper bushes on a tiny island between Africa and their home island of Sicily. They suspected that volatile organic compounds emitted by crucifer seedlings were attracting the pests. So Colazza contacted a friend he’d done a sabbatical with 20 years ago, Jocelyn G. Millar at the University of California, Riverside, to help with structure identification. They published a study in 2018 concluding that the attractant was a diterpene hydrocarbon, but they couldn’t identify the compound (PLoS ONE 2018, DOI: 10.1371/journal.pone.0209870).
“The first time around we did not have enough sample to run the 2-D NMR [nuclear magnetic resonance] experiments that were required to work out the structural details” of the compound that the bugs were flocking to, Millar says. So the team in Italy collected volatile organic compounds from thousands of seedlings, a larger sample than in 2018. That yielded enough material to identify the diterpene as brassicadiene. Coauthor Stefan Schulz’s group at Technical University of Braunschweig did calculations to predict which enantiomer of brassicadiene is biologically active.
Depending on the insect and plant species, as well as concentration, diterpenes can be repellent or toxic or both, a mechanism for plants to defend themselves from being eaten by insects. So what to make of the fact that brassicadiene is irresistible to the invasive bug? Phyllis Coley, an expert in plant defenses at the University of Utah, says this isn’t surprising. Most plants protect themselves with a chemical arsenal, she says, so insects specialize and eat plants for which they have evolved detoxification mechanisms. “And to find the correct host plant in a sea of other species that would be toxic to them, they have frequently evolved the ability to detect olfactory signals, like brassicadiene, that are specific to their preferred host plants,” she explains. “It would be interesting to know what the stink bug feeds on in its native habitat, and if those plant species also emit brassicadiene,” she adds.
Brassicadiene’s structure is likely too complex for it to be cost-effectively chemically synthesized in the multikilogram amounts needed to serve as bait for traps, Millar says, but because some plant species related to cauliflower and broccoli lack the compound, “breeding brassicadiene out of crops would seem to be a much better option.”