‘Toxic bait’ from Indian pitcher plants lures hungry insects to their doom
December 15, 2023 by IAPPS
Nectar produced on and around the traps is laced with a neurotoxin that may drug ants in addition to drawing them in
Pitcher plants in the genus Nepenthes thrive in places where they shouldn’t. There’s very little nitrogen in the Southeast Asian and Australian soils where they grow—but they do just fine, thanks to a macabre source for this essential nutrient: the dissolved flesh of small animals, mostly insects, that slip into their bulbous traps.
A new study suggests why Nepenthes is so effective at catching its victims: It produces a sweet nectar containing a potent neurotoxin that could make them lose their balance at the pitcher’s edge. The work, published as a preprint on bioRxiv this month, is the first known example of nectar acting both as a lure and a poison.
The finding is intriguing, says University of Bristol researcher Ulrike Bauer, who has studied Nepenthes plants for nearly 2 decades but who wasn’t involved with the research. “The nectar has really been neglected for a long time,” she says, and the idea that it contains compounds that “drug” insects is plausible. Still, she and others would like clearer evidence that the toxin originates from the nectar—and that it really accounts for the unlucky ants’ falls.
Phytochemist Sabulal Baby has been studying carnivorous plants—“the most unique life forms on Earth,” he says—for more than a decade. He and his colleagues at the Jawaharlal Nehru Tropical Botanic Garden and Research Institute had previously discovered that the rims of the Indian pitcher plant Nepenthes khasiana are fluorescent and that newly opened traps emit carbon dioxide—features that attract insects. Because they knew the plants also produce nectar on and around their traps, which acts as a lure, they decided to examine it more closely.
In other plants, such extrafloral nectar isn’t designed to harm insects. The liquid’s high sugar content appeals to ants, whose presence—and aggression—wards off potential herbivores. But when Baby and colleagues teased out the contents of the nectars of N. khasiana and several other pitcher plants growing in their institute’s botanic garden, they found something unexpected. The nectars contained (+)–isoshinanolone—a compound that interferes with the activity of an enzyme called acetylcholinesterase, which prevents the buildup of the neurotransmitter acetylcholine between neurons. Too much acetylcholine in lab animals leads to muscle cramps, weakness, blurry vision, and paralysis.
And indeed when Baby and his colleagues examined ants that had drowned in the pitcher fluid of N. khasiana, they found almost no acetylcholinesterase activity in their tissues. Ants collected on the plant’s exterior showed more of this activity. This indicates that the nectar inhibits the insects’ locomotion, Baby says, making them temporarily clumsy and more likely to tumble into a pitcher. The nectar is a “toxic bait,” he says. “Prey capture by these pitchers is a story of total deception.”
Bauer doesn’t agree that the relationship is that unbalanced. The nectar isn’t so potent that the plants catch every ant that imbibes; many of the insects are able to shrug off its effects and make it home to deliver the sweet treat. “Sugar is an important food resource for ants, because it’s very energy dense,” Bauer notes. Meanwhile, worker ants are relatively expendable. “It’s a good deal for the ant colony to sacrifice some workers, as long as the workers that survive bring in enough sugar to offset that loss.”
It wouldn’t be completely unheard of for nectar to manipulate insects to the plant’s benefit, says Martin Heil, an expert on ant-plant interactions and extrafloral nectar with the Mexican Center for Research and Advanced Studies of the National Polytechnic Institute.
That said, he finds the data provided—that drowned ants exhibited high levels of acetylcholinesterase inhibition—are circumstantial at best. He would like to see experiments examining live ants before and after consuming the nectar to be convinced that the fluid has a real impact on prey capture.
Bauer says that in her work, she hasn’t seen ants with impaired movement after drinking pitcher plant nectar, though others have, and she notes the effect need not be dramatic to benefit the plant. She also hasn’t worked with N. khasiana specifically, and nectar components vary between species—a fact she can personally confirm from tasting the nectars of several pitcher plants.
Bauer notes that the authors’ method for nectar sampling—which involved rinsing cut sections of plant to collect the fluid—could have introduced intracellular compounds. So future work should confirm that (+)–isoshinanolone is in the nectar that the ants consume, not just the plant’s tissues.
Still, she wouldn’t be too surprised if it is. Pitcher plants have “such an amazing diversity of tricks for how to trap insects” that toxic nectar would hardly be the strangest.
ABOUT THE AUTHOR
Christie Wilcox is the Newsletter Editor for Science.