First posted on 05-31-2012
Plants and pollinators have co-evolved over millions of years. During that time, plants developed myriad means of attracting bees, flies and other animals to spread pollen and keep the plant species thriving. In similar fashion, pollinators developed specialized methods of collecting plant nectar, as well as a little pollen while they were at it.
Recently, entomologists at the University of Arizona discovered a new sensory channel used by some insects in this complicated plant and animal interaction. They found that at least one common species of hawkmoth can detect even tiny differences in humidity when it hovers near a plant bloom. The hawkmoth uses this information to decide whether enough nectar is inside the bloom to warrant an extended stay.
“Traditionally, most research on plant-pollinator interactions has focused on static cues like floral scent, color or shape,” said Martin von Arx, one of the lead scientists on the team. “All this time, evaporation from nectar was right under our noses, but few people ever looked. We were able to show that the insects actually perceive this cue, and it allows them to directly assess the reward that they might get from the flower.”
Those other cues gave little if any information to pollinators about nectar levels inside individual blooms. Scent, for example, is more dependent on petals than nectar. But humidity as it evaporates from nectar offers a real and meaningful signal to potential visitors.
For a hawkmoth out for an early evening banquet, time is precious. Too much spent at unproductive sites could be the difference between life and death. The act of hovering in front of a plant with its tongue, or proboscis extended, is an exhausting endeavor, one that expends a great deal of the moth’s energy. And reaching inside to gather all those sweet offerings leaves the moth exposed to predators.
“The metabolic cost of hovering in hawkmoths is more than 100 times that of a moth at rest,” said Goggy Davidowitz, another team member. “This is the most costly mode of locomotion ever measured. An individual hawkmoth may spend 5-10 seconds evaluating whether a flower has nectar, multiply that by hundreds of flowers visited a night, and the moth is expending a huge amount of energy searching for nectar that may not be there. The energy saved by avoiding such behavior can go into making more eggs. For a moth that lives only about a week, that is a very big deal.”
The team measured humidity levels inside a sealed container of flowers, and discovered that the air just above an opening bloom was slightly higher in humidity than the surrounding air. They found that the bloom’s nectar tube was partially responsible as it emitted a plume of water vapor.
The scientists even placed artifical flowers into the chamber, only to find that the moths hovered over and extended their tongues into those artificials that had slightly higher humidity levels than the ambient air.
The research team measured moth sensitivity to humidity, and found that the hawkmoth could sense humidity elevations as low as 4 percent above ambient levels. Even air disturbances caused by the fluttering of many moths inside the chamber failed to alter their sensitivity to humidity changes.
“It was really exciting to see their high sensitivity to humidity in that they can perceive such a minute amount of difference in such a dynamic environment,” von Arx said.
From the flower’s point of view, it has to compete for the attention of many pollinators but doesn’t want to expend a great deal of energy doing so. Often, plants seem to oversell their nectar stockpiles through dishonest advertising in terms of scent, color, or other attributes that have little to do with nectar levels. But plants that are selective about which pollinators to attract have to change tactics.
“If you’re one of only a few flowers and there are lots of pollinators out there, you don’t have to be honest about how much nectar you have because they’ll visit anyway,” von Arx said. “But if you want the attention of just a few, you really have to go all out. So by saying, ‘Hey, come here, I have lots of nectar,’ you’re giving a faithful signal about an actual benefit that the pollinators can perceive and evaluate.
“I think in this case we showed that honesty makes sense in this system, because plants pollinated by hawkmoths are often pollinator-limited.”
Humidity plays an important role in the insect world, according to von Arx. Entomologists understand how humidity impacts something like the selection of suitable habitat, but its role had not been studied with respect to foraging for food.
“As creatures who use vision and olfaction, humans think in odors and shape, and color,” von Arx said. “We are biased by what we can perceive. We know that moths have hygroreceptors on the tips of their antennae, but they remain a mystery for the most part. We know a lot about olfactory receptors, mechanoreceptors and vision. The insect eye has been studied in and out. But hygroreception? We still don’t really know how that actually works.”
This is an evening primrose flower as it appears to the human eye (left) and to a hawkmoth (right). if nectar is present, an invisible plume of humidity emanates from the flower, telling the moth if it warrants a visit.
Photos courtesy of Robert A. Raguso, Cornell University, and Martin von Arx.
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