Night-fliers and the High Cost of Jimsonweed’s Pollination
By Jennifer Graber*
Desert nature lovers recognize Datura wrightii, or Jimsonweed, by its large white funnel-shaped flowers. This night-blooming gem can be found in the many arroyos cutting through the mesquite grassland along Madera Canyon Road in the summer months. Turn over one of its leaves and you might be lucky enough to find a robust, bright-green Manduca sexta (tobacco hornworm) caterpillar contentedly devouring the plant, undeterred by the poisonous defensive chemicals stored up in its leaves. To most plants, the cost incurred by a voracious herbivore such as Manduca sexta would be devastating. But the Datura is prepared for the attack. It has stored resources in its massive root that it can allocate to new leaf production. Why does the plant go to such extremes to accommodate a destructive caterpillar? As it turns out, these caterpillars undergo metamorphosis and emerge as powerful night-flying hawkmoths that, after visiting the plant’s impressive nectar-filled flowers, go on to transport its pollen. Although the plant can self-pollinate, it needs the moths for cross-pollination, which probably results in heartier seedlings than those that have been self pollinated. So the plant cuts the losses it incurs from the caterpillars, pays high energy costs to produce massive flowers to attract Manduca adults, and, in the end, may come out ahead with a “home-grown” effective pollinator that ensures it of reproductive success.
But that’s not the end of the story. In their efforts to quantify the costs and benefits to both partners in this relationship, researchers and a team of students from the University of Arizona working on the Santa Rita Experimental Range have discovered a new twist. Studies of hawkmoths east of Madera Canyon on the Santa Rita Experimental Range, as well as in Box Canyon, have provided them with critical new information. This is how it works. Researchers use ultraviolet lights to attract moths. They capture the moths and delicately remove pollen from their long tongues before releasing them back into the wild. They then make microscope slides from the pollen collected from the moths’ tongues and look at the pollen grains under the microscope in an effort to identify what plant species the moths have been visiting. Pollen slides produce a valuable source of data, because pollen grains are unique to different plant families. Much to their surprise, researchers have found that Manduca sexta is not particularly loyal to its “generous” host plant. For much of the season, it appears the adult moths lay their eggs on Datura but rely almost exclusively on Agave (found in mid elevations in the Santa Ritas) as a nectar source. Is Datura being fooled into feeding Manduca caterpillars--and producing expensive nectar-laden flowers--while its pollinators fly on by, headed up canyon to visit Agave instead? And if so, how does this affect the reproductive success of Datura? These are some of the questions we are trying to answer at various field sites in the foothills of the Santa Ritas. So as you are driving to Madera Canyon, look out for the researchers, with their clipboards and water bottles in hand, attempting to understand the interactions between these two charismatic desert dwellers. And if you hear a hummingbird zoom by in Madera Canyon at dusk, look again. If it’s not a Manduca, it’s likely one of the many other hawkmoth species of Southeastern Arizona.
*Jennifer Graber is a participant in the Undergraduate Biology Research Program and the Datura project. Southeastern Arizona has one of the most diverse Sphingidae (hawkmoth) communities in North America. For more information and photographs of hawkmoth caterpillars and adults visit:
Moths of SE Arizona at www.nitro.biosci.Arizona.edu/zeeb/butterflies/mothlist.html
Moths of North America at www.npwrc.gov/resource/distr/lepid/moths/mothsusa.htm
The Datura project is an NSF-funded grant obtained by researchers Dr. Judith Bronstein, Dr. Goggy Davidowitz, and Dr. Travis Huxman of the University of Arizona.
Tobacco Hornworm - larva of the Hawkmoth
Adult Hawkmoth - also called Sphinx Moth