Want your bodyguards to stick around? Give them lodging. Some plants seem to do just that in the form of tiny pockets and hair tufts on the undersides of leaves, offering the shelter necessary to house a population of plant-protecting bugs, report researchers at the University of California, Davis.
By simulating these naturally occurring shelters, known as "leaf domatia," on cotton plants, the researchers reduced the populations of cotton-eating spider mites and boosted cotton yield by 30 percent. These results, offering the first experimental evidence that plants and certain bugs both benefit from the presence of domatia, are reported as scientific correspondence in the June 5 issue of the journal Nature.
"Domatia are wildly common and take on many forms, ranging from very simple cavities to complex hair-tufted structures," explained Anurag Agrawal, a UC Davis doctoral candidate, who conducted the study with entomology professor Richard Karban. "We think that predatory insects and mites use domatia both for protection from their enemies and for improved microclimatic conditions," he said.
Domatia were originally described in the late 1800s by Swedish naturalist Axel Lundstrom, who proposed that these structures offer mutual benefits to both the plants and to certain protective insects and mites. His notion lay dormant until about a decade ago when a few research groups around the world began to probe its validity.
One of the striking features of domatia is that they seem to be inhabited strictly by insects and mites that feed on other bugs rather than on plants, further evidence of a mutually beneficial relationship between plants and the domatia residents.
Exploration of this phenomenon was a natural for researchers in the laboratory of Richard Karban, who has been studying the relationship between plant-eating insects and their plant hosts for some 15 years. He has been particularly interested in cotton and the pinhead-sized spider mites, which are among the most severe cotton pests for California's $1 billion cotton industry. The mites essentially spit on the cotton leaf to pre-digest it, then proceed to slurp it up.
"Spider mites, the third largest pest of cotton nationwide, have been difficult to control because they produce a new generation every seven to 10 days and are resistant to many chemical pesticides," Agrawal said.
Most growers apply sulfur, oils and mostly chemical miticides in an attempt to control spider mites. With the use of two pesticide sprays, 5 percent of the cotton yield is usually lost to spider mites. If untreated, a cotton field can be thoroughly devastated by the mites.
Commercial cotton varieties have no domatia, so Karban and Agrawal decided to see what would happen if they added some of these tiny structures to the cotton leaves. They glued dots of cotton fibers to the undersides of cotton leaves growing in the field and found that the spider mites' natural enemies - the western flower thrip, the big-eyed bug and the minute pirate bug - readily took up residence in the simulated domatia. Furthermore, yield of cotton bolls was boosted 30 percent in cotton plants that had domatia added to their leaves early in the season.
The researchers also tried adding the simulated domatia to field cotton in the middle of the growing season. In those plants, levels of spider mites remained below the pest status. However, the addition of the domatia at that time appeared to have the biggest effect on populations of the big-eyed bugs and minute pirate bugs, both of which colonize later in the season. The populations of thrips, which colonize earlier in the growing season, did not appear to benefit as much from the addition of the domatia at that time.
"These results suggest that domatia may offer a natural biological control strategy as an alternative to the use of pesticides," said Agrawal. "Leaf domatia are found on wild varieties of several crop species damaged by spider mites, so we're hopeful that domatia might be introduced to commercial varieties through traditional breeding or genetic engineering."
The researchers suspect that domatia might be helpful in controlling spider mite populations, not only in cotton, but also in almonds, walnuts, coffee and avocadoes.
Agrawal and Karban will continue their research, comparing use of domatia to induced resistance as biological controls for spider mites in cotton. To jump-start the plant's defense mechanisms, they have previously induced resistance in cotton plants experimentally with naturally occurring plant chemicals or by briefly introducing mites during the early stages of growth.
Funding for the domatia study was provided by the U.S. Department of Agriculture and the National Science Foundation.