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Spider silk reduces plant damage by insect pests, finds Miami ecologist; and spiders don't have to be present

11/30/2012

Long jawed orb weaver (Tetragnatha) (photo courtesy M. Larrivee)
Long jawed orb weaver (Tetragnatha) (photo courtesy M. Larrivee)
In the presence of spider silk, insect plant pests eat less plant material, and the spider doesn’t have to be around to see this effect, according to a recent study by Miami University evolutionary ecologist Ann Rypstra and colleague Christopher Buddle of McGill University.

Their results suggest that spider silk signals herbivores in ways that influence plant production. If so, the introduction of silk might contribute to natural pest control programs. Their study is published in the Feb. 23, 2013 issue of the Royal Society journal Biology Letters, and online ahead of print Nov. 28.

“This work is fascinating because it shows that spiders have a much more important role in agricultural systems than we realized before,” said Rypstra, Distinguished Professor of Zoology, and Buddle, associate professor of forest insect ecology at McGill.

“Spiders do not have to be present to cause insect pests to eat less - as long as they were there, and produced silk as they moved through their environment, their potential prey will live in a 'landscape of fear' or, the insect pest is living in fear of silk.”

“Spiders are important in agricultural systems because they eat many insect pests that in turn eat valuable crops. Spiders also leave behind silk as they move through an agricultural field - sometimes this silk is there because it was part of a web that was constructed to catch prey, or sometimes spiders leave silk in the form of a 'drag-line' - a kind of silk that is a sort of safety-line for a spider as it moves around,” explained the researchers. “Whatever the means, the agricultural landscape contains plants, their insect pests, spiders and spider silk.

In this study, Rypstra and Buddle hypothesized that herbivores - the Japanese beetle and Mexican bean beetle - would associate silk with predation risk and reduce their consumption in its presence.

Beetles, Beans, Spiders and Silk: How the Study was Conducted

Bean plant leaf damaged by beetles (photo courtesy Ann Rypstra)
Bean plant leaf damaged by beetles (photo courtesy Ann Rypstra)
Rypstra and Buddle used a combination of experiments in a laboratory setting, and in a field-setting. The two pest species - the Japanese beetle and the Mexican bean beetle - were allowed to eat either leaflets of or whole plants of bush-style snap beans.

The plants or leaflets were either left alone or were adorned with five strands of spider silk or with five strands of silkworm silk. The silkworm silk was tested to determine whether the beetles might respond to any silk instead of silk produced by spiders.

To extract the spider silk, they allowed a long-jawed orb-web spider to hang from its drag-line, and wound its silk around a stick as the spider bobbed up and down.

“We found that when spider silk was on the plants, the insects inflicted less damage compared to when there was no silk,” Rypstra and Buddle said. “The silkworm silk also caused the insects to feed less, but the effect with silkworm silk was less than with spider silk.”

Spider Silk and Beetles’ “Landscape of Fear”

“The pest insects (the beetles) in our study system recognize the silk is coming from a potential predator (the spider), and this means they alter their behavior, or live in fear! This work fits within the broader literature about the landscape of fear,” Buddle and Rypstra explained.

“The idea here is that prey are shifting their behaviors depending on predators. Fear induces behavioral changes in prey; they are scared and this fear has real and measurable effects.”

“Something Special About Spider Silk” Causes Pest Insects to Eat Less Plants

Japanese beetle on bean plant leaf (photo courtesy Ann Rypstra)
Japanese beetle on bean plant leaf (photo courtesy Ann Rypstra)
These experiments “tell us that there is something very special about spider silk, and it causes pest insects to eat less plants,” said the researchers.

Their results show that insect pests that feed on plants in agroecosystems may be living in “a landscape of fear that is brought on by one of the most common substances produced by spiders – the silk. This silk acts as an important cue for the insect pests and they eat less plant material because of this.”

This research also shows the added value of spiders in agroecosystems; conservation of spiders, or even habitat manipulations to encourage spiders to live in agroecosystems, could have many pay-offs.


Related Research

Rypstra is co-author on two other journal articles published recently:

“The Nutritional Content of Prey Affects the Foraging of a Generalist Arthropod Predator” published in the online journal PLoS One Nov. 8.

Co-authors are Jason Schmidt (Miami Ph.D. ‘11, currently post-doctoral scholar, University of Kentucky Invertebrate Ecology Laboratory); Shawn Wilder (Miami Ph.D. ‘07, currently postdoctoral fellow and ARC Discovery Early Career Fellow, 2013, at School of Biological Sciences, University of Sydney, Australia); and Peter Sebastian, former NSF Research Experience for Undergraduates program participant with Rypstra.

“Experience with a Chemotactile Predation Cue" published in the Dec. 2012 issue of Ethology, and online Nov. 19. Co-author is Michael Sitvarin, doctoral student in zoology.

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