Biologists Team Up to Study
Impact of Spiders on Farms

Nov. 23, 1998
Contact for photos or information: Chris Curran
(513) 556-1806 (O)

Cincinnati -- UC biologists are collaborating with faculty at Miami University and the University of Kentucky to revive a long- standing ecological concept and apply it to agricultural pest control. It is the concept of "guilds," which has been around since the turn of the century but was formally described in a 1967 paper by Richard Root of Cornell University.

A guild is a group of organisms that exploits a similar resource in a similar way. For example, several types of insect- eating birds might actually comprise multiple guilds: a guild which catches insects out of the air, a guild which picks insects off foliage, and a guild which forages along the ground.

"This concept has been applied to a variety of different kinds of organisms," said Uetz. "I think I was the first person to use the term in arachnology, but many people thought about it." Uetz's original work was difficult, because the quantity of data and the complexity of the ecological relationships were enormous.

Two new developments have allowed him to revisit the problem: the existence of more powerful computer database programs and the establishment of the Ohio Spider Web, a consortium of Ohio arachnologists with ties to biologists in three other states.

Uetz recently presented a symposium paper co-authored with Al Cady of Miami University-Middletown and Juraj Halaj of the University of Kentucky explaining the new use of the guild concept. "We went to the literature and looked at all the families of spiders that are found in agricultural fields. We took basic information about those they build a web? Do they live on the ground? Are they active during the day or night? How do they capture their prey? We put all that into a very large data matrix and subjected it to a computer analysis. The computer told us what guilds the spiders belonged to."

The method used is very similar to that used by scientists comparing evolutionary relationships and DNA sequences across taxa. Instead of gene sequences, Uetz used ecological information to develop dendrograms or cluster trees. "That shows us groups that are similar and groups that are distinct. We can use that as a hypothesis to test real field data against," said Uetz.

Finally, Uetz used the computer to compare guilds across a variety of agricultural field types. "One of the striking findings was that the number of species in each guild seems to be relatively constant across all crop types despite the fact that these crop types are very different. Rice is a very grassy crop grown in very wet environments, while wheat is a grassy crop grown in dry environments. We have soybeans which are foliage plants and corn which is very tall. This seems to suggest a pattern that within an agricultural system there is a certain amount of ecological room for different kinds of spiders."

If there is a true, repeatable pattern, that increases the likelihood that the guild concept could be used to improve agricultural pest control. "You could enhance spiders in a particular guild to go after a particular kind of pest based on its life cycle," explained Uetz.

In a related project funded by the U.S. Department of Agriculture, biologists working at the Miami University Ecological Research Center in southwest Ohio have already demonstrated one technique for enhancing spiders populations with a guild. It's called the refugia concept. "When you plow a field in the spring or fall, you tear up everything that's there and disturb the ground and scatter the species that are there to thefour winds. They usually die off. We've been putting out shelters made of straw to attract them, and it does. It increases their numbers about 30-fold. They reproduce in there and move back out on the field as the field grows up. There's some evidence that in areas around these shelters, there's a lower level of pests and a better yield of crops."

In one study, there were 5-37 times more spiders compared with non-sheltered plots. The number of species present tripled, insect damage was reduced 33 percent, and biomass yields increased 21 percent.

For the farmer, those figures add up to a significant savings on pesticides. "Anything we can do to promote biological pest control or integrated pest management means you're saving money for the farmer," said Al Cady of Miami University-Middletown.

The Ohio Spider Web consortium plans to continue the work. For example, Uetz says it is important to compare areas with patchy agricultural fields intermixed with population centers and the mega-farms of the Plains states. He would also like to see if the guild patterns hold up in natural ecosystems as well as the artificial agricultural ones.

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