- Summer 1994 "Update" Newsletter Article -
Spider research
Vineyard studies show myriad creatures willing to help growers control pests
From CATI Publication #940701
Copyright © 1994. All rights reserved.
Five years of insect studies in some uniquely-maintained San Joaquin Valley grape vineyards have provided a team of CSU, Fresno entomologists with “net-fulls” of new data about the creatures that dwell there.
Among other things, researchers have learned that the seemingly quiet vineyards under study hide a teeming population of hungry carnivores - spiders - that spend most of the day and night hunting and devouring their insect prey.
Collection and identification efforts have revealed that in general, spiders, from a number of biological families, were by far the most populous inhabitants of the vineyards. Spider densities ranged anywhere from two to 10 times the densities of other arthropods and insects, reported William O’Keefe, a research associate for the Viticulture and Enology Research Center, and one of the technicians for the project.
Following collection of more than 100,000 specimens, research leader and entomology Professor Mark Mayse remains convinced that spiders are a farmer’s friend, not an enemy.
“Some people worry that spiders get hungry and eat the vine. But that’s not what happens. Every spider is a carnivore - period,” Mayse said in explaining their habits in the vineyard.
Cover crops, used in conjunction with little or no pesticides, enable millions of beneficial arthropods, such as spiders, to thrive in a vineyard, helping to control populations of serious pests such as the western grapeleaf skeletonizer (WGLS), variegated leaf hopper (VLH), and omnivorous leafroller (OLR).
All the vineyards under study are being managed with low or no pesticide input and cover crops, including combinations of legumes (trefoil, strawberry clover, white clover, allysum and poppy); grasses (perennial ryegrass and fescue); winter oats; vetch; faba bean; buckwheat; and resident vegetation (weeds).
Most commonly found spiders were identified by biological family and include Trachelas and Chiracanthium (“sac” spiders); Theridion (comb-footed spider); and Pardosa (wolf spider). They vary in size and method of capturing their prey. Some, known as runner-hunters, stalk and run down their prey; others spin webs which serve as traps.
Exactly what types of prey spiders eat is a question yet to be fully answered, Mayse said. A primary goal of the research has been simply to determine what types of spiders and other insects are present in vineyards being managed with the use of cover crops.
“A lot of information is lacking on the players and their roles. We are just beginning to clarify who the key players are,” Mayse said in explaining research objectives.
Although the study did not seek specifically to determine what spiders eat, researchers did make some general observations.
“While hefty Trachelas and Chiracanthium probably travel the inner vine areas of the canopy, tiny Theridion spiders were often found out on the leafy fringes of the canopy within their small webs on the leaf undersides. Whenever a Theridion was found on a leaf, there were very few VLH and GLH,” OÔKeefe noted. “Chiracanthium is suspected to be an important grape cluster inhabitant. If so, lepidopteran larvae (OLR) and mealybug pests would have a natural enemy within the grape clusters,” he also observed.
A variety of other beneficial insects inhabited the vineyards along with the spiders, researchers found. They include Orius (pirate bugs); Nabis (damsel bug); and chalcidoids, e.g., Anagrus (tiny parasitic wasps). All these beneficials typically prey on other insects.
In addition to WGLS, VLH and OLR, an assortment of other plant-eating insects also found suitable living conditions in the vines or cover crop. They included false chinch bugs, flies, moths, caterpillars, mites, weed bugs, weevils, aphids, crickets, earwigs, midges, stink bugs, darkling beetles, and, of course, cockroaches.
Several methods were used by field researchers O’Keefe and Deborah Dexter-Mendez to obtain specimens. They included 1) pitfall traps - quart-size cups partially filled with antifreeze and buried so that the top of the cup is even with the ground level; 2) band traps - strips of corrugated cardboard wrapped around vine trunks; 3) sweepnet - a heavy-duty butterfly net collectors would sweep back and forth across the top of the cover crops; and 4) shake cloth - where one person holds a framed, two-foot square catch cloth under the canopy while a partner grasps the main vines and shakes the canopy vigorously.
Commercial vineyards where the samples were taken were located in west Fresno and Madera counties. Grape varieties included Chenin blanc, Grenache, and French Colombard.
The use of cover crops to promote beneficial insects and spiders has proven an effective method of managing vineyards with reduced pesticide use, Mayse noted. “It is becoming apparent that cover crops can be a powerful tool in the broader management of vineyard agroecosystems. Clearly, innovative farmers are succeeding with this approach,” he said.
The researchers will complete a publication this summer offering more details on the types of arthropod communities associated with various cover crops and grape varieties. Information on obtaining the publication will be provided in the fall issue of Update.{ page top }