Weaving a Web of Vineyard Spiders
GrapeGrower Magazine, (July 2003)
by Devin Carroll
Spiders are often the most abundant predators in vineyards. Although few studies document their role in grape pest management, most observers agree that some species are important predators of many pests, particularly leafhoppers and caterpillars. Other spiders may retard biological control by preying on other predators. A few spiders may become such a nuisance at harvest that growers ask for control measures.
A Cooperative Extension brochure, Spiders in San Joaquin Valley Grape Vineyards, has good pictures and descriptions of most common vineyard spiders.
One beneficial group of spiders hunts without a web by blindly wandering over foliage until they bump into food. The yellowish agrarian sac spider, Cheiracanthium inclusum, an important predator of leafhopper nymphs and caterpillars such as omnivorous leafroller, ties a leaf together with silk and stays inside with its eggs in April and May. These spiders often shake out of clusters at harvest time. The similar Trachelas pacificus, dark brown in front with a pale abdomen, is common under bark where it lays its eggs in a papery white disk. At night it hunts on the foliage for leafhoppers, mealybugs, caterpillars, and other prey.
Several species of tiny spiders build webs on single leaves. The most common are two species of Theridion, which can occur on 20% or more of leaves. Hoppers seem to be their preferred prey, mainly nymphs but some adults. They also eat small flies and a few mites, ants and beetles. Their abundance and feeding habits make them key hopper predators. Dictyna has a similar web and appearance, but is not related. They eat some adult leafhoppers, but more small flies, including the beneficial tiger fly, Coenosia. The dwarf spider, Erigone dentosa, eats leafhopper nymphs, small flies, small moths, and mites.
The thick sheet webs of the funnel web weavers, Hololena nedra, located around the trunk or in thick foliage, snare many hopper adults, flies, and midges. The spiders hide in a tunnel to one side, and run out on top of the web to catch prey. The webs sometimes cover bunches and need to be wiped off table grapes at harvest. Babies appear in April and build tiny webs on single leaves, often catching aphids, bark lice, and small flies.
Many other spiders occur in vineyards, but only these three groups appear important to leafhopper control.
Some spiders eat more predators than pests. The lynx spider, Oxyopes scalaris, eats a few hoppers, but more ants, flies, and spiders including Trachelas, Erigone, and smaller Oxyopes. I have seen them eat lady beetles, predaceous flies (Syrphidae), and damsel bugs. The small and very common jumping spider, Metaphidippus vitis, also eats other spiders. They love thrips and eat many small flies including tiger flies, but they refuse to eat leafhoppers.
The orbweb weavers, Neoscona oaxacensis, bother tractor drivers with their webs built across the rows. Adults eat moths, flies, lacewings, and an occasional katydid. Juveniles build smaller webs and catch some leafhopper adults, small flies, aphids, and winged ants.
The best way to encourage spiders is to allow them something to eat. Although many sprays aimed at insects have little toxicity to spiders, they wipe out the leafhoppers and other insects that spiders need for food. Most spiders have a single generation per year and overwinter in the vineyard. If they build up one year, they will be there the next. Growers who avoid spraying can achieve a balanced system with a steady, low level of hoppers and other pests. The spiders maintain themselves on flies and other prey, and protect the vineyard when hoppers come in from the neighbor’s place.