orb weaver; spiders prey on insect pests

As much as earthworms are evidence of fertile, well-aerated soil, spiders are a sign of ongoing pest control in the garden. Spiders provide an around- the-clock security service that puts the clamps on leafhoppers, aphids, caterpillars, horseflies and ants. A spider eats two times its weight in insects every day.
Spiders belong to the class arachnida, which includes scorpions, ticks and mites. Spiders are distantly related to insects but vastly different from them. Spiders have eight legs, while insects have six. But this is just the beginning of the spider’s story.
All spiders produce silk, but not all spin webs. Orb weavers are the most noticeable garden spiders because of their large size, enormous webs and the bright, symmetrical patterns on their abdomens. Webs are used not only for catching insects. Most spiders have poor vision and use their webs as a means of communication. This is especially true in mating, since a courting male must vibrate a female’s web in just the right way to distinguish himself from a trapped insect.
Aside from webs, spiders use their silk for making nests, protecting their eggs and binding up insect prey. Combfooted spiders live in the lower branches of trees and in the corners of our rooms, from which they keep a watch out for insect intruders. By contrast, hunting spiders are webless creatures that capture their prey by ambushing or jumping on them.
Spiders are the only animals whose food is digested outside their bodies. This is accomplished by depositing digestive enzymes onto their prey, which liquefies their tissues prior to consumption.
Spiders feed not only on insects, but on other spider species, as well as on their own kind. In a University of Kentucky study, when predators of the common wolf spider were removed from a soybean field, wolf spider mortality was still high, the result of cannibalism. At Oklahoma State University, the density of spiders in red clover – used as a cover crop in pecan orchards – was not affected by a rise in aphid density. Usually, there is a direct correlation between density of prey and predator (such as aphid and ladybug), since predators will be fruitful and multiply when their prey is abundantly available. But not in this case. Although spiders eat aphids, they seem just as interested in keeping their own numbers in check by self-consumption.
There are several ways of attracting spiders to the garden. Aside from mulch, which provides the ideal nesting ground for hunting spiders, cover crops, hedgerows, and windbreaks are welcome havens for spiders. The point of growing a diversity of plants around your garden is to keep the beneficial insects and spiders there after a crop is harvested. In a study at the University of Nebraska, crops near windbreaks had more spiders, lady beetles and parasitic wasps and fewer insect pests as compared to crops not sheltered by windbreaks.
Although a spider’s diet is primarily insect pests, beneficial insects form a small part of it. The greatest enemy of spiders is the same as that of most insects: the parasitic wasp.
The bite of a female black widow spider (the one with the orange hourglass on its abdomen) may cause nausea and a fever for about eight hours, and the bite of the brown recluse or violin spider requires medical attention. Otherwise, the spiders you see are good for the garden and for you. Tarantulas are practically harmless; they are the Methuselahs of the spider world, with a life span of up to 20 years, while common garden spiders are lucky to live past their first birthday.
There is no more irrational fear than that of spiders. These virtually blind creatures have no interest or even awareness of human beings. Yet only recently has research begun on the benefit of spiders to the environment. Spiders are an indispensable part of garden life, and living would be considerably duller without them.

Photo credit: tlindenbaum / Foter.com / CC BY-NC-ND