Integrated Pest Management Guide

SPIDERS

Spiders (class Arachnida) are not insects (class Insecta) but are more closely related to mites. The most obvious difference between spiders and insects is the number of pairs of legs. Spiders, like mites, have four pairs of legs whereas insects have three. Spiders do not metamorphose from larva to pupa to adult like insects do. When they hatch, juvenile spiders just look like miniature adults. Adult female and male spiders often differ considerably in size. Female spiders are larger than males of the same species. All spiders are predacious and feed mainly on insects. They play an important role in restraining insect populations.

Spiders are mostly generalist predators. They prey on any kind of insect they are able to catch. In crops like cranberries, both web-spinners and hunting spiders contribute to the reduction of aphids, flies, gnats, caterpillars and other harmful insects.

Most web-spinners are unable to catch moths easily because of the moths' ability to shed the scales on their wings. While the webs of most web-spinners are designed to catch air-borne and jumping insects, the webs of some species adhere to the ground and trap crawling insects.

Compared with web-dwelling spiders, individual hunters (wolf and crab spiders) are not very efficient predators and do not catch nearly as much prey as they consume when fed in captivity. Their significance as predators stems from the fact that hunting spiders occur in vast numbers in small areas and, as a group, destroy great quantities of insects. Wolf spiders are effective pest control agents and, like beneficial insects, are adversely affected by insecticides. Crab spiders walk mostly sideways like fiddler crabs. Most crab spiders are less than 1 cm (0.4 inch) long. They are predators, lying in wait to ambush their prey, then injecting a venom that quickly immobilizes them.

In cranberry fields, wolf spiders of the genus Pardosa and crab spiders of the genera Xysticus and Tibellus have been found preying upon live fireworm moths in experimental field cages. Wolf spiders are active day and night.

Harvestmen, also called daddy long-legs, are arachnids like spiders. Spiders have two body parts, two fangs, and produce silk. Harvestmen have one oval body part, no fangs, do not produce silk and have eight extremely slender legs. Harvestmen are rarely found indoors, preferring moist areas outside. They feed on plant juices or dead insects, but some species could feed on living insects.

List of species found by Ursula Dole in cranberry beds in the Fraser Valley, in 1990
 

Wolf spider	Crab spider

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