Understanding spiders and mites as natural enemies of field pests

Spiders are carnivorous and can predate a variety of prey. Some make webs, others ambush their prey, and others actively hunt.

Spiders are arachnids, not insects. They have four pairs of legs, no wings and no antennae. They have only two body regions. Harvestmen are most likely to be confused with spiders. In harvestmen, the two parts of the body are broadly joined, whereas in spiders, the body is clearly divided into two parts that are joined by a narrow stalk. Harvestmen have only two eyes, whereas spiders have six to eight eyes.

Spiders and harvestmen are generalist predators and, consequently, may capture other pest natural enemies. They provide background levels of control, but the more mobile species only settle in locations with sufficient prey, so exhibit some response to prey densities.

The generation time varies hugely between species, from short-lived species with multiple generations per year to those that produce one generation per year and live for several years. Eggs are laid in clutches inside a silk sac. They emerge as spiderlings, which, in some species, are cared for by the female. Spiderlings pass through several moults before reaching adulthood. Some spiders are capable of dispersing over large distances by spinning a long thread on which they are carried by air currents; this is known as ballooning.

Spiders are carnivorous and are unique in having a spinner near the hind end of the abdomen, which produces silk. Some spiders spin webs that are used as snares.

Others do not spin webs, but instead lie in wait for their prey or actively hunt. Web-spinning spiders are more effective predators because their webs can continue to catch prey after they have been abandoned. Spiders and harvestmen can be an important component of bird diets.

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For web-spinning species, vegetation, or a soil surface with a diverse architecture, provides more opportunities to build webs. Leaving trash on a cloddy surface helps retain spiders within fields in the autumn, while the presence of arable weeds provides a suitable structure in the spring and summer.

Wolf spiders occur predominantly along field margins and penetrate less than 60 m into fields. Splitting fields using beetle banks or similar vegetative strips can help improve coverage within fields.

Spiders are highly vulnerable to pyrethroid insecticides.

• Largest European family
• Highly abundant, especially cereal crops
• Small grey or black–bodied spiders, 1.2–7.2 mm long
• Hunting or web building
• Drift through air on strands of silk

• Hunting spiders, mostly at ground level but occasionally on low vegetation
• Mostly brownish, but attractively marked and clothed in dense hairs, 3.5–18.0 mm long
• Most abundant in field headlands
• Good runners and can jump
• Females conspicuous from May/June onwards, with brown or greenish-blue egg sacs

• Highly effective predators of spider mites owing to their voracity (20 spider mites per day), short generation time (1 week) and high fecundity (40–60 eggs per female)
• Some species are reared and released for biological control
• May also feed on other small insects, honeydew and pollen the latter boosting their reproductive capability
• More susceptible to insecticides than their spider mite prey