Spiders
Primary prey: aphids, mites, moths, flies, beetles, etc.
Predatory stages: nymphs and adults
Description and life cycle
Spiders are not insects. One of their major characteristics is that they have 8 legs (insects have 6 legs). While insects have three body parts (head, thorax and abdomen) spiders have only two major parts. The front part is called cephalothorax and is a body region made up of the head and thorax joined together. The hind part is called abdomen. Spiders never have wings.
Many species of spiders exist. They can be divided in two major groups, depending on the method they use to catch their prey. One group consist of hunting spiders that actively go around in search for their prey. They are very mobile and spend a lot of time searching for their prey. The other group of spiders make webs and then wait for prey to become trapped. they are especially important as predators of flying insects such as adults of moths.
Both the hunting spiders and the web-making spiders are very common and important as predators in vegetable and fruits crops as well as in rice fields.
Population density of spiders depend on the availability of prey insects. If their is a lot of prey available, the female spiders will lay more eggs. So when a pest population increases, the spider population will quickly respond and also increase in size.
The number of eggs spiders can lay varies from a few to several hundreds, depending on the species. Some spiders carry the eggs in a little sac until the young spiders hatch from the eggs (e.g. wolf spiders - Lycosidae). Others guard the location where the eggs are deposited (e.g. Lynx spiders) or place the egg mass in the web or on leaves, covered with fluffy silk. Spiders may live up to 4 months, depending on the species.
Effectiveness
Spiders are very voracious predators. It depends on the species how many prey they can eat on a day. Some spiders can eat for example 5 large insects in a day, or many more small ones. Spiders that make webs usually will trap more insects than they can eat.
Because spiders can feed on a wide range of insects, they will also be present in the field in the absence of pest insects. If there are no pest insects they just switch to other food sources. This has the advantage that when pests arrive in the filed, the spiders are already there.
Conservation
Mulching, especially organic mulch, can increase the number of spiders. Spiders can hide in the layer of mulch and they will also find a lot of alternative prey there, such as springtails (Collembolans) and other small insects that feed inside the mulch.
Leaving some pockets of weeds between fruit trees has a similar advantage. The insects that are associated with the weeds form a source of food for the spider population.
Spiders are easily killed by broad-spectrum insecticides and acaricides. Avoid the use of pesticides as much as possible.
Mass production of spiders is very difficult because in captivity spiders will feed on each other (cannibalism).
See: references