Spiders are one of the commonest predator groups found in
agro-ecosystems in South Africa. They have special adaptations towards a
predatory way of life. Their soft abdomens enable them to consume large amounts
of food in relatively short periods of time, while their rate of predation may
greatly increase during short periods when plentiful supply of food is
available. They have an exceedingly high resistance to starvation, which enables
them to survive and maintain normal reproduction during periods of low prey
availability. This is accomplished by the ability to decrease their metabolic
rate. During their lifecycle, which varies from 9 months to 25 years, all
instars feed actively as predators. Most spiders are polyphagous and feed on a
variety of prey but some are specialists. Predation is not limited to adults
only but includes the egg and larval or nymphal stages. Insect predators and predacious mites have received much
attention in biological control programmes of mite pests, while spiders appear
to have been neglected. The natural occurrence of adult and immature spider
species on plants places them in close contact with plant feeding mites.
Laboratory experiments done at ARC-PPRI suggest that a variety of mites found on
cultivated crops, are consumed by spiders while field experiments showed that
spider populations have the potential to reduce a population of mites in certain
crops to such a level that an substantial increase in yield may be expected.
Spiders can also inhibit the initial build-up of prey
populations. The use of spiders as biological control agents will demand the
conservation and possibly the augmentation of spiders in cultivated fields. This
presents difficulties, as rearing techniques are as yet prohibitively
time-consuming and expensive. Husbandry of spiders should rather be accomplished
by means of environmental manipulation through adapted farming practices. Further reading:Dippenaar-Schoeman, A.S. 1976. An ecological
study of the spider population in strawberries with special reference to the
role of Pardosa crassipalpis Purcell (Araneae: Lycosidae) in the control of
Tetranychus cinnabarinus (Boisduval). MSc thesis-Rand Afrikaans University 119
pp. Dippenaar-Schoeman, A.S. 1977. The biology of
Pardosa crassipalpis Purcell (Araneae: Lycosidae). J. ent. Soc. sth.
Afr. 40: 225-236. Dippenaar-Schoeman, A.S. 1979. Spider
communities in strawberry beds: seasonal changes in numbers and species
composition. Phytophylactica 11: 1-4. Dippenaar-Schoeman, A.S. 1979. A simple
technique for study feeding behaviour of spiders on mites. Bull. Br.
arachnol Soc. 4(8): 349. Van den Berg, A.M. & Dippenaar-Schoeman, A.S.
1991. Spiders, predacious insects and mites on South African cotton.
Phytophylactica 23: 85-86. | 
