Spiders - The African Farmer's Best Friend

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Spiders - The African Farmer's Best Friend

Dr Ansie Dippenaar-Schoeman

Termite-eating spider (Ammoxenus amphalodes) feeding on harvester termite.

Africa has a rich fauna of spiders with more than 6000 known species. All spiders are predators and feed on a wide prey range consisting mainly of invertebrates, but can also include prey like fishes, birds or even reptiles. Reviews on the role of spiders in agro-ecosystems indicate an increasing interest in, and recognition of, spiders as natural control agents of insects and mites in crops. Spiders form part of a complex predatory community in crops and are important in regulating pest species in both commercial and smaller developing farms.

Research undertaken over a 30-year period at the Plant Protection Research Institute (PPRI), an Institute of the Agricultural Research Council (ARC) in Pretoria has shown that spiders are amongst the very first predacious arthropods to colonize newly planted crops and their numbers build up gradually as plant density and prey numbers increase. Their diversity increases, as plants grow, to form a community which consists of different species occupying complementary niches. The buffering effect of spiders may be important in keeping pests species at endemic levels and make outbreaks less likely to occur in the first place. This buffering effect of spiders can be achieved through the combined activities of a variety of species in a given habitat. Therefore, diversity can be expected to increase the potential of spiders as predators in agro-ecosystems.

Grass orb-web spider (Neoscona moreli) feeding on a moth.

Observations have shown that most spider species are polyphagous (they can feed on many different foods) and feed on a variety of available prey such as mites, aphids, thrips and termites. Predation is not limited to the adult stage only, but includes the egg and larval or nymphal stages as well. Various spider species use different foraging modes and hunting strategies to catch prey. Due to the differences in feeding behaviour, various stages of pests are preyed upon by different spider species. Smaller prey like mites and thrips provide an important supply of food to especially the young spiderlings. Spiders do not only prey on pest species, but their presence on plants can cause disturbance and indirect mortality as high as 30% can occur with larvae falling to the ground and dying. Comprehensive surveys, to determine the species diversity, the numerically dominant species and their abundance are essential before experiments on their effectiveness can be conducted.

Why spiders are such good predators

Spiders have extensible abdomens, which enable them to consume large amounts of food in relatively short periods. Their rate of predation may greatly increase during short periods when plentiful supply of food is available.
They are exceptionally well adapted to survive in nature. They have high resistance to starvation as they are able to lower their metabolic rate; this enables them to survive during periods of low prey densities especially early in the season and helps them to survive harsh weather conditions and to withstand extremes of climate.
They have a long life span, which varies from 9 months to 25 years and all instars feed actively as predators.
As mentioned spiders have different hunting strategies (guilds) and are found in a wide variety of habitats. They can be divided into wandering or web building spiders. The wandering spiders occupy different habitats in a crop system such as ground or vegetation with specific species living on the bark, leaves, flowers or seeds of a plant.
The web dwellers construct their different kind of webs on or between the plants.
Spiders are easily distributed by wind and ballooning is the primary mode by which spiders colonize cultivated fields.

The Spider Research Centre at the ARC- Plant Protection Research Institute has been involved in research on spiders in different agro-ecosystems. On cotton more than 5000 spiders were collected represented by 127 species from 31 families, which were sampled over a 10-year period. It was found that the spiders represented about 70% of all predators present in the cotton. Studies showed that the spiders prey on mites, aphids, leaf hoppers, bollworm moths and larvae that attack the cotton plants.

Salticid jumping spider (Thyene sp.) feeding on a fly.

A total of 5059 spiders were collected from strawberry fields represented by 32 species from 14 families. The most abundant species was the wolf spider (Pardosa crassipalpis) and studies showed that this spider mainly preys on red spider mites. The presence of spiders in the strawberry beds compared to beds without spiders also resulted in an increase in the yield.

Over a period of 25 years spiders were sampled from 10 sites in citrus-growing areas in South Africa. One hundred and thirty four genera were collected represented by 198 species from 35 families. Spiders are important predators of orchard pests such as thrips, citrus scale and citrus psylla.

Over a year 2770 spiders were sampled from macadamia orchards in the Mpumalanga Province represented by 80 species from 21 families. The jumping spiders (Salticidae) represented 73% of all spiders collected and they prey on a variety of pest species such as thrips, mites, midges and flies.

From avocado orchards more than 3715 specimens were collected represented by 90 species from 26 families. The jumping spiders represented 31% of all spiders collected followed by crab spiders (24%). Crab spiders prey on a variety of small pest species such as aphids, red spider mites and thrips.

A total of 32 spider species were collected from tomatoes, represented by 10 families. In contrast with other predators, the hairiness of the tomato plants did not seem to affect the spiders. On tomatoes spiders prey on red spider mites, the first two larval stages of the bollworm, as well as on aphids.

Crab spider (Misumenops rubrodecoratus) feeding on a fruit fly.

At ARC-PPRI spiders are marketed as the "farmers best friend" The use of spiders as biological control agents depends mainly on the conservation and increase in numbers through the use of agricultural practices such as selective spraying rather than on mass rearing and release. The extent to which spiders can contribute to agricultural pest control, is however limited by the disruptive effect of insecticide applications. The following practices could help to conserve spider populations to the benefit of the farmer: selective use of pesticides to prevent elimination of natural enemies; restricting insecticide usage during crucial periods in the life cycle of the pest; limiting spray application to midday when spiders are less active and shelter; application of pesticides as spot treatments to permit spiders to recolonize treated areas immediately.

More information:

Dr Ansie Dippenaar-Schoeman (DippenaarA@arc.agric.za)
or Almie van den Berg (VDBergAM@arc.agrica.za)
ARC-Plant Protection Research Institute, Private Bag X134
Queenswood 0121 South Africa

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