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| December 2007 |
Feature |
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Cotton on to the value of spidersAnsie Dippenaar-Schoeman and Almie van den Berg
Cotton is one of South Africa's five major field crops and its annual production is still increasing. A number of insect and mite pests cause problems in the production of cotton. However, a range of natural enemies found on cotton crops, of which spiders are one of the commonest, attack these pests and can be used in integrated pest management (IPM) programmes.
In an overview of studies undertaken on cotton by ARC-Plant Protection Research Institute from 1979-1997 a checklist of all the spider species found in five cotton growing areas was compiled which forms part of the South African National Survey of Arachnida (SANSA) in agro-ecosystems. The primary purpose of the study was to determine what spiders species are commonly found in cotton fields, their abundance throughout the year, where they occur on the cotton plant and the pest species they prey on. A total of 127 different spider species were collected from cotton, represented by 31 spider families. This compares well with the spider assemblages found in cotton fields in other parts of the world. Spiders are found on all parts of the cotton plant but are usually not easily seen because they are so well camouflaged. A large number of the species are only active at night and will hide during the day.
Spiders use different methods to catch their prey. During this study it was found that 38 % of the species found on cotton build webs on or in between the plants. These webs are used to catch flying and jumping insects. The most common type of web found in the cotton fields were orb-webs made by the family Araneidae, of which 18 different species were collected. It was observed that they prey on the larvae and moths of the cotton bollworm (Helicoverpa armigera). Other web dwellers, the sheet-web spiders (Linyphiidae) spin small webs over the cotton leaves and prey on red spider mite species and bollworm larvae. Members of the cob-web family Theridiidae build their irregular webs on, between and around the leaves, near the flowers and bracts of the cotton plant. They prey on red spider mites, the first three larval stages and adult stage of the bollworm, leafhoppers and aphids.
A large number of spiders do not build webs, but wander around on the plant and soil surface in search of prey. They are known as wandering spiders and the ARC-PPRI team found that 62 % of all the spiders collected from cotton were wanderers. One of the dominant species collected, the black-palp wolf spider (Pardosa crassipalpis) was collected from all five cotton-growing areas. They are well known predators of red spider mites and cotton bollworm. Overseas it was found that wolf spiders play an important role in biocontrol on cotton and they have been found to prey on at least 30 different insect species.
Another important wandering spider found on cotton is the sac spider (Cheiracanthium spp.). They are free running, aggressive night hunters and will kill any prey they encounter. They prey on a variety of cotton pests species and it was found that they not only prey on the larval stages and adults but also consume high numbers of bollworm eggs over a 24 hour period.
Feeding studies in Israel showed that they have a very high capacity for feeding on red spider mites and can kill on average 27.5 mites per day. Other wandering spiders that were commonly found on cotton were the lynx spiders (Oxyopidae), jumping spiders (Salticidae) and crab spiders (Thomisidae). Field and laboratory experiments indicated that they all prey on cotton pest species. ConclusionFarmers must regard spiders as some of their best friends in combating harmful pests. The study has shown that spiders are common and occur in high numbers in South African cotton fields. They are one of the first predators to colonize newly planted cotton fields and their numbers build up gradually as plant density and prey numbers increase. They are found everywhere on the cotton plant, especially at night and are searching for food most of the time. Their food includes most of the cotton pest species and they not only prey on one stage of the prey but eggs, larvae, pupae and moths are fed on. Their different hunting strategies enable them to obtain different kinds of prey. Although a single spider species may not be able to control a single pest species, spider assemblages as seen here, consisting of 137 species, can be effective in stabilizing pest populations. Their role as part of a complex predatory community may be important in regulating small numbers of pest species early in the season and in-between peaks of pest species activity. This may keep pests at endemic levels early in the season and prevent or prolong outbreaks from occurring in the first place. Because they are found everywhere, the various families are affected differently by pesticides. Their presence in cotton fields should be encouraged and steps taken to protect them from harmful chemicals. More information:
Further reading Van den Berg, A.M. & Dippenaar-Schoeman, A.S. 1991. Spiders, predacious insects and mites on South African cotton. Phytophylactica 23: 85-86. Dippenaar-Schoeman, A.S., Van den Berg, A.M. & Van den Berg, A. 1999.
Spiders in South African cotton fields: species diversity and abundance (Arachnida:
Araneae). African Plant Protection 5: 93-103.
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For more information contact Prof
A.S. Dippenaar, ARC-Plant Protection
Research Institute, P/Bag X134, Pretoria 0001