Spider ecology in pistachio orchards in South Africa

English: As part of a larger biomonitoring project on pistachio nuts, Pistaciae vera L. (Anacardiaceae), a new crop in South Africa, spiders (Arachnida: Araneae) were surveyed over a 2-year period (January 2001 to December 2002) at orchards on the farms Green Valley Nuts (GVN) and Remhoogte (REM) in the Prieska district, Northern Cape Province, South Africa. This study aimed to determine the diversity of spiders in the tree canopies, ground covers, and soil surface, and aspects of the biology and pest control potential of common species present. The various studies were conducted in the orchards GVN 1 (8 yrs old, 16 ha), GVN 19 (5 yrs old, 16 ha), and REM(9 yrs old, 1.5 ha). A review was conducted to assess the effects of pesticide management on the spider fauna of orchards. It was found IPM, organic and unsprayed orchards supported a much greater diversity and abundance of spiders than conventionally managed orchards. The effects of particular pesticides on spiders were also assessed. Since the present study was conducted in IPM orchards one could expect spiders to play an important role as predators of arthropod pests. During the 2-year survey of arboreal spiders, 200 trees were sampled per orchard primarily using insecticide fogging with dichlorvos as a knockdown agent. A total of 18 families and 87 species were collected. Numbers and diversity were highest in REM (n=2202, 70 spp.), followed by GVN 1 (n=2051, 64 spp.) and GVN 19 (n=1550, 47 spp.). Orchard age has a significant effect on spider abundance and diversity. The jumping spider, Heliophanus pistaciae Wesolowska (Salticidae), was strongly dominant, comprising an average of 53.8% of the fauna. Field observations on three common spiders, H. pistaciae, Cheiracanthium furculatum Karsch (Miturgidae) and Neoscona subfusca (c. L. Koch) (Araneidae), found them to prey on minor pest aphids (Aphididae), false chinch bugs (Lygaeidae), leafhoppers (CicadeIIidae), thrips (Phlaeothripidae) and leaf beetles (Chrysomelidae), indicating that they have potential as biological control agents. Endosulphan and parathion applications apparently had a minimal impact on the arboreal spiders, but further research is necessary to clarify this. In the ground covers, 55 species in 14 families were collected between July 2001 and July 2002 (10 samples, 2000 sweeps/orchard). Numbers and diversity were highest in GVN 1 (n=631, 40 spp.), then REM (n=580, 35 spp.) and GVN 19 (n=549, 36 spp.). Ground cover composition significantly affect spider abundance, but not diversity. The lynx spider, Peucetia viridis (Blackwall) (Oxyopidae), dominated the fauna (29.3%), and H. pistaciae was also common (23.4%). Common spiders also preyed on the same pest organisms listed above in the ground covers, indicating that predation in this stratum may limit pest populations before they can reach damaging levels on the main crop. In a comparison of the epigeic fauna of the three orchards and undisturbed grassland ("veld"), pitfall traps (all sites) and active searching (orchards only) were used to determine the diversity and abundance of spiders from August 2001 to July 2002. Pitfall catches were highest and most diverse in the veld (n=1112, 56 spp.), followed by REM(n=704, 35 spp.), GVN 1 (n=560, 26 spp.) and GVN 19 (n=428, 25 spp.). The dominant species in the orchards was the sheet-weaver, Ostearius me/anopygius (0. P.-Cambridge) (Linyphiidae, 30% in the orchards), and the ground spider, Asemesthes /ineatus Purcell (Gnaphosidae) in the veld (29.1%). Orchard disturbances had a significant negative effect on the abundance and diversity of epigeic spiders. Families dominating the pitfall trapping (Linyphiidae, Gnaphosidae, Lycosidae and Salticidae) also dominated active searching, but their relative abundance varied between methods. In regard to its high abundance in pistachio orchards, the biology of H. pistaciae was studied to create a better understanding of its role in pest control. Arboreal populations peaked between December and March, depending on the year of sampling. Ground cover populations peak in early summer (December- January), which suggests a vertical migration to the tree canopies early in summer. Various aspects of the feeding and reproductive behaviour of this species were observed in the laboratory, and described. Egg production in the field was greatest in early summer, but declined in subsequent months. An average of 12.6 eggs are produced per egg sac (n=88). An egg parasite, Odontacolus sp. (Scelionidae), and an undetermined polysphictine ichneumonid wasp, parasitic on adult females of H. pistaciae only, were associated with this spider. The predation potential of H. pistaciae on the minor pest Nysius natalensis Evans (Lygaeidae) was assessed in laboratory and field tests. Predation rates were compared with vinegar flies, Drosophila melanogaster Meigen (Drosophilidae). Predation rates were significantly higher for both male and female spiders on D. melanogaster than N. natalensis during both days of the 48-hour-long tests. Predation of N. natalensis increased significantly for both sexes during the second day of the tests. Preference tests indicated a high preference of D. melanogaster, with increasing capture of N. natalensis with time. In the field, female H. pistaciae (n=8) preyed on a mean of 1.38 N. natalensis in 24 hrs. Factors affecting prey capture rates may be prey size, palatability and activity. The present study showed spiders to be a diverse and abundant group of generalist predators in pistachio orchards, with 143 species representing 31 families collected in the surveys of the three strata. They may play an important role in the suppression of minor pests before they reach levels that may be damaging to pistachio trees and nuts. Further research is needed to clarify their predatory impact on pests, effects of pesticides, and the relative abundance and diversity of spiders in other orchard crops in the Prieska district.