Spider ecology in pistachio orchards in South Africa

[摘要] English: As part of a larger biomonitoring project on pistachio nuts, Pistaciae veraL. (Anacardiaceae), a new crop in South Africa, spiders (Arachnida: Araneae)were surveyed over a 2-year period (January 2001 to December 2002) atorchards on the farms Green Valley Nuts (GVN) and Remhoogte (REM) in thePrieska district, Northern Cape Province, South Africa. This study aimed todetermine the diversity of spiders in the tree canopies, ground covers, and soilsurface, and aspects of the biology and pest control potential of common speciespresent. 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 onthe spider fauna of orchards. It was found IPM, organic and unsprayed orchardssupported a much greater diversity and abundance of spiders thanconventionally managed orchards. The effects of particular pesticides on spiderswere also assessed. Since the present study was conducted in IPM orchards onecould expect spiders to play an important role as predators of arthropod pests.During the 2-year survey of arboreal spiders, 200 trees were sampled perorchard primarily using insecticide fogging with dichlorvos as a knockdownagent. A total of 18 families and 87 species were collected. Numbers anddiversity 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 onspider abundance and diversity. The jumping spider, Heliophanus pistaciaeWesolowska (Salticidae), was strongly dominant, comprising an average of53.8% 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), falsechinch bugs (Lygaeidae), leafhoppers (CicadeIIidae), thrips (Phlaeothripidae) andleaf beetles (Chrysomelidae), indicating that they have potential as biologicalcontrol agents. Endosulphan and parathion applications apparently had a minimalimpact on the arboreal spiders, but further research is necessary to clarify this.In the ground covers, 55 species in 14 families were collected betweenJuly 2001 and July 2002 (10 samples, 2000 sweeps/orchard). Numbers anddiversity 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 affectspider 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 abovein the ground covers, indicating that predation in this stratum may limit pestpopulations before they can reach damaging levels on the main crop.In a comparison of the epigeic fauna of the three orchards andundisturbed grassland (veld), pitfall traps (all sites) and active searching(orchards only) were used to determine the diversity and abundance of spidersfrom August 2001 to July 2002. Pitfall catches were highest and most diverse inthe 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 orchardswas 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 significantnegative effect on the abundance and diversity of epigeic spiders. Familiesdominating the pitfall trapping (Linyphiidae, Gnaphosidae, Lycosidae andSalticidae) also dominated active searching, but their relative abundance variedbetween 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 theyear of sampling. Ground cover populations peak in early summer (December-January), which suggests a vertical migration to the tree canopies early insummer. Various aspects of the feeding and reproductive behaviour of thisspecies were observed in the laboratory, and described. Egg production in thefield was greatest in early summer, but declined in subsequent months. Anaverage of 12.6 eggs are produced per egg sac (n=88). An egg parasite,Odontacolus sp. (Scelionidae), and an undetermined polysphictine ichneumonidwasp, parasitic on adult females of H. pistaciae only, were associated with thisspider.The predation potential of H. pistaciae on the minor pest Nysius natalensisEvans (Lygaeidae) was assessed in laboratory and field tests. Predation rateswere compared with vinegar flies, Drosophila melanogaster Meigen(Drosophilidae). Predation rates were significantly higher for both male andfemale spiders on D. melanogaster than N. natalensis during both days of the48-hour-long tests. Predation of N. natalensis increased significantly for bothsexes during the second day of the tests. Preference tests indicated a highpreference of D. melanogaster, with increasing capture of N. natalensis withtime. 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 ofgeneralist predators in pistachio orchards, with 143 species representing 31families collected in the surveys of the three strata. They may play an importantrole in the suppression of minor pests before they reach levels that may bedamaging to pistachio trees and nuts. Further research is needed to clarify theirpredatory impact on pests, effects of pesticides, and the relative abundance anddiversity of spiders in other orchard crops in the Prieska district.

[发布机构] University of the Free State