Effect of Bt-cotton cultivation on spider (Arachnida : Araneae) populations near Marble Hall, Mpumalanga, South Africa

Abstract

Bt -cotton, containing and expressing genes from the soil bacterium, Bacillus thuringiensisl/Ig var. kurstaki , is toxic specifically to lepidopteran larvae, but little is known about the impact it could have on predators such as spiders. A survey to determine the effect of Bt -cotton and endosulfan application on spider populations was conducted over two cotton-growing seasons (2001/2002 and 2002/2003) on a farm near Marble Hall in South Africa. Plant-dwelling spiders (lIgnl/Ig = 227) were counted over the two seasons while scouting the plants, but were not identified. Ground-dwelling spiders ( n = 3777) were collected with pitfall traps in both seasons and identified to species level. The ground dwellers were represented by 21 families, 49 genera and 54 species. During the first season a total of 2431 spiders was collected from the pitfall traps, 945 from Bt -cotton and 1486 from non- Bt -cotton (control) fields. In the second season a total of 781 was collected, 416 from Bt -cotton and 365 from non- Bt -cotton fields. A total of 565 spiders was collected from a non- Bt -cotton field sprayed with endosulfan during the second season. The Lycosidae ( n = 2359) represented 62.5 % of all spiders collected in the pitfall traps, followed by the Theridiidae ( n = 757) with 20.0% and Linyphiidae ( n = 342) with 9.1 %. Steatoda erigoniformisl/Ig (Theridiidae) ( n = 744) was the most abundant species, representing 19.7%of all the spiders collected, followed by Pardosa clavipalpisl/Ig (Lycosidae) ( n = 624) with 16.5 %, an undetermined lIgTrabeal/Ig sp. (Lycosidae) ( n= 592) with 15.7 % and another lycosid, Pardosa crassipalpisl/Ig ( n = 543), with 14.4 %. Neither Bt -cotton nor the application of endosulfan had a marked or persistent negative impact on ground- or plant-dwelling spiders in the field.