Effects of plantation residue management on the community structure of wattle regeneration invertebrate pests in South Africa

Abstract

There is a limited availability of land for the expansion of South African black wattle plantations. One way to increase productivity is to ensure the survival of seedlings during regeneration, thereby increasing the stocking of compartments. Soil invertebrate pests constitute one of the important causes of seedling mortality. The effect of different plantation residue management practices on the incidence of these pests was unknown. Fourteen trials were planted on previous wattle sites over six growing seasons. Seven different plantation residue management regimes (windrowed–burnt–weeded, windrowed–burnt–ripped, fallow [mowed or manually weeded], windrowed–burnt–old arable, windrowed–burnt–closer spacing, and windrowed–broadcast–herbicide) were appraised using multivariate analysis. Seedlings were evaluated monthly after planting for a period of six months. Stressed, damaged and dead seedlings were uprooted and inspected to determine the cause of death. Members of the soil invertebrate pest complex included whitegrubs and cutworms that generally had a higher pest status than millipedes, nematodes, grasshoppers, ants, false wireworms, termites and crickets. A higher incidence (1.44%) of cutworm damage was observed in the windrowed–burnt–ripped and fallow sites. However, there was a greater infestation of soil invertebrate pests on sites where the plantation residue was windrowed–burnt–weeded or ‘broadcast’ (20.34%) than in the other treatments (windrowed–burnt–ripped or fallow; 2.36%). The addition of a rip treatment to a depth of about 50 cm in the windrow and burn regime significantly reduced the infestation of soil invertebrate pests, especially whitegrubs. Seedlings that were planted at a closer spacing in windrowed and burnt sites also had a lower incidence of soil invertebrate pest damage (7.79%). Regeneration of an old arable site in Seven Oaks had a sporadically high incidence of nematodes (11.58%). These findings have important management implications because windrow and burn are frequent plantation residue management practices in wattle silviculture. Insecticide application is the alternative option, although its use is restricted by Forest Stewardship Council guidelines.