Status of seedling establishment pests of Acacia mearnsii De Wild. (Mimosaceae) in South Africa

Abstract

There are about 106 690 ha of Acacia mearnsii (wattle) plantations in South Africa. Wattle was previously grown mainly for the commercial potential of its bark (a source of tannin extract) but is now also managed on a short rotation for pulpwood. Clear-felled sites are continually being regenerated. Although considerable research has been done on the post-establishment insect pests of wattle, little is known about the incidence and status of seedling establishment pests. Fourteen trials were planted, on previous wattle sites, over six growing seasons from 1990/91 to 1999/00. Seedlings were evaluated monthly after planting for one year. Stressed, damaged and dead seedlings were uprooted and inspected to determine the cause of death. About 9% to 51% of seedlings failed to establish during wattle regeneration, and the incidence of damage by seedling pests ranged from about 2% to 30%. At sites where the plantation residue was windrowed and burnt, the average incidence of seedling establishment pests was about 20%, and the average total failure of wattle seedlings to establish was about 34%. Whitegrubs (larvae of Coleoptera : Scarabaeidae : Melolonthinae, Rutelinae) were the dominant and economically most important seedling establishment pests (average incidence of about 13%), followed by cutworms (larvae of Lepidoptera : Noctuidae) whose average incidence of about 4% was similar to that of grasshoppers (Orthoptera : Acrididae, Pyrgomorphidae) (about 2%) and millipedes (Diplopoda : Juliformia) (about 1%). Other seedling establishment pests included termites (Isoptera : Termitidae, Hodotermitidae), tipulid larvae (Diptera : Tipulidae), wireworms (Coleoptera : Elateridae), false wireworms (Coleoptera : Tenebrionidae), crickets (Orthoptera : Gryllidae), ants (Hymenoptera : Formicidae) and nematodes (Nematoda : Heteroderidae, Trichodoridae). Nematodes were sporadically important (about 12%) in an old arable wattle site. Although the prophylactic and corrective application of insecticides was widely used to control these pests at planting, their routine use in certified plantations now contravenes the Forest Stewardship Council guidelines.