Biological control of cactus weeds of minor importance in South Africa

Abstract

The origin, distribution, weed status and biological control of 13 species of alien invasive cacti of lesser economic importance in South Africa are reviewed. (The biological control of two major cactus weeds, Opuntia aurantiaca Lindley and Opuntia ficus-indica (L.) Miller, is fully reviewed elsewhere.) The species include two in the genus Austrocylindropuntia ( A. exaltata (Berger) Backeberg and A. salmiana (Parmentier)); Cereus peruvianus (L.) Miller; three in the genus Cylindropuntia ( Cyl. imbricata (Haworth) Knuth, Cyl. leptocaulis (De Candolle) Knuth and Cyl. rosea (De Candolle) Backeberg); Eriocereus martinii (Labouret) Riccobono; five in the genus Opuntia ( O. dellenii (Ker-Gawler) Haworth, O. lindheimeri Englemann, O. spinulifera Salm-Dyck, O. stricta (Haworth) Haworth and O. vulgaris Miller); and, lastly, Pereskia aculeata Miller. Total success has been achieved in biocontrol against Cyl. leptocaulis and O. vulgaris using the cochineal insects Dactylopius tomentosus (Lamark) and Dactylopius ceylonicus (Green), respectively. Introduced insect agents, together with sporadic chemical or mechanical control, have arrested the invasion of E. martinii (where a pseudococcid Hypogeococcus festerianus (Lizer y Trelles) was the agent used), Cyl. imbricata and Cyl. rosea (using D. tomentosus in both cases), and O. lindheimeri (using Dactylopius opuntiae (Cockerell)). Recognition of these cacti as weeds, before they became widely distributed or of much economic importance, and timely initiation of biological control, coupled with the locality of the weed species in hot dry areas that enhance the debilitating effects of cochineal insects on the plants, seem to have contributed most to the successes. Relatively isolated, refuge populations of the weeds that escape colonization by the imported insect agents, together with the inimical, but mandatory herbicidal control practices that often kill the natural enemies, seem to have been the two main reasons for reduction in the levels of biocontrol achieved.