V.C. Moran

Biological control of prickly pear, Opuntia ficusindica (Cactaceae), in South Africa

Abstract The outstanding success of a cochineal insect, Dactylopius opuntiae (Cockerell), and, to a far lesser extent, a phycitid moth, Cactoblastis cactorum (Bergroth), in biocontrol of the prickly pear, Opuntia ficus-indica (L.) Miller, in South Africa was fully reviewed in 1978. The present account updates this information. Today, the weed infests less than 100 000 ha (of the 900 000 ha originally colonized) in the cooler, higher rainfall areas where D. opuntiae is relatively ineffective. Cactoblastis cactorum is able to kill only the small prickly pear plants and suffers lossess because of ant predation, climatic extremes and, partly, to host plant incompatibility. Integrated chemical and biological control methods are described which allow O. ficus-indica to be managed as necessary and also to be exploited as a drought fodder for stock, as a fruit and vegetable source for humans, and as a host plant for the commercial production of dye stuff from the cochineal, Dactylopius coccus O. Costa. As a result of successful biological control over the past 50 years, the public has forgotten the former severity of the prickly pear weed problem in South Africa. The residual O. ficus-indica populations are now highly valued and the biological control agents themselves are regarded as pests. The consequences of these changed perceptions are discussed.

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.

Biological control of Sesbania punicea (Fabaceae) in South Africa

Abstract Sesbania punicea (Cav.) Benth., a South American leguminous shrub or small tree, has recently become a weed, principally of river banks and of wetlands, throughout South Africa. Three weevil species have been introduced for its biological control: a florivorous apionid, Trichapion lativentre (Beguin-Billecocq), that was inadvertently introduced in the 1970s, and two curculionids that were first released in 1984, a seed-feeder, Rhyssomatus marginatus Fahraeus, and a stem borer, Neodipiogrammus quadrivittatus (Olivier). The reproductive potential of S. punicea has been markedly reduced throughout South Africa by T. lativentre and, to a lesser extent, by R. marginatus. In the vicinity of release sites, many large S. punicea plants have been killed by N. quadrivittatus and the density of at least one infestation has been reduced to acceptable levels within 4 years. A critical assessment of this biocontrol programme is presented.

Biological control of jointed cactus, Opuntia aurantiaca (Cactaceae), in South Africa

Abstract Jointed cactus, Opuntia aurantiaca Lindley, has been a problem weed in South Africa for nearly a century. Its taxonomic status, history of introduction, and chemical and biological control were reviewed in 1979. The present account updates this information and deals with recent South African research on the ecology of O. aurantiaca and its biological control agents, and on the management of the weed. This review places some emphasis on the primary agent, the cochineal insect Dactylopius austrinus De Lotto (Homoptera: Dactylopiidae), with a shorter commentary on the phycitid moth Cactoblastis cactorum (Bergroth) that has also become established on jointed cactus. Research on three other introduced moths, Tucumania tapiacola Dyar (Phycitidae), Mimorista pulchellalis Dyar (Pyraustidae) and Nanaia sp. Heinrich (Phycitidae), is abstracted. Of the latter three species, only M. pulchellalis has become established, but its role in biological control is insignificant. It is concluded that: (i) the utility of insects as biological agents against jointed cactus has probably been fully exploited; (ii) the importance and threat of O. aurantiaca as a weed in South Africa may have been overestimated; (iii) future research emphases should be on the role of pathogens as biocontrol agents and on the formulation of area-specific integrated management procedures involving D. austrinus and herbicides.

Promising results from the first biological control programme against a solanaceous weed (Solanum elaeagnifolium)

Two leaf-feeding chrysomelid beetles, Leptinotarsa texana and L. defecta, have been successfully established on Solanum elaeagnifolium, a solanaceous weed inadvertently introduced into South Africa from North America during the early 1900s. Unlike L. defecta, which remains localised and scarce, L. texana has proliferated at a number of release sites. Surveys of the damage caused by L. texana on S. elaeagnifolium showed that in some situations the beetles reached densities where the host plants were completely stripped of leaves, flowers and bark, whereas fruits were not eaten by the beetles. Longer term studies in plots with and without L. texana showed that even at relatively low levels of abundance of the beetles, the growth of S. elaeagnifolium was suppressed and the capacity of the plants to produce fruits was severely curtailed. As a result L. texana has the potential to be an effective biological control agent of S. elaeagnifolium in South Africa and elsewhere.

Localized failure of a weed biological control agent attributed to insecticide drift

Abstract In South Africa, a bud-feeding apionid weevil, Trichapion lativentre (Beguin Billecocq), is one of three introduced insect herbivores that are responsible for the successful biological control of Sesbania punicea (Cav.) Benth., a leguminous weed of South American origin. With few exceptions, the annual seed crop of S. punicea has declined by more than 98% since T. lativentre became established on the weed during the late 1970s. However, there are several localized infestations of S. punicea in the Olifants river valley where T. lativentre has been much less successful than elsewhere in the country, and where pod and seed production by S. punicea is still prolific. In this valley, S. punicea often occurs in close proximity to citrus orchards, many of which are sprayed with organophosphate insecticides in spring and early summer (August–October). In the vicinity of sprayed citrus orchards, populations of T. lativentre remained low during the early summer (the main period of bud-burst for S. punicea ) and weevil numbers only increased later, when insecticide treatments in the orchards had almost ceased. As a result, the incidence of damaged flower-buds was lowest, and pod production was highest, on S. punicea plants in the vicinity of sprayed citrus orchards. The negative effects of the insecticides, as measured by higher levels of pod production on the weed, decreased exponentially with increasing distance up to 300 m from the nearest sprayed orchard.