RESUMO
Although records show that the loss of susceptibility of pests to biological control agents is an exceedingly rare event, there are certain behavioural and ecological settings that may well predispose to it. In general, these circumstances rarely converge. Such a critical combination of factors could possibly occur in agroecosystems based on incomplete transplants imported from elsewhere. It can be argued that such ecosystems lack the biodiversity required to confer biotic resistance to invasive species and this can result in spectacularly high and damaging pest densities. Through exactly the same mechanism, introduced control agents such as parasitoid wasps, similarly can prove to be very successful in producing persistently very high levels of parasitism of pests, leading to triumph. However, this feeling may be short-lived. When success is based on very high selection pressure on the host pest species this could have the potential to lead to the evolution of resistance to the control agent. This is particularly so should it coincide with factors such as a lack of pest host refugia, parasitoid parthenogenetic reproduction, versus pest sexual reproduction, as well as suppression based on a narrow range of natural enemies. In effect, the very thing that can lead to spectacular success can eventually become the basis for failure. For the purposes of illustration, these considerations are illustrated via what seems to be a developing cause for concern about biological control in New Zealand's pastures.
RESUMO
Ryegrass staggers is a seasonal mycotoxicosis of grazing livestock characterised by tremors, in coordination and a staggering gait almost unaccompanied by physical lesions. Deaths occur only as a consequence of accident or starvation. Outbreaks, in summer and autumn, occur only on pasture in which endophyte (Neotyphodium lolii)-infected perennial ryegrass (Lolium perenne) predominates and usually on which animals are grazed intensively. Animals recover when moved to a different type of grazing or after rain has promoted pasture growth. The disease was recognised for 80 years before its cause was discovered as a consequence of a grazing trial of sheep on three ryegrass cultivars which happened to have three different levels of endophyte infection. The endophyte was first formally described as Acremonium loliae, later corrected to Acremonium lolii, and was finally placed in the genus Neotyphodium. It produces a number of secondary metabolites of which lolitrem B is the principal one causing ryegrass staggers symptoms. Ergopeptides are also produced which cause heat stress and lack of productivity. N. lolii is symptomless in the plant, seed borne and grows intercellularly in the aerial parts, mainly in reproductive tillers and leaf sheaths but sparsely in leaf blades. It dies in stored seed and infection rates of different ryegrass cultivars have depended on seed storage times during their production. In addition, N. Lolii produces insect feeding deterrents, among them peramine, which protects infected plants from pest predation. Because of this, control of ryegrass staggers by elimination of endophyte-infected ryegrass is not feasible in areas in which insect predation is a serious pasture problem. However, N. lolii strains vary in the secondary metabolites they produce allowing the selection of strains that produce desirable metabolites. By inoculating such strains into uninfected ryegrass plants it is possible to produce cultivars which do not cause ryegrass staggers but resist insect predation. This review aims to provide a comprehensive summary of the current understanding of the N. lolii / L. perenne symbiosis, the toxins it is known to produce, their effects on animals and plants and the strategies used to control their ill effects while maximising their beneficial ones.
