New Zealand experience is that perennial ryegrass in most old pastures is normally infected at very high frequency.However,in the south of NZ,infection rates are lower and some old pastures are nearly free 0f50 two vere less cted,but 29 tha ted perenni ryegras naopnyte (Piggot New Zealand pastures have higher levels of infection than those studied overseas.Of 54 onulations collected in north est Spain,15 were free of endophyte (Oliveira Castro 1998).and only 12 populations were 60%or more infected.Similar results were observed with French populations(Ravel et al.,1994).These infection levels are lower than those observed in southern NZ(Widdup Ryan,1992). 2.3 The Effects of Endophvte on Various Aspects of Pasture Yield Production Endophyte is essential to the thrift of perennial ryegrass pastures in NZ because it ryegrass nvertebra ests ASW 1982 (Mortime d tidge ely to d or Hunt.1983) those ent are pr at al rass stems bore less vigo ously into infected plants (Prestidge Gallag er,1988) observed to be dar ed in late summ in swards but was observed in infected swards (Popay&Mainland 1991).Under pressure from ASW,the frequency of endophyte infection increases rapidly as non-infected plants die and are replaced by expansion of infected plants (Hume Brock,1997;Prestidge et al.,1984). ASW has been shown to be an important factor in pasture failure in Canterbury (Goldson 1982)and much of the North Island(Barker et al.,1984b).Yield responses of 50-60%in mixed ryegrass-wnite insectici 1982 1n19 pastu 1991) was estimated a costi (FI it ophyte was shown t pro seed ilit genetic or varietal diffe 1982,and thougn Endophvte also protects perennial n rass from other invertebrates (Prestidge Ball 1993).Black beetle (Heteronychus arator)is intermittently a serious pest in northern NZ (East et al.,1981).Endophyte-free ryegrass can be quickly destroyed by black beetle (Lee et al.,1982),but endophyte deters feeding by the adults,and egg and larval numbers are reduced(Ball Prestidge,1992).Various nematode species,some endo-parasitic. others free-living in the root zone,are found in ryegrass pastures,and there is some evidence that populations are affected by endophyte infection (Eerens et al., 1998b Stewart et al.,1993).There is very little NZ information on the importance of grass in pastures (Watson et al., 1986, Yeates Prestidge, 1986 sture m aly bug (B s poae)is an &Bal,1999 Mealy bug presence is trivial n pasture PAGE 5 New Zealand experience is that perennial ryegrass in most old pastures is normally infected at very high frequency. However, in the south of NZ, infection rates are lower, and some old pastures are nearly free of endophyte (Widdup & Ryan, 1992). Of 50 populations examined, two were less than 20% infected, but 29 populations were more than 60% infected. Naturally occurring hybrid ryegrass may also be infected with the perennial ryegrass endophyte (Piggot et al., 1988). New Zealand pastures have higher levels of infection than those studied overseas. Of 54 populations collected in north-west Spain, 15 were free of endophyte (Oliveira & Castro, 1998), and only 12 populations were 60% or more infected. Similar results were observed with French populations (Ravel et al., 1994). These infection levels are lower than those observed in southern NZ (Widdup & Ryan, 1992). 2.3 The Effects of Endophyte on Various Aspects of Pasture Yield & Production Endophyte is essential to the thrift of perennial ryegrass pastures in NZ because it protects ryegrass from invertebrate pests, notably Argentine stem weevil (ASW) (Listronotus bonariensis) (Mortimer & di Menna, 1983; Prestidge & Ball, 1993; Prestidge et al., 1982). Adult ASW are less likely to feed on infected ryegrass (Barker et al., 1984d; Rowan & Gaynor, 1986). Less eggs and larvae are present in infected swards (Gaynor & Hunt, 1983), those present survive less well (Barker et al., 1984c) and larvae attacking ryegrass stems bore less vigorously into infected plants (Prestidge & Gallagher, 1988). In Manawatu, 70% of tillers were observed to be damaged in late summer in endophyte-free swards but negligible damage was observed in infected swards (Popay & Mainland, 1991). Under pressure from ASW, the frequency of endophyte infection increases rapidly as non-infected plants die and are replaced by expansion of infected plants (Hume & Brock, 1997; Prestidge et al., 1984). ASW has been shown to be an important factor in pasture failure in Canterbury (Goldson, 1982) and much of the North Island (Barker et al., 1984b). Yield responses of 50-60% in mixed ryegrass-white clover swards were achieved by protecting from ASW with insecticide (Kain et al., 1982). In 1991, pasture damage by ASW was estimated as costing NZ $46-$200m annually (Prestidge et al., 1991). Before endophyte was shown to provide protection, it had been noted that different seed lines of perennial ryegrass differed in their vulnerability to attack (Kain et al., 1982), and this was thought to indicate genetic or varietal differences. Endophyte also protects perennial ryegrass from other invertebrates (Prestidge & Ball, 1993). Black beetle (Heteronychus arator) is intermittently a serious pest in northern NZ (East et al., 1981). Endophyte-free ryegrass can be quickly destroyed by black beetle (Lee et al., 1982), but endophyte deters feeding by the adults, and egg and larval numbers are reduced (Ball & Prestidge, 1992). Various nematode species, some endo-parasitic, others free-living in the root zone, are found in ryegrass pastures, and there is some evidence that populations are affected by endophyte infection (Eerens et al., 1998b; Stewart et al., 1993). There is very little NZ information on the importance of grass￾associated nematodes in pastures (Watson et al., 1986; Yeates & Prestidge, 1986). Pasture mealy bug (Balanococcus poae) is an insect frequently present in pastures but of unknown importance. It is sensitive to the presence of endophyte (Pearson, 1989; Pennell & Ball, 1999). Mealy bug presence is trivial in pastures intensively infected with PAGE 5