S U M M A R YThe host range and virulence of five insect baculoviruses (two multiply-enveloped nuclear polyhedrosis viruses (MNPVs) from Agrotis segetum and Mamestra brassicae; one singly-enveloped NPV from Plusia gamma and two granulosis viruses (GVs) from A . segetum and Pieris brassicae) were studied for seven lepidopterous pests of temperate agriculture ( A . segetum, Agrotis exclamationis, Lacanobia oleracea, M. brassicae, Noctua pronuba, P. gamma and Pieris rapae).None of the viruses killed all species but M. brassicae MNPV failed to infect only P. rapae. The other viruses were restricted to the homologous host, or members of its genus or subfamily. In all examples except A . segetum GV, the median lethal dose for the most susceptible host, was less than 22 virus inclusion bodies and median lethal times for all infections ranged from 5 . 5 to 16.6 days. The low susceptibility of A . segetum and other noctuids to GV infections is discussed in relation to the structure of inclusion bodies and the nature of the infectious unit in baculoviruses. A l B 3V6 0 1984 Association of Applied Biologists 30 G . P . A L L A W A Y A N D C . C . PAY"! structural types of occluded baculoviruses, for larvae of seven lepidopterous pest species; the noctuid moths Agrotis segetum, A . exclamationis, Lacanobia oleracea, Mamestra brassicae, Noctua pronuba, Plusia (= Autographa) gamma, and the pierid butterfly Pieris rapae. These insects are all pests of temperate agriculture with overlapping geographical and ecological ranges and could therefore be exposed, in nature, to the same virus infections. The species also exhibit a range of taxonomic relationships from intrageneric to interfamilial. The five viruses chosen include examples of the three main occluded baculovirus structural forms (MNPV, SNPV, GV). All the viruses were isolated from one or another of the host species tested in this study, except P. brassirae GV. This is one of a complex of related GVs, all of which are more infective for P. rapae than P. brassicae (Crook, 1 9 8 1~) . P. rapae, the species used in this study, is probably a frequent natural host for the GV.The comparative virulence of the viruses in homologous and alternative hosts was examined bq measuring their median lethal doses (LDsos) and median lethal times (LTsos) in neonate larvae. Neonate larvae are generally more susceptible to viruses than older larvae (Evans, 198 1 : Payne, Tatchell & Williams, 1981) and may, therefore, be particularly important for initiating viral infections within larval populations. This larval stage is ideal for comparing the susceptibility of different species since large numbers of larvae can be readily obtained at the same stage of development. Restriction enzyme analysis of viral DNA was used for virus identification in cross-transmission tests. EcoRI, a restriction enzyme isolated from the bacterium E.ccherichia coli (Nathans & Smith, 1975), was used to digest purified viral DNA and I ) h A fragment profiles are presented of those virus isolates ( A . segetum GV, P. g...