Scaling up tests on virulence of the cassava green mite fungal pathogen Neozygites tanajoae (Entomophthorales: Neozygitaceae) under controlled conditions: first observations at the population level

Virulence of entomopathogens is often measured at the individual level using a single host individual or a group of host individuals. To what extent these virulence assessments reflect the impact of an entomopathogen on their host in the field remains largely untested, however. A methodology was developed to induce epizootics of the cassava green mite fungal pathogen Neozygites tanajoae under controlled conditions to evaluate population-level virulence of two (one Beninese and one Brazilian) isolates of the entomopathogen-which had shown similar individual-level virulence but different field impacts. In unrepeated separate experiments we inoculated mite-infested potted cassava plants with either 50 or 25 live mites (high and low inoculum) previously exposed to spores of N. tanajoae and monitored the development of fungal infections for each isolate under the same conditions. Both isolates caused mite infections and an associated decline in host mite populations relative to the control (without fungus) in all experiments, but prevalence of the fungus varied with isolate and increased with inoculum density. Peak infection levels were 90% for the Beninese isolate and 36% for the Brazilian isolate at high inoculum density, and respectively 17% and 25% at low inoculum density. We also measured dispersal from inoculated plants and found that spore dispersal increased with host infection levels, independent of host densities, whereas mite dispersal varied between isolates. These results demonstrate that epizootiology of N. tanajoae can be studied under controlled conditions and suggest that virulence tests at the population level may help to better predict performance of fungal isolates than individual-level tests

Parasitoid complex of diamondback moth in south-east Queensland: first records of Oomyzus sokolowskii (Hymenoptera : Eulophidae) in Australia

Surveys for parasitoids attacking larvae and pupae of Plutella xylostella L. (Lepidoptera: Plutellidae) were conducted in the Brassica agro-ecosystem of the Lockyer valley, south-east Queensland between 1999 and 2002. Parasitised hosts were collected from infested plants placed in Brassica crops during the growing season (February-October) and from the natural P. xylostella population on the common weed Rapistrum rugosum L. (Brassicaceae) during the summer production break (November-January). Five species of primary parasitoids were recorded and they ranked in order of abundance: Diadegma semiclausum Hellen (Hymenoptera: Ichneumonidae) > Diadromus collaris Gravenhorst (Hymenoptera: Ichneumonidae) = Apanteles ippeus Nixon (Hymenoptera: Braconidae) > Brachymeria phya Walker (Hymenoptera: Chalcididae) = Oomyzus sokolowskii Kurdjumov (Hymenoptera: Eulophidae). In crops, parasitoids were reared from 23 of the 30 P. xylostella cohorts studied; D. semiclausum was collected at 21 sites, O. sokolowskii at eight sites, D. collaris at six sites, A. ippeus at three sites and B. phya at a single site. The incidence and abundance of hosts parasitised by D. semiclausum increased markedly in winter and early spring months and parasitism rates as high as 95% were recorded. Diadromus collaris and O. sokolowskii were recovered at low levels throughout 2000 and 2001 but parasitism rates never exceeded 11% or 8%, respectively. Parasitism rates are compared with other mortality factors and the contribution that each species of parasitoid makes to the biological control of the pest in the region is considered. Oomyzus sokolowskii has also been reared from P. xylostella collected in Tasmania and Western Australia. This study is the first report of the incidence O. sokolowskii in Australia; however, its wide distribution indicates that its establishment is not a recent event

Challenges in modelling complexity of fungal entomopathogens in semi-natural populations of insects

The use of fungal entomopathogens as microbial control agents has driven studies into their ecology in crop ecosystems. Yet, there is still a lack of understanding of the ecology of these insect pathogens in semi-natural habitats and communities. We review the literature on prevalence of fungal entomopathogens in insect populations and highlight the difficulties in making such measurements. We then describe the theoretical host-pathogen models available to examine the role that fungal entomopathogens could play in regulating insect populations in semi-natural habitats, much of the inspiration for which has been drawn from managed systems, particularly forests. We further emphasise the need to consider the complexity, and particularly the heterogeneity, of semi-natural habitats within the context of theoretical models and as a framework for empirical studies. We acknowledge that fundamental gaps in understanding fungal entomopathogens from an ecological perspective coupled with a lack of empirical data to test theoretical predictions is impeding progress. There is an increasing need, especially under current rapid environmental change, to improve our understanding of the role of fungi in insect population dynamics beyond the context of forestry and agriculture