Jørgen Eilenberg

Trade-Offs in Group Living: Transmission and Disease Resistance in Leaf-Cutting Ants

Sociality can be associated with significant costs due to the increased risk of disease transmission. However, in some organisms the costs may be offset by benefits due to improvements in defences against parasites. To examine this possible trade–off between infection risk and disease resistance, we used Acromyrmex leaf–cutting ants and the entomopathogenic fungus Metarhizium anisopliae as the model system. Ants exposed to the parasite were found to have substantially improved survival when they were kept with nest–mates, while the cost of being in a group in terms of increased disease transmission was very low. The efficiency of transmission is described by the transmission parameter, which decreased with increasing host density showing that transmission rates are inversely density dependent. Both grooming and antibiotic secretions appeared to be important in resistance against the parasite, with the defences of small workers being particularly effective. The results indicate that leaf–cutting ant colonies may have much greater resistance to disease than would be predicted from the high densities of host individuals within them. Unlike most organisms, group living in these ants may actually be associated with a net benefit in terms of disease dynamics.

Pathogenicity of the Hyphomycete Fungi Verticillium lecanii and Metarhizium anisopliae to the Western Flower Thrips, Frankliniella occidentalis

The pathogenicity of several isolates of the hyphomycete fungi Verticillium lecanii and Metarhizium anisopliae to Frankliniella occidentalis was investigated. Treatment of adult thrips with M. anisopliae resulted in at least 94% mortality at 7 days post-inoculation. In contrast, V. lecanii isolates only gave mortalities of between 20 and 70%. Detailed studies were made on the most virulent isolate of M. anisopliae (275) to determine its efficacy at different doses and temperatures. At 23 C the LC was ca. 3 105 conidia ml-1 after 5 days and the LTs were 50 50 3 and 4.5 days at 10 7 and 106 conidia ml-1 respectively. Temperature influenced fungal virulence to adult thrips; the LT at 18 and 20 C was ca. 4 days and at 23 or 26 C it was ca. 3 days. 50 Larvae were less susceptible to infection than adult thrips (27% versus 100% mortality), presumably due to the inoculum being shed with the exuvium during ecdysis. Conidia of M. anisopliae isolate 275 germinated rapidly on the surfaces of larvae, pupae and adults...

Effects of farming system, field margins and bait insect on the occurrence of insect pathogenic fungi in soils

Abstract A method for baiting soil samples with Delia floralis larvae was developed, and a systematic survey was conducted on soils from northern Norway for insect pathogenic fungi, using D. floralis and Galleria mellonella larvae as bait. The occurrence of insect pathogenic fungi in soils from arable fields and adjacent field margins of conventionally and organically managed farms was compared. The study showed a significantly higher occurrence of insect pathogenic fungi in soils from arable fields of organically managed farms. No significant differences in the occurrence of insect pathogenic fungi were, however, found between the field margins of the two cropping systems. Fungal species identified in the study were Beauveria bassiana, Fusarium merismoides, Metarhizium anisopliae and Tolypocladium cylindrosporum. T. cylindrosporum was found more frequently when using D. floralis as the bait insect than when using G. mellonella.

Community composition, host range and genetic structure of the fungal entomopathogen Beauveria in adjoining agricultural and seminatural habitats

Although intensively investigated for biological control of insect pests, little is known about the ecology of the fungal entomopathogenic genus Beauveria in natural or agricultural habitats. In this study, we used molecular phylogenetic and genotypic information to infer species diversity, reproductive potential and genetic structure of Beauveria occurring within a single arable field and bordering hedgerow in Denmark. Isolates were sampled from cultivated field and hedgerow soils, from insects harbouring latent fungal infections, and from the phylloplanes of three plant species common in the hedgerow flora. A nuclear phylogeny of this local Beauveria assemblage resolved seven phylogenetic species, including (i) five phylogenetic species within Beauveria bassiana sensu stricto; (ii) Clade C, a taxonomically uncharacterized species that is morphologically indistinguishable but phylogenetically distant from B. bassiana s.s.; and (iii) Beauveria brongniartii. All seven species were present throughout the hedgerow habitat, including as infections in insects. Significantly, only B. bassiana s.s. phylogenetic species Eu_1 was isolated from tilled soils. Mating type polymerase chain reaction assays demonstrated that all five B. bassiana s.s. phylogenetic species possess bipolar outcrossing mating systems. Of these, only the Eu_1 population contained two mating types; however, a 31:2 skew in MAT1:MAT2 mating types suggests a low frequency of sexual reproduction in this population. The four remaining B. bassiana s.s. phylogenetic species were fixed for single mating types and these populations are evidently clonal. Multilocus microsatellite genotyping revealed polymorphism in all five phylogenetic species of B. bassiana s.s.; however, all show evidence of clonal genetic structure.

The Evolution of Invasiveness in Garden Ants

It is unclear why some species become successful invaders whilst others fail, and whether invasive success depends on pre-adaptations already present in the native range or on characters evolving de-novo after introduction. Ants are among the worst invasive pests, with Lasius neglectus and its rapid spread through Europe and Asia as the most recent example of a pest ant that may become a global problem. Here, we present the first integrated study on behavior, morphology, population genetics, chemical recognition and parasite load of L. neglectus and its non-invasive sister species L. turcicus. We find that L. neglectus expresses the same supercolonial syndrome as other invasive ants, a social system that is characterized by mating without dispersal and large networks of cooperating nests rather than smaller mutually hostile colonies. We conclude that the invasive success of L. neglectus relies on a combination of parasite-release following introduction and pre-adaptations in mating system, body-size, queen number and recognition efficiency that evolved long before introduction. Our results challenge the notion that supercolonial organization is an inevitable consequence of low genetic variation for chemical recognition cues in small invasive founder populations. We infer that low variation and limited volatility in cuticular hydrocarbon profiles already existed in the native range in combination with low dispersal and a highly viscous population structure. Human transport to relatively disturbed urban areas thus became the decisive factor to induce parasite release, a well established general promoter of invasiveness in non-social animals and plants, but understudied in invasive social insects.

Natural occurrence of Bacillus thuringiensis on cabbage foliage and in insects associated with cabbage crops

Bacillus thuringiensis was isolated from the phylloplane of organically grown cabbage in one field during two growth seasons (1992‐93). The frequency of B. thuringiensis varied between 0.02 and 0.67 of the total B. cereus/B. thuringiensis population, with an average of 0.11. Characterization of the B. thuringiensis isolates from foliage showed that the majority (64% of 150 isolates) belonged to serovar kurstaki, had bipyramidal crystals and toxicity towards Pieris brassicae and/or Trichoplusia ni. Other serovars were also found on the foliage but occurred at very low frequencies (one to three isolates of each serovar). Bacillus thuringiensis was also isolated from insects associated with the cabbage crop (Pieris rapae (Lep.), Delia radicum (Dip.), Syrphidae ribesii (Dip.) and Aleochara bilineata (Col.)), which were collected alive at different developmental stages in the same field. Serologically these isolates were assigned to the serovars kurstaki, aizawai, tochigiensis, colmeri and indiana/colmeri.

Evolutionary Interaction Networks of Insect Pathogenic Fungi

Lineages of insect pathogenic fungi are concentrated in three major clades: Hypocreales (several genera), Entomophthoromycota (orders Entomophthorales and Neozygitales), and Onygenales (genus Ascosphaera). Our review focuses on aspects of the evolutionary biology of these fungi that have remained underemphasized in previous reviews. To ensure integration with the better-known domains of insect pathology research, we followed a conceptual framework formulated by Tinbergen, asking complementary questions on mechanism, ontogeny, phylogeny, and adaptation. We aim to provide an introduction to the merits of evolutionary approaches for readers with a background in invertebrate pathology research and to make the insect pathogenic fungi more accessible as model systems for evolutionary biologists. We identify a number of questions in which fundamental research can offer novel insights into the evolutionary forces that have shaped host specialization and life-history traits such as spore number and size, somatic growth rate, toxin production, and interactions with host immune systems.

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.

Soil as an environment for winter survival of aphid-pathogenic Entomophthorales

The survival of Pandora neoaphidis was studied for both discharged primary conidia and hyphal bodies inside aphid cadavers after storage on moist soil at different temperatures. The activity of the inoculum was quantified by the ability to produce replicate conidia as well as the ability to infect aphids. No effect of inoculum type was found. Conidia were produced after storage for at least 32 days at 20C, 64 days at 10C, and 96 days at 5C. Inoculum retained the ability to initiate infections in aphids after storage for at least 14 days at 20C, 32 days at 10C, and 64 days at 5C. Morphological studies of the inoculum suggest that P. neoaphidis may survive unfavorable conditions as thick-walled conidia also known as loricoconidia. Furthermore, P. neoaphidis and Conidiobolus obscurus were documented for the first time in field-collected soil in early spring by baiting the soil with aphids. We hypothesize that germination of overwintering inoculum is stimulated by host-induced factors since inoculum apparently responded to the presence of aphids. 2003 Elsevier Science (USA). All rights reserved.

Time-Concentration Mortality of Pieris brassicae (Lepidoptera: Pieridae) and Agrotis segetum (Lepidoptera: Noctuidae) Larvae from Different Destruxins

Abstract Mortality of first instars of Pieris brassicae (L.) and Agrotis segetum (Schiffermüller) exposed to crude destruxin extracts per os was analyzed by time-concentration-mortality regressions based on the complementary log-log (CLL) model, and was compared with the mortality from pure destruxins A and E and the synthetic analog hpy-6 destruxin E. The model described the temporal course of mortality for both insects exposed to the different destruxins. Based on estimated LC50 and LT50 values, P. brassicae was much more susceptible to destruxins than A. segetum. Destruxin E seemed to be the most potent against P. brassicae, followed by the synthetic analog hpy-6 destruxin E, and destruxin A was least active. The results of the toxicity of pure destruxins against P. brassicae provide a basis for further investigations on destruxins as control agents against this insect. A. segetum larvae were only weakly susceptible to destruxins and the observed mortality of the larvae seemed to be a result of starvation caused by an antifeedant effect of the destruxins rather than by direct toxicity. Overall, the per os bioassays point toward a potential for possible future use of destruxins as control agent against lepidopteran pest larvae. The results do, however, also document, that the activity level of destruxins depends highly on the target insect species.