O. N. Yaroslavtseva

More than a colour change: insect melanism, disease resistance and fecundity

A ‘dark morph’ melanic strain of the greater wax moth, Galleria mellonella, was studied for its atypical, heightened resistance to infection with the entomopathogenic fungus, Beauveria bassiana. We show that these insects exhibit multiple intraspecific immunity and physiological traits that distinguish them from a non-melanic, fungus-susceptible morph. The melanic and non-melanic morphs were geographical variants that had evolved different, independent defence strategies. Melanic morphs exhibit a thickened cuticle, higher basal expression of immunity- and stress-management-related genes, higher numbers of circulating haemocytes, upregulated cuticle phenoloxidase (PO) activity concomitant with conidial invasion, and an enhanced capacity to encapsulate fungal particles. These insects prioritize specific augmentations to those frontline defences that are most likely to encounter invading pathogens or to sustain damage. Other immune responses that target late-stage infection, such as haemolymph lysozyme and PO activities, do not contribute to fungal tolerance. The net effect is increased larval survival times, retarded cuticular fungal penetration and a lower propensity to develop haemolymph infections when challenged naturally (topically) and by injection. In the absence of fungal infection, however, the heavy defence investments made by melanic insects result in a lower biomass, decreased longevity and lower fecundity in comparison with their non-melanic counterparts. Although melanism is clearly correlated with increased fungal resistance, the costly mechanisms enabling this protective trait constitute more than just a colour change.

Can insects develop resistance to insect pathogenic fungi

Microevolutionary adaptations and mechanisms of fungal pathogen resistance were explored in a melanic population of the Greater wax moth, Galleria mellonella. Under constant selective pressure from the insect pathogenic fungus Beauveria bassiana, 25th generation larvae exhibited significantly enhanced resistance, which was specific to this pathogen and not to another insect pathogenic fungus, Metarhizium anisopliae. Defense and stress management strategies of selected (resistant) and non-selected (susceptible) insect lines were compared to uncover mechanisms underpinning resistance, and the possible cost of those survival strategies. We hypothesize that the insects developed a transgenerationally primed resistance to the fungus B. bassiana, a costly trait that was achieved not by compromising life-history traits but rather by prioritizing and re-allocating pathogen-species-specific augmentations to integumental front-line defenses that are most likely to be encountered by invading fungi. Specifically during B. bassiana infection, systemic immune defenses are suppressed in favour of a more limited but targeted repertoire of enhanced responses in the cuticle and epidermis of the integument (e.g. expression of the fungal enzyme inhibitor IMPI, and cuticular phenoloxidase activity). A range of putative stress-management factors (e.g. antioxidants) is also activated during the specific response of selected insects to B. bassiana but not M. anisopliae. This too occurs primarily in the integument, and probably contributes to antifungal defense and/or helps ameliorate the damage inflicted by the fungus or the host’s own immune responses.

Synergistic action of entomopathogenic hyphomycetes and the bacteria Bacillus thuringiensis ssp. morrisoni in the infection of Colorado potato beetle Leptinotarsa decemlineata

A synchronous coinfection of the Colorado potato beetle Leptinotarsa decemlineata (Say) with the entomopathogenic bacteria Bacillus thuringiensis ssp. morrisoni Bonnifoi & de Barjak var. tenebrionis Krieg et al. and hyphomycete Metarhizium anisopliae (Metsch.) Sorokin or Beauveria bassiana (Bals.) Vuill leads to the rapid death of 95–100% of larvae. The bacteria arrest the nutrition of insects, while the fungal spores kill the weakened larvae. The synergistic effect of two pathogens is recorded at a relatively low hyphomycete titer (1–5 × 106 conidia/ml) and is evident in the mortality dynamics at all larval ages. These bacterial and fungal pathogens display no antagonism on artificial nutrient media. This microbial complex is highly efficient under natural conditions (80–90% larval mortality rate and no plant defoliation).

The activity of nonspecific esterases and glutathione-S-transferase in Locusta migratoria larvae infected with the fungus Metarhizium anisopliae (Ascomycota, Hypocreales)

The activity of nonspecific esterases and glutathione-S-transferase in whole body homogenates, hemolymph plasma, and fat body of the larvae of the locust Locusta migratoria was analyzed during development of infection with the fungus Metarhizium anisopliae. The lethal dose of the fungus (LC80) was found to enhance the activity of detoxifying enzymes in the whole body homogenate of the larvae on the 3rd day after infection. The activity of nonspecific esterases and glutathione-S-transferase in the plasma and fat body of the infected larvae increased on the 3rd day but dropped to the control levels by the 6th day, during the acute period of infection. The detoxifying enzymes may participate in defense reactions at the early stage of the acute fungal infection.

An increase in the immune system activity of the wax moth Galleria mellonella and of the Colorado potato beetle Leptinotarsa decemlineata under effect of organophosphorus insecticide

Several aspects of immune response in insects treated with an organophosphate (pirimiphosmethyl) have been assessed. Both humoral (phenoloxidase activity) and cellular (hemocyte count and encapsulation rate) immunity changes have been studied in larvae of the representatives of two insect orders, the Colorado potato beetle Leptinotarsa decemlineata (Coleoptera, Chrysomelidae) and the wax moth Galleria mellonella (Lepidoptera, Pyralidae). A direct contact with sublethal and half-lethal doses of the insecticide results in stimulation of immune reactions: phenoloxidase activity and encapsulation rate are heightened, and hemocyte count increases.

Change in the temperature preferences of Beauveria bassiana sensu lato isolates in the latitude gradient of Siberia and Kazakhstan

The radial growth of twenty isolates of the entomopathogenic fungus Beauveria bassiana sensu lato from different natural zones of Western Siberia and Kazakhstan (from 65 to 43°N) was tested under different temperatures (5–35°C). It was shown that the thermotolerance of the fungal isolates increased significantly from the north to south. The cold activity of the cultures did not significantly correlate with the latitude of origin and the sum positive temperatures of the regions. A distinct group of the steppe thermotolerance isolates was shown by the analysis of genomic polymorphism using seven intermicrosatellite DNA markers (ISSR). The steppe isolates had high levels of virulence to the wax moth Galleria mellonella and the Colorado potato beetle Leptinotarsa decemlineata at high temperatures (>30°C) compared to that of the forest-steppe isolates. The obtained data indicate that the use of isolates from the steppe zone will be most promising for the insect pest control under the conditions of continental and arid climate.

Insecticidal and immunosuppressive effect of ascomycete Cordyceps militaris on the larvae of the Colorado potato beetle Leptinotarsa decemlineata.

The immunosuppressive and insecticidal activity of culture of the entomopathogenic fungus Cordyceps militaris on the larvae of the Colorado potato beetle Leptinotarsa decemlineata has been established for the first time. It was found that the peroral effect of the fungal culture resulted in dose-dependent decrease in survival, delayed in development time and molting, decreases in the total hemocyt counts, increased activity of phenoloxidases in the hemolymph, and reduced activity of the enzyme in the cuticle, as well as increased sensitivity of larvae to the fungus Beauveria bassiana at the level of the synergistic effect.

Dominating species of entomophilous ascomycetes anamorphs in West Siberia, Primorsky krai, and Kyrgyzstan

The paper studies mycobiota of the dead insects in West Siberia, Primorsky krai, and Kyrgyzstan. Ascomycetes anamorphs of 13 genera are revealed. In all regions Beauveria bassiana (Bals.-Criv.) Vuill. dominated comprising on average 68% of the total number of isolates. The fungus hosts list the insects of 7 orders and 32 families with Coleoptera, Lepidoptera, and Hemiptera dominating. The rarely found entomopathogens include Tolypocladium inflatum Gams (primarily on Lepidoptera), Metarhizium anisopliae (Metschn.) Sorokin (on Coleoptera). The mortality rate of the insects due to micromycetes is observed mainly on enzootic level. The study of the pathogenic properties of the dominating species (B. bassiana) show the absence of specificity of its environmental isolates for a number of representatives of Orthoptera, Lepidoptera, Coleoptera, and Diptera.

Immunological mechanisms of synergy between fungus Metarhizium robertsii and bacteria Bacillus thuringiensis ssp. morrisoni on Colorado potato beetle larvae

The synergistic effect between the entomopathogenic fungus Metarhizium robertsii and a sublethal dose of the bacterium Bacillus thuringiensis ssp. morrisoni var. tenebrionis was studied in terms of immune defense reactions and detoxification system activity of the Colorado potato beetle, Leptinotarsa decemlineata, fourth instar larvae. Bacterial infection led to more rapid germination of fungal conidia on integuments. We found a significant decrease of cellular immunity parameters, including total hemocyte count and encapsulation response, under the influence of bacteria. Phenoloxidase activity in integuments was increased under bacteriosis, mycosis and combined infection compared to controls. However, phenoloxidase activity in the hemolymph was enhanced under bacteriosis alone, and it was decreased under combined infection. Activation of both nonspecific esterases and glutathione-S-transferases in the hemolymph was shown at the first day of mycosis and third day of bacteriosis. However, inhibition of detoxification enzymes was detected under combined infection. The suppression of cellular immunity and detoxification reactions in Colorado potato beetle larvae with a sublethal dose of bacteria is discussed as a reason for synergy between B. thuringiensis and M. robertsii.

Local epizootics caused by teleomorphic cordycipitoid fungi ( Ascomycota: Hypocreales ) in populations of forest lepidopterans and sawflies of the summer-autumn complex in Siberia

Unique epizootic loci were revealed in the summer-autumn complex of the multispecies communities of forest lepidopterans and sawflies in two regions of Novosibirsk oblast and in the Baikal region at the border of the Buryat Republic and Irkutsk oblast. Mass insect mortality was caused by two species of cordycipitoid fungi: Cordyceps militaris and Cordyceps sp. At least 30 species from 7 families of millers (Macroheterocera) and sawflies from the family Cimbicidae were found to be C. militaris hosts. Lepidopterans from the family Thyatiridae are the hosts of the second species, Cordyceps sp. Total mortality was noted for the condition close to optimal for pupation, i.e., in places of mass pupa accumulation. It may be assumed on the basis of the results of the laboratory experiments that, under natural conditions, host insects may be infected by ascospores and conidia at the anamorphic stage.