Efficacy of the Applied Natural Enemies on the Survival of Colorado Potato Beetle Adults

Isolation and identification of entomopathogenic fungi strains for Colorado potato beetle (Leptinotarsa decemlineata) control

AIMS

The Colorado potato beetle, Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae) is the most widespread insect pest that causes major economic losses, especially on potatoes. Due to heavy insecticide use, this species now resists most pesticides, posing a significant control challenge. Frequent pesticide application also harms non-target organisms, the environment, and human health. Hence, utilizing biocontrol agents like entomopathogenic fungi (EPF) offers a viable alternative for pest management. The aim of this study was to identify and characterize new EPF strains isolated from soil samples and evaluate their efficacy against adult L. decemlineata under laboratory conditions.

METHODS AND RESULTS

Soil samples were collected in potato fields or uncultivated areas adjacent to the field in the Czech Republic and the EPF strains were isolated using a modified Tenebrio bait method. A total of 20 fungal strains were isolated and identified using morphological and molecular markers based on the 28S rRNA, ITS, and elongation factor 1-alpha gene sequences as Beauveria bassiana (Bals.-Criv.) Vuill., Beauveria brongniartii (Sacc.) Petch, and Cordyceps fumosorosea (Wize) Kepler, B. Shrestha & Spatafora (Hypocreales: Cordycipitaceae), Purpureocillium lilacinum (Thom.) Luangsa-ard, Houbraken, Hywel-Jones & Samson (Hypocreales: Ophiocordycipitaceae), Metarhizium brunneum (Petch), and Metarhizium robertsii Bisch., Rehner & Humber (Hypocreales: Clavicipitaceae). The bioassays revealed high variability among virulence of these strains against L. decemlineata with the shortest median time to death (LT50 = 5 days) in M. robertsii strain MAN3b.

CONCLUSIONS

Results shown that some EPF strains, particularly those of genera Metarhizium, can be promising biocontrol agents against the Colorado potato beetle.

Interactions between Entomopathogenic Fungi and Entomopathogenic Nematodes

Entomopathogenic fungi and entomopathogenic nematodes are globally distributed soil organisms capable of infecting and killing a vast variety of insects. Therefore, these organisms are frequently used as biocontrol agents in insect pest management. Both entomopathogenic fungi and nematodes share the soil environment and thus can infest and compete for the same insect host; however, natural co-infections are rarely found due to the cryptic soil environment. Our current knowledge on their interactions within hosts mainly comes from laboratory experiments. Because of the recent trend of combining biocontrol agents to increase their efficacy, many studies have focused on the co-application of different species of EPF and EPNs against various insect pests with variable outcomes ranging from synergistic effects and additive effects to antagonism. In addition, the effect on the development and reproduction of each pathogen varies from normal reproduction to exclusion, and generally the outcomes of the interactions are dependent on pathogen and host species, pathogen doses, and the timing of infection. The present review aims to summarize the current knowledge on the interactions of entomopathogenic fungi and nematodes within an insect host and to estimate the possible effects of the interactions on natural pathogen populations and on their use in biocontrol.

Role of adipokinetic hormone in the Colorado potato beetle, Leptinotarsa decemlineata infected with the entomopathogenic nematode Steinernema carpocapsae

The effects of the entomopathogenic nematode Steinernema carpocapsae on the Colorado potato beetle (CPB) Leptinotarsa decemlineata and the involvement of adipokinetic hormone (AKH) in the responsive reactions were examined in this study. It was observed that nematode application doubled the amount of AKH (Peram-CAH-I and Peram-CAH-II) in the central nervous system of L. decemlineata, indicating mobilization of anti-stress reactions in the body. Furthermore, the external co-application of Peram-CAH-II with the nematode significantly increased beetle mortality (5.6 and 1.8 times, 1 and 2 days after application, respectively). The mechanism underlying this phenomenon was investigated. As the effect on gut characteristics was equivocal, it was assumed that the nematodes profited from the observed mobilization of metabolites from the fat body into the Peram-CAH-II-induced hemolymph. This phenomenon supplied nematodes with a more nutrient-dense substrate on which they propagated. Furthermore, Peram-CAH-II lowered vitellogenin expression in the fat body, particularly in males, thus limiting the anti-pathogen defense capacity of the protein. However, there could be other possible mechanisms underpinning this chain of events. The findings could be theoretically intriguing but could also aid in developing real insect pest control methods in the future.

Steinernema Africanum n. Sp. (Rhabditida, Steinernematidae), a New Entomopathogenic Nematode Species Isolated in the Republic of Rwanda

Alternatives to hazardous insecticides are urgently needed for an environmentally friendly and effective management of insect pests. One such option is the use of entomopathogenic nematodes (EPN). To increase the availability of EPN with potential for biocontrol, we surveyed agricultural soils in the Republic of Rwanda and collected two Steinernema isolates. Initial molecular characterization showed that they represent a new species, for which we propose the name S. africanum n. sp. To describe this new species, we reconstructed phylogenetic relationships, calculated sequence similarity scores, characterized the nematodes at the morphological level, conducted crossing experiments, and isolated and characterized their symbiotic bacteria. At the molecular level, S. africanum n. sp. is closely related to S. litorale and S. weiseri. At the morphological level, S. africanum n. sp. differs from closely related species by the position of the nerve ring and also because the stoma and pharynx region is longer. The first-generation males have ventrally curved spicules with lanceolate manubrium and fusiform gubernaculum and the second-generation males have rounded manubrium and anteriorly hook-like gubernaculum. Steinernema africanum n. sp. does not mate or produce fertile progeny with any of the closely related species.