Efficacy of Bacillus thuringiensis (var. kurstaki) Against Diamondback Moth (Plutella xylostella L.) Eggs and Larvae on Cabbage Under Semi-Controlled Greenhouse Conditions

Entomopathogen-based biopesticides: insights into unraveling their potential in insect pest management

Global food security is a critical challenge to fulfill the demands of an exponentially growing population. To date, growers rely on chemicals; the broad-spectrum application of synthetic molecules leads to environmental contamination, resistance development, residual toxicity, pest resurgence, and a detrimental effect on human health and cattle. Crop production needs to be improved considering environmental and human health concerns to ensure food security. Furthermore, economically important crops are prone to attack by insect pests, causing considerable yield losses. Microbes are an eco-friendly, versatile alternative, and a potential candidate for combatting destructive pests below the economic injury level and improving the plant's health and productivity. Several microbial pathogens, including parasites, predators, parasitoids, pollinators, and many beneficial microorganisms, possess toxic properties against target organisms but do not cause harm to the non-target organisms. Entomopathogens (ENMs) have great potential for pest suppression due to their remarkable properties. Bacteria are host-specific, but fungi have a broader host range and can be significantly affected by both soil-dwelling and terrestrial insect pests. Virulent pathogens cause mortality in target insect pests known as ENMs and can penetrate through natural openings, ingestions, and integuments to cause a possible effect on target insect pests. The objective of using ENMs is to sustain productivity, improve environmental health, reduce pesticides, and conserve natural resources. Moreover, research is ongoing to discover other possible aspects, especially exploring potential ENMs. Therefore, there is a need for identification, isolation, and bioformulation to overcome the existing issues. This study is mainly focused on the status of bio-formulations, pathogenicity, their mode of action, and the potential application of different types of microbial formulations for sustainable pest management.

A major forest insect pest, the pine weevil Hylobius abietis, is more susceptible to Diptera- than Coleoptera-targeted Bacillus thuringiensis strains

BACKGROUND

The pine weevil (Hylobius abietis) is a major forest regeneration pest causing high levels of seedling mortality and economic losses. Current management relies on silviculture, stem coatings and insecticides. Here we evaluated for the first time the effects of Bacillus thuringiensis (Bt) strains on H. abietis adults: two producing the Coleoptera-targeted toxins Cry3Aa (Bt tenebrionis NB-176) and Cry8Da (Bt galleriae SDS-502), and one producing the Diptera-targeted Cry10A (Bt israelensis AM65-52). Choice and nonchoice assays using individual and mixtures of Bt formulations, containing these strains respectively, were conducted.

RESULTS

We found that Bt had toxic and lethal effects on H. abietis, but effects varied with strain and formulation concentration. The Diptera-targeted Bt israelensis had the most negative effects on weevil weight, feeding and mortality (70-82% feeding reduction, 65-82% greater mortality than control), whereas the effect was lower for the Coleoptera-specific Bt tenebrionis (38-42%; 37-42%) and Bt galleriae (11-30%; 15-32%). Reduced weevil feeding was observed after 3 days, and the highest mortality occurred 7-14 days following Bt exposure. However, we found no synergistic toxic effects, and no formulation combination was better than Bt israelensis alone at reducing consumption and survival. Also, pine weevils were not deterred by Bt, feeding equally on Bt-treated and non-Bt treated food.

CONCLUSION

There is potential to develop forest pest management measures against H. abietis that include Bt, but only the Diptera-targeted Bt israelensis would provide effective seedling protection. Its Diptera-specificity may need reconsideration, and evaluation of other Bt strains/toxins against H. abietis would be of interest. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

An entomopathogenic bacterium strain, Bacillus thuringiensis, as a biological control agent against the red palm weevil, Rhynchophorus ferrugineus (Coleoptera: Curculionidae)

BACKGROUND

The red palm weevil (RPW), Rhynchophorus ferrugineus, is an invasive wood-boring insect that damages palms and sugarcane. Bacillus thuringiensis (Bt) is an entomopathogenic bacterium which has been modified into various strains and widely used in pest management. The aim of this study was to evaluate the susceptibility of RPW to the HA strain of Bt.

RESULTS

Five concentrations of Bt bioassays were used on RPW eggs, second instars and fourth instars. Average egg hatching rates exceeded 85% using Bt suspensions or distilled water. Hatch times were extended significantly using higher Bt concentrations. For second instar larvae, the LC₅₀ was 4.92 × 10⁹ CFU mL^-1 15 d after feeding; the LT₅₀ values decreased with each higher concentration. The corrected mortality of second instars increased significantly with increased concentrations after 15 d, ranging from 16.97% to 94.32%. Significant differences occurred in the boring activity of fourth instars when dipped in Bt suspensions or crawling on treated sugarcane. Bacterial infection in dead larvae was confirmed using molecular techniques.

CONCLUSION

Our results indicated that Bt can be used in RPW control as a potential biological control agent and can effectively reduce palm trees damage. © 2016 Society of Chemical Industry.