Toxicity of microbial insecticides toward the non‐target freshwater insect Chironomus xanthus

Abstract

BACKGROUND Commercial formulations based on Bacillus thuringiensis subs. kurstaki (Btk) and Beauveria bassiana (Bb) are commonly used microbial insecticides in Brazil and other tropical regions. However, and despite being considered environmentally friendly, their use generates concerns regarding possible adverse ecological effects in freshwater ecosystems. Here, we evaluate the effects of these bioinsecticides on the tropical aquatic dipteran Chironomus xanthus under laboratory conditions. RESULTS After laboratory exposures to these compounds 48‐h median lethal concentration (LC50) values of 1534 μg a.i./L for Btk and of 6.35 μg a.i./L for Bb were estimated. Chronic assays revealed different sublethal effects: Btk‐based bioinsecticide exposure reduced C. xanthus growth [lowest observed effect concentration (LOEC) was 126 μg a.i./L for head width], decreased emergence rate (LOEC = 8 μg a.i./L) and increased immunological response (LOEC = 50 μg a.i./L) measured as total hemocyte count in larvae hemolymph. Exposure to low concentrations of Bb‐based insecticide also reduced C. xanthus growth (LOEC = 0.07 μg a.i./L for larvae body length measurements), and emergence rate (LOEC = 0.28 μg a.i./L), despite no clear effects on the total hemocyte counts. CONCLUSION Our results suggest that low concentrations of Btk and Bb bioinsecticides are toxic to C. xanthus. Given their widespread use and occurrence in tropical freshwater systems, research is needed to evaluate the potential effects of these compounds concerning natural freshwater insect communities and ecosystem functioning. © 2019 Society of Chemical Industry

The combined effects of microbial insecticides and sodium chloride on the development and emergence of Chironomus xanthus

“…After the larvae of C. xanthus were exposed to 8, 20, 50 and 126 μg a. i./L of Btk, there were reductions in adult emergence of 76%, 72%, 60% and 56% compared to control. 12 One week after application, 1740 International Toxic Units (ITU)/L of Bti in mesocosm caused a 59% reduction in emergence compared to the control, 55 which is in good agreement with our study. Ecotoxicological data concerning the effects of Bb on nontarget species are still scarce in the literature, with only a few studies showing the toxicity of this fungus to larvae of target mosquitoes that are carriers of disease-causing pathogens.…”

“…The mean time to emergence of adult C. xanthus exposed to Btk and Bb (in the absence of NaCl) was significantly affected at all concentrations tested, whereas at similar concentrations Dornelas et al 12 . did not observe a significant effect on time to emergence for either bioinsecticide.…”

“…The control of insect pests in sustainable agriculture has led to the development of biologically based compounds with low environmental impact 2–4 . The interest in biological control products to eliminate pests has gained much attention since the 1980s as promising environmentally friendly and sustainable options to overcome the undesirable effects of chemical pesticides 5–13 …”

“…Chironomids have a short life cycle and in their larval stages live in contact with sediments, playing an important role in the transformation of particulate organic matter in addition to serving as food for higher trophic levels 36,37 . The tropical species C. xanthus has been shown to be sensitive to Btk (LC 50 = 1534 μg a.i./L) and Bb (LC 50 = 6.35 μg a.i./L) 12 …”

A temporal perspective on aquatic subsidy: Bti affects emergence of Chironomidae

“…We initially hypothesized lighter individuals due to stress-induced energy allocation from storage to increased maintenance costs and repair mechanisms (Kooijman, 2000;Saraiva et al, 2020;Sokolova et al, 2012). In fact, several studies documented higher energetic costs as a consequence of immunological or biochemical stress responses leading to retarded growth and delayed emergence of individual species (Bordalo et al, 2020;Saraiva et al, 2017;Silvestre Pereira Dornelas et al, 2020). Thus, increased resource availability in a much more complex benthic and in particular chironomid community within our FPMs could have concealed potential energy-demanding effects of Bti.…”