Responses of benthic macroinvertebrate communities to a Bti-based insecticide in artificial microcosm streams

Effects of chironomid larvae density and mosquito biocide on methane and carbon dioxide dynamics in freshwater sediments

Small lentic water bodies are important emitters of methane (CH4) and carbon dioxide (CO2), but the processes regulating their dynamics and susceptibility to human-induced stressors are not fully understood. Bioturbation by chironomid larvae has been proposed as a potentially important factor controlling the dynamics of both gases in aquatic sediments. Chironomid abundance can be affected by the application of biocides for mosquito control, such as Bti (Bacillus thuringiensis var. israelensis). Previous research has attributed increases in CH4 and CO2 emissions after Bti application to reduced bioturbation by chironomids. In this study, we separately tested the effect of chironomid bioturbation and Bti addition on CH4 production and emission from natural sediments. In a set of 15 microcosms, we compared CH4 and CO2 emission and production rates with high and low densities of chironomid larvae at the bioturbating stage, and standard and five times (5x) standard Bti dose, with control sediments that contained neither chironomid larvae nor Bti. Regardless of larvae density, chironomid larvae did not affect CH4 nor CO2 emission and production of the sediment, although both rates were more variable in the treatments with organisms. 5xBti dosage, however, led to a more than three-fold increase in CH4 and CO2 production rates, likely stimulated by bioavailable dissolved carbon in the Bti excipient and priming effects. Our results suggest weak effects of bioturbating chironomid larvae on the CH4 and CO2 dynamics in aquatic ecosystems. Furthermore, our results point out towards potential functional implications of Bti for carbon cycling beyond those mediated by changes in the macroinvertebrate community.

Impact across ecosystem boundaries - Does Bti application change quality and composition of the diet of riparian spiders?

Emerging aquatic insects link aquatic and adjacent terrestrial food webs by subsidizing terrestrial predators with high-quality prey. One of the main constituents of aquatic subsidy, the non-biting midges (Chironomidae), showed altered emergence dynamics in response to the mosquito control agent Bacillus thuringiensis var. israelensis (Bti). As riparian spiders depend on aquatic subsidy, they may be affected by such changes in prey availability. Thus, we conducted a field study in twelve floodplain pond mesocosms (FPMs), six were treated with Bti (2.88 × 10⁹ ITU/ha, VectoBac WDG) three times, to investigate if the Bti-induced shift in chironomid emergence dynamics is reflected in their nutritional value and in the diet of riparian spiders. We measured the content of proteins, lipids, glycogen, and carbohydrates in emerged Chironomidae, and determined the stable isotope ratios of female Tetragnatha extensa, a web-building spider living in the riparian vegetation of the FPMs. We analysed the proportion of aquatic prey in spiders' diet, niche size, and trophic position. While the content of nutrients and thus the prey quality was not significantly altered by Bti, effects on the spiders' diet were observed. The trophic position of T. extensa from Bti-treated FPMs was lower compared to the control while the aquatic proportion was only minimally reduced. We assume that spiders fed more on terrestrial prey but also on other aquatic organisms such as Baetidae, whose emergence was unaffected by Bti. In contrast to the partly predaceous Chironomidae, consumption of aquatic and terrestrial primary consumers potentially explains the observed lower trophic position of spiders from Bti-treated FPMs. As prey organisms vary in their quality the suggested dietary shift could transfer previously observed effects of Bti to riparian spiders conceivably affecting their populations. Our results further support that anthropogenic stressors in aquatic ecosystems may translate to terrestrial predators through aquatic subsidy.

Benthic macroinvertebrate community shifts based on Bti-induced chironomid reduction also decrease Odonata emergence

Chironomid larvae (Diptera: Chironomidae) often dominate aquatic macroinvertebrate communities and are a key food source for many aquatic predators, such as dragonfly and damselfly larvae (Odonata). Changes in aquatic macroinvertebrate communities may propagate through terrestrial food webs via altered insect emergence. Bacillus thuringiensis israelensis (Bti)-based larvicides are widely used in mosquito control but can also reduce the abundance of non-biting chironomid larvae. We applied the maximum field rate of Bti used in mosquito control three times to six mesocosms in a replicated floodplain pond mesocosm (FPM) system in spring for two consecutive years, while the remaining six FPMs were untreated. Three weeks after the third Bti application in the first year, we recorded on average a 41% reduction of chironomid larvae in Bti-treated FPMs compared to untreated FPMs and a shift in benthic macroinvertebrate community composition driven by the reduced number of chironomid, Libellulidae and Coenagrionidae larvae (Odonata). Additionally, the number of emerging Libellulidae (estimated by sampling of exuviae in the second year) was reduced by 54% in Bti-treated FPMs. Since Odonata larvae are not directly susceptible to Bti, our results suggest indirect effects due to reduced prey availability (i.e., chironomid larvae) or increased intraguild predation. As Libellulidae include species of conservation concern, the necessity of Bti applications to their habitats, e.g. floodplains, should be carefully evaluated.

Larval mosquito management and risk to aquatic ecosystems: A comparative approach including current tactics and gene-drive Anopheles techniques

Genetic engineering of mosquitoes represents a promising tactic for reducing human suffering from malaria. Gene-drive techniques being developed that suppress or modify populations of Anopheles gambiae have the potential to be used with, or even possibly obviate, microbial and synthetic insecticides. However, these techniques are new and therefore there is attendant concern and uncertainty from regulators, policymakers, and the public about their environmental risks. Therefore, there is a need to assist decision-makers and public health stewards by assessing the risks associated with these newer mosquito management tactics so the risks can be compared as a basis for informed decision making. Previously, the effect of gene-drive mosquitoes on water quality in Africa was identified as a concern by stakeholders. Here, we use a comparative risk assessment approach for the effect of gene-drive mosquitoes on water quality in Africa. We compare the use of existing larvicides and the proposed genetic techniques in aquatic environments. Based on our analysis, we conclude that the tactic of gene-drive Anopheles for malaria management is unlikely to result in risks to aquatic environments that exceed current tactics for larval mosquitoes. As such, these new techniques would likely comply with currently recommended safety standards.

Microplastics in freshwater sediments: Effects on benthic invertebrate communities and ecosystem functioning assessed in artificial streams

The high levels of microplastics (MPs) found in freshwaters, particularly in riverine sediments, may impose a threat to the macroinvertebrate communities with possible consequences at ecosystem-level. The present study aimed to assess the effects of a mixture of different sizes of polyethylene microplastics (PE-MPs) on the composition and structure of macroinvertebrate communities and key-functions, such as primary production and leaf litter decomposition. MPs were mixed in the sediment at three different concentrations (0.1, 1, and 10 g kg^-1) already found in freshwater sediments to enhance the relevance of the work. After eight days of exposure to PE-MPs, the observed changes in macroinvertebrate community structure were mostly due to the reduction in the abundance of deposit-feeders and grazers that were reduced by ca 31-50% and 34-39%, in the two highest MPs concentrations respectively, in comparison with the control treatment after 8 days of exposure. MPs internal concentrations were detected only in organisms exposed to plastic particles within artificial streams with chironomids and mayflies presenting higher MPs internal levels (average of 115 particles/individual found in chironomids, 166/individual for Baetis sp. and 415 particles/individual for Ephemerella sp.) suggesting higher ingestion of plastic microparticles. Nevertheless, the alterations in the community structure did not translate into impairments in the functional endpoints analysed, leaf litter decomposition and primary production, that were expected due to possible sub-lethal effects (e.g., feeding inhibition) on detritivores and grazers. This study represents one of the few assessments of MPs effects on freshwater benthic macroinvertebrate community structure and the first that simultaneously considered ecosystem-level functional endpoints. Further research combining different microplastics and longer exposure periods are needed to raise knowledge on potential ecological consequences of MPs to freshwaters.

Lethal and sub-lethal activity of Brevibacillus laterosporus on the mosquito Aedes albopictus and side effects on non-target water-dwelling invertebrates

The biocidal potential of Brevibacillus laterosporus against mosquitoes of major medical importance has been widely documented, but its effects on non-target invertebrates are still poorly known. In this study, we determined the lethal and sub-lethal effects of B. laterosporus strain UNISS 18, an entomopathogenic bacterium known for its effectiveness against synanthropic Diptera, on the larvae of the Asian tiger mosquito Aedes albopictus, a vector of several pathogens to humans. Moreover, we compared the larvicidal activity with the lethal action on the invasive snail Physella acuta and on two non-target water-dwelling species: the mayfly Cloeon dipterum, and the harlequin fly Chironomus riparius. B. laterosporus exhibited significant lethal effects on all the tested species with a concentration-dependent activity. However, the susceptibility varied among species, with a higher susceptibility of Ae. albopictus (LC₅₀ = 0.16 × 10⁷ spores mL^-1) than the other species (LC₅₀ = 0.31, 0.33, and 0.30 × 10⁷ spores mL^-1 for C. dipterum, C. riparius, and P. acuta, respectively). While 1st instar mosquito larvae were very susceptible to the bacterial infection, no effects on preimaginal development stages and adult emergence were observed at sub-lethal spores' concentrations. Even if the efficacy of B. laterosporus against Ae. albopictus and the invasive freshwater snail P. acuta is promising for their control, the susceptibility of non-target beneficial aquatic insects, highlights the need of accurate evaluations before applying B. laterosporus for pest management in water environments.