Effects of imidacloprid on the survival and biomarker responses of Eristalis tenax larvae (Diptera: Syrphidae): a comparative study between indoor and outdoor exposures.

Imidacloprid is a widely used pesticide in agriculture. It is being found in aquatic ecosystems in agricultural regions. This study aimed to evaluate its effects on the survival rates, acetylcholinesterase (AChE) and catalase (CAT) responses of larval Eristalis tenax hoverflies. The larvae were exposed for 3, 7 and 14 days to increasing concentrations of imidacloprid (0, 0.1, 0.5 and 2 mg L-1) both indoors at a constant temperature of 20 °C and outdoors under varying environmental conditions. The results revealed that indoors and outdoors, the mortality of E. tenax significantly increased with increasing imidacloprid concentration and duration of exposure. Median lethal concentrations (LC50) varied from 0.03 to 0.17 mg L-1 depending on the duration and conditions of exposure. Indoors, AChE activity decreased in all the treatments for all three exposure durations, whereas outdoors the decrease was observed after the short (3-day) and long (14-day) exposure durations. AChE inhibition ranged from 6% to 62% (indoors) and 12% to 62% (outdoors). Variations in CAT activity were observed for both experimental setups, with a decrease outdoors in larvae exposed to 0.5 mg L-1 for 7 days and a gradual dose-dependent increase indoors for exposure lasting 3 and 7 days. This study sheds light on the potential ecological implications of imidacloprid contamination which may cause the decline of aquatic insect populations and pollination rates, leading to disruptions of the food chain and the overall decline of aquatic and terrestrial ecosystem health.

Local-Scale DNA Barcoding of Afrotropical Hoverflies (Diptera: Syrphidae): A Case Study of the Eastern Free State of South Africa.

The Afrotropical hoverflies remain an understudied group of hoverflies. One of the reasons for the lack of studies on this group resides in the difficulties to delimit the species using the available identification keys. DNA barcoding has been found useful in such cases of taxonomical uncertainty. Here, we present a molecular study of hoverfly species from the eastern Free State of South Africa using the mitochondrial cytochrome-c oxidase subunit I gene (COI). The identification of 78 specimens was achieved through three analytical approaches: genetic distances analysis, species delimitation models and phylogenetic reconstructions. In this study, 15 nominal species from nine genera were recorded. Of these species, five had not been previously reported to occur in South Africa, namely, Betasyrphus inflaticornis Bezzi, 1915, Mesembrius strigilatus Bezzi, 1912, Eristalinus tabanoides Jaennicke, 1876, Eristalinus vicarians Bezzi, 1915 and Eristalinus fuscicornis Karsch, 1887. Intra- and interspecific variations were found and were congruent between neighbour-joining and maximum likelihood analyses, except for the genus Allograpta Osten Sacken, 1875, where identification seemed problematic, with a relatively high (1.56%) intraspecific LogDet distance observed in Allograpta nasuta Macquart, 1842. Within the 78 specimens analysed, the assembled species by automatic partitioning (ASAP) estimated the presence of 14-17 species, while the Poisson tree processes based on the MPTP and SPTP models estimated 15 and 16 species. The three models showed similar results (10 species) for the Eristalinae subfamily, while for the Syrphinae subfamily, 5 and 6 species were suggested through MPTP and SPTP, respectively. Our results highlight the necessity of using different species delimitation models in DNA barcoding for species diagnoses.

Dietary deprivation reduces the deleterious effects of carbaryl on the survival and activity of both catalase and acetylcholinesterase in earthworms.

Dietary restriction (DR) and dietary deprivation (DD) have been shown to be significantly beneficial in terms of lifespan gains and stress alleviation in invertebrate and vertebrate species. Such beneficial effects, however, have yet to be clearly assessed in the presence of chemical stressors. We conducted a comparative evaluation of the toxicity of carbaryl in Eisenia fetida individuals subjected to a full diet (FD), DR and DD. For 14 days, groups of ten worms subjected to FD received 5 g oatmeal, those subjected to DR received 2.5 g oatmeal, and those subjected to DD received 0 g oatmeal weekly. We evaluated concentrations of 0, 7, 14 and 28 mg carbaryl.kg-1 soil and measured effects on survival, reproduction, biomass and biomarkers (Catalase- CAT and acetylcholine esterase- AChE). Carbaryl caused a total inhibition of reproduction in all the treatments. For each diet level, the 14-day LC50 s were higher than 28 mg.kg-1, but the 14-day LC20 s for the earthworms subjected to FD, DR, and DD were 11.24, 20.51 and > 28 mg.kg-1, respectively. This showed that the toxicity of carbaryl consistently decreased with the reduction in nutrients. Carbaryl caused a significant weight loss in the worms subjected to FD in the 7 mg.kg-1 treatment (P = 0.0065). Such weight loss was not found in any of the other treatments and diets. Both CAT and AChE were significantly inhibited in the two highest treatments (P = 0.0071 and P = 0.0073, respectively). Interestingly, the earthworms subjected to DD showed relatively lower biomarker inhibition, indicating a greater tolerance to oxidative and neurotoxic stresses in these starved earthworms. For all endpoints investigated, aside from reproduction, the starved earthworms fared better under carbaryl toxicity than those given the other diets. Overall, a positive correlation was observed between the amount of food and chemical toxicity as mortality rates, AChE and CAT inhibition increased with the increased amount of nutrients given to the worms. These results show that, in the presence of a chemical stressor, the beneficial effects of DR and DD were variably manifest for select lifecycle parameters and biomarker responses, further suggesting dietary reduction as a non-genetic intervention that could help extend lifespan and alleviate stress even under a chemical insult.

Developmental performance of Eristalis tenax larvae (Diptera: Syrphidae): Influence of growth media and yeast addition during captive rearing.

In holometabolous insects, life-history characteristics can vary in response to diet. The main aim of this contribution was to examine which diet best promotes larval development and survival of the aquatic saprophagous hoverfly Eristalis tenax L. (Diptera: Syrphidae). This study was motivated by a need to optimize techniques for rearing these insects in captivity. We studied how adding yeast to several rearing media based on animal droppings or decaying plant material affected development and survival in captive larvae, and whether these effects could be optimized depending on the amount of yeast and the rearing medium. In addition, premature exit of larvae was examined in two medium volumes to investigate differences in pupation. Larvae in yeast supplemented rabbit growth medium pupated and emerged faster than those in horse and antelope growth media. A high number of adult females emerged when compared to males, and both seemed to have a shorter developmental period in yeast supplemented growth media. Pupal survival was significantly greater in a mixture of droppings and plant organic matter, and a high medium volume of 140 ml (p < 0.05). Between 10% and 17% of larvae prematurely exited the aquatic medium in high (140) and low (70 ml) medium volumes, respectively. These results provide additional information that may be crucial for the successful mass rearing of E. tenax in captivity, and suggest that apart from the addition of yeast, growth medium quality and volume may be limiting factors for the production of large colonies of adults.

Habitat components and population density drive plant litter consumption by Eudrilus eugeniae (Oligochaeta) under tropical conditions.

The ingestion of organic and mineral materials by earthworms is a prominent functional role that has profound consequences for the decomposition and stabilization of soil organic matter. To investigate the litter consumption of the African nightcrawler earthworm Eudrilus eugeniae under different tropical conditions, we used DNA barcoding to identify specimens of E. eugeniae collected from sites across the Adamawa region in Cameroon, and studied the influence of habitat suitability (soil properties), soil moisture, litter type, and population density on litter consumption. A total of four litter consumption experiments were carried out using soils collected from refuse disposal sites, agricultural lands, and savannahs dominated by the Mexican sunflower Tithonia diversifolia. The results revealed that litter consumption significantly increased in the refuse disposal and agricultural soils as opposed to the Mexican sunflower (T. diversifolia) soil, a cow dung enriched substrate, and a sterile soil horizon from the savannah (P < 0.05). The optimum moistures for litter consumption were between 24% and 50%. Litter type did not affect the consumption rate of the earthworms (P > 0.05). We observed a general positive density-dependent consumption with litter mass loss increasing with increasing density. Our results suggest that E. eugeniae has a strong direct effect on the decomposition of plant materials than expected from previous estimations, and that litter consumption rates are determined by several habitat components and population density.