Sex-specific alterations in adaptive responses of Chironomus columbiensis triggered by imidacloprid chronic and acute sublethal exposures.

The use of imidacloprid is a common pest control practice in the Neotropical region. However, the imidacloprid unintended sublethal effects on Neotropical aquatic non-target arthropods and undesirable consequences for aquatic environments remain unclear. Here, we assessed the susceptibility of Chironomus columbiensis (Diptera: Chironomidae) larvae to the neonicotinoid imidacloprid and evaluated whether sublethal exposure types would trigger sex-dependent adaptive responses (e.g., emergence, body mass, reproduction, wing morphology). We conducted a concentration-mortality curve (96 h of exposure) and established chronic and acute sublethal exposure bioassays. While chronic sublethal exposures consisted of exposing individuals during their entire larval and pupal stages, the acute sublethal exposures represented a single short duration (24 h) exposure episode during either the first or fourth larval instar. Our results revealed that chronic sublethal exposure reduced the body mass of males, while acute sublethal exposures during the first instar resulted in heavier males than those that were not exposed to imidacloprid. Chronic exposure also reduced the reproduction of males and females, while the acute sublethal exposure only affected the reproduction of individuals that were imidacloprid-exposed on their later larval instar. Chronic and acute sublethal exposures did differentially affect the wing properties of C. columbiensis males (e.g., increased size when chronically exposed and highly asymmetric wings when acutely exposed in early larval phase) and females (e.g., highly asymmetric wings when chronically and acutely exposed). Collectively, our findings demonstrated that imidacloprid can cause unintended sublethal effects on C. columbiensis, and those effects are dependent on sex, exposure type, and developmental stage.

Chironomus columbiensis (Diptera: Chironomidae) as test organism for aquatic bioassays: Mass rearing and biological traits.

Chironomidae are aquatic insects that have become a model in laboratory tests to evaluate toxic effect of different pollutants. The use of chironomids as test organisms depends on standard protocols for the rearing under laboratory conditions and expanding knowledge on its biology. We standardize a culture of C. columbiensis and analyze its life cycle under laboratory conditions. The experiments were carried out with different treatments for water, temperature, and food. As a result, a protocol was proposed for the rearing of C. columbiensis under the following conditions: semi-soft and reconstituted water, shredded paper towels as a substrate, soft aeration, temperature of 22 ± 2°C, fed with 0.3 g TetraMin® twice a week and weekly renewal of 50% of the water. Under these conditions, the duration of the life cycle was 17-31 days, with a continuous growth of 1.16 to 14.05 mm in the larval stage and an average of 530 eggs per oviposition. In comparison with other species in the family, this study indicated that C. columbiensis is a species with a short life cycle, sensible to changing conditions (e.g., temperature and food), wide distribution, so is a good organism for being used in aquatic bioassays.

Exposures to deltamethrin on immature Chironomus columbiensis drive sublethal and transgenerational effects on their reproduction and wing morphology.

Sublethal exposure to insecticides can trigger unintended responses in non-target insects that may disrupt reproductive and developmental performances of these organisms. Here, we assessed whether sublethal exposure to the pyrethroid insecticide deltamethrin in early life had sublethal and transgenerational effects on the reproduction (i.e., fecundity and fertility) and wing morphology of Chironomus columbiensis, an aquatic insect used as a water quality indicator. We first conducted concentration-response bioassays to evaluate the susceptibility of C. columbiensis larvae to deltamethrin. Our results revealed that deltamethrin toxicity was approximately 7-fold higher when C. columbiensis larvae where exposed to 96 h (LC50 = 0.17 [0.15-0.20] µg/L) than to 24 h (LC50 = 1.17 [0.97-1.43] µg/L). Furthermore, the sublethal exposures (at LC1 = 0.02 µg/L or LC10 = 0.05 µg/L) of immature C. columbiensis resulted in lower fecundity (e.g., reduced eggs production) and morphometric variation wing shapes. Further reduction in fertility rates (quantity of viable eggs) occurred at deltamethrin LC10 (0.05 µg/L). Almost 80% of the fecundity was recovered with only a single recovery generation; however, two subsequent recovery generations were not sufficient to fully recover fecundity in C. columbiensis. Specimens recovered from 98.5% of wing morphometric variation after two consecutive generations without deltamethrin exposure. Collectively, our findings demonstrates that sublethal exposure to synthetic pyrethroids such as deltamethrin detrimentally affect the reproduction and wing shape of C. columbiensis, but also indicate that proper management of these compounds (e.g., concentration and frequency of application) would suffice for these insects' population recovery.