Brewer s yeast

The test collembolan species, A. kimi (formerly known as Paronychiurus kimi), was collected from a paddy field in Korea in 1996 [23 (link)]. The A. kimi population was cultured in a plastic petri dish (9.0 cm in diameter and 1.5 cm in height) filled with approximately 0.5 cm depth of moist substrate comprised of plaster of Paris, activated charcoal, and distilled water at a ratio of 4:1:4 by volume in a dark growth chamber at 20 ± 1 °C. Brewer’s yeast (Sigma-Aldrich, St. Louis, MO, USA) was provided weekly as food. To obtain age-synchronized collembolans, the eggs laid by hundreds of adults were transferred to a fresh moist substrate using a fine-haired brush. After the eggs hatched, the juveniles were reared under the same conditions and used for all subsequent experiments.

Allonychiurus kimi colonies were first collected from Incheon, South Korea (32.267°N, 127.433°E) on 15 September 1996. Then, this collembolan population has been maintained at the Ecology & Toxicology Laboratory, Korea University, Seoul, South Korea (accession number KUETCOLC001) for approximately 25 years (Lee, Lee et al., 2021 (link)). The A. kimi colony was maintained in a plastic Petri dish (90 mm × 15 mm (diameter × height) (D × H)) filled with 0.5 cm deep substrate comprise of plaster of Paris, activated charcoal, and distilled water at a ratio of 4:1:4 by volume at 20 ± 1°C with continuous darkness (Wee, Lee, Kim, Son et al., 2021 (link)). Distilled water was added periodically, and the plastic Petri dishes were aerated weekly to maintain the fresh condition of the substrate. Brewer's yeast (Sigma‐Aldrich) was added weekly as food. To obtain cohorts of A. kimi, eggs laid by hundreds of adults on the same day were transferred to plastic Petri dish with a fresh moist substrate using a fine hair brush. The eggs were hatched after 10 days, and the juveniles were maintained under the same condition for 6 weeks until they develop into adults. Then, the adults were used for all subsequent experiments.

In the current study, efficacy of two different larval diets was compared with that of the IAEA-recommended larval diet. Stock solution of the first diet (D₁) was formulated by dissolving 25 g of dry fish powder meal (available at the Peliyagoda fish market, Peliyagoda, Sri Lanka) in 100 mL of distilled water. The dry fish powder was comprised of fish, shellfish, and mollusks, with crude protein (19% wt/wt), crude fat (5% wt/wt), and ash (4% wt/wt), as specified by the manufacturer. Meanwhile, the second larval diet (D₂) was prepared by dissolving 21.5 g of dry fish powder meal and 3.5 g brewer's yeast in 100 mL of distilled water. Stock slurry of the third larval diet (D₃) was prepared according to the recommended compositions of the International Atomic Energy Agency as the control by dissolving 50% tuna meal (12.5 g) (T.C. Union Agrotech, Thailand), 36% bovine liver powder (9.0 g) (MP Biomedicals, Santa Ana, CA), and 14% brewer's yeast (3.5 g) (Sigma Aldrich Inc., St. Louis, MO) in 100 mL of distilled water [13 (link)]. The homogenized stock slurries of the three diets were stored at -20°C to prevent the degradation of diet components and microbial proliferation [18 (link)]. The initial concentrations of the prepared stock slurries were considered 100% throughout the study.