Laboratory culture of Chironomus tentans for use in toxicity testing: optimum initial egg-stocking densities

The midge Chironomus tentans Fabricius is a commonly used freshwater invert ebrate in sediment toxicity tests. Rigorous laboratory culturing techniques are needed to provide organisms of uniform quality and known age for use i n testing and for the continuation of the culture itself. This study was co nducted to determine the effect of initial culture stocking density on: (1) post-hatch (larval) dry weight, body length and head-capsule width at 10 a nd 20 days; (2) time to emergence; (3) number and sex of emergent adults; ( 4) number of larvae and pupae at test termination (day 42 post hatch); and (5) adult dry weight. Three egg stocking densities were used 690 (1.1 eggs cm(-)2), 1043 (1.7 eggs cm(-2)) and 1463 (2.4 eggs cm(-2)). Mean weight of larvae at 10 days in high density tanks (0.13 mg/organism) was significantl y higher (P=0.003) than both the medium and low density tanks (0.10 and 0.0 9 mg/organism, respectively). No significant differences between the three stocking densities were observed for the body length or head-capsule width at either 10 or 20 days post-hatch. Although not statistically significant, larval dry weight decreased with increased stocking density at day 20. A s ignificantly ( P=0.02) greater number of females (173 +/- 28) emerged from the low stocking density compared to both the medium and high stocking dens ities (123 +/- 45 and 118 +/- 54, respectively). Peak adult emergence for t he low and medium stocking densities occurred between days 22 and 25 post-h atch, whereas peak adult emergence occurred between days 30 and 33 for the high stocking density. Survival relative to the initial number of eggs stoc ked was significantly greater (P=0.007) in the low density treatment compar ed to that in either the medium or the high density treatments. Mean adult weight exhibited an inverse relationship with initial stocking densities. A t test end, there was not a significant difference in the mean number of or ganisms surviving and emerging in the three density levels. The central ten dency for number of organisms surviving for all three treatments was 504 or ganisms per tank (0.82 organisms cm(-2)). The results of this experiment su ggest that an optimal egg stocking density of 1.0 egg cm(-2) (similar to 60 0 eggs/tank) be used with the feeding rate identified. This would ensure un iform larvae at the appropriate developmental stage (2nd-3rd instar) needed for toxicological research/testing (e.g. 10 days post-hatch), as well as p roducing sufficient emergence of males and females for future culture estab lishment.