Effects of Chironomus tentans larval growth retardation on adult emergence and ovipositing success: implications for interpreting freshwater sediment bioassays

A laboratory study was conducted with Chironomus tentans to assess the significance of growth retardation of third-to fourth-instar larvae over a 10-d test period on long-term survival, adult emergence, and ovipositing success. Data were intended to provide interpretive guidance for the commonly used growth endpoint in 10-d sediment bioassays with C. tentans. Larval growth was controlled by using six feeding levels ranging from 0.2 to 5.9 mg dry weight Tetrafin® fish food per day. Mean 10-d survival was 88% at all feeding levels, but larval growth decreased significantly (p<0.05) with each decrease in feeding level. Cumulative successful emergence of adult C. tentans decreased significantly with decreasing larval growth. Mean times to emergence always increased with decreasing growth rates and effects were generally more pronounced for females than males. At the lowest 10-d mean growth where successful emergence occurred (0.27 mg), the times to emergence doubled relative to the times observed at the highest 10-d mean growth (1.03 mg). Ten-day larval growth retardation was strongly correlated with reduction in adult emergence success (r² = 0.96). Growth retardations 64% resulted in 86–100% reductions in adult emergence. Growth retardation in the range of 35 to 50% equated with comparable percent reductions in adult emergence success. Although fewer females successfully emerged at 10-d growths of 0.37 and 0.74 mg relative to the highest 10-d growth (1.03 mg), ovipositing success of these females did not appear to be adversely affected by either their slower growth rates or their lower mean dry weights (0.62 and 0.99 mg, respectively). Growth of second generation larvae did not appear to be affected by maternal growth rate, but rather appeared to be solely correlated with their own feeding level.