The potential of native damselflies (Odonata: Zygoptera) as bioindicators of insecticide contamination of freshwater

Abstract

This thesis investigated if and how New Zealand damselflies (Odonata: Zygoptera) could be used as bioindicators of insecticide contamination of freshwater.

Two damselfly species are common throughout New Zealand: Xanthocnemis zealandica and Austrolestes colensonis. The sensitivity of larvae of both species to azinphos-methyl and carbaryl was estimated. X. zealandica was more sensitive to both compounds than was A. colensonis and was chosen for more detailed studies. The LC₅₀ values for azinphos-methyl did not correlate with the larval instar. The most susceptible stage was instar 7 and the least susceptible were instars 2 and 13. In contrast, LC₅₀ values for carbaryl were related to larval instar.

Fluctuating asymmetry (FA) is defined as minor, nondirectional deviations from bilateral symmetry and has been suggested as a biomonitoring tool. FA in the wings of adult X. zealandica was analysed as a potential indicator as described below.

Exposure of the last instar of X. zealandica to carbaryl at 40 ppb increased the level of FA compared with the control. Larvae that had already completed more than half their development when the experiment started did not show significant differences in the level of FA. However, damselflies that were exposed for more than half the final instar showed increased levels of FA compared with controls. The exposure of the penultimate larval instar to carbaryl at 100 ppb killed more than 90% of the individuals, whereas no increased mortality occured at 10 ppb. However, this lower concentration increased the level of FA.

In a replicated, artificial-pond experiment, carbaryl at 100 ppb reduced emergence success of X. zealandica 10 days after application, whereas carbaryl at 10 ppb had no effect after more than 120 days. Levels of FA were not affected by carbaryl at 10 ppb in this system over any experimental time period.

To investigate the practicability of FA as a bioindicator in the field, levels of asymmetry in damselflies caught at sites of high pesticide usage were compared with those from control sites. Evolutionary and behavioural ecology of damselflies suggests that FA differs between different sub-groups of damselfly populations. The importance of these differences in relation to pollution-induced asymmetry has hardly been addressed. Hypotheses (H1 - H3) on how evolutionary and behavioural ecology could influence the level of FA in different sub-groups of damselfly populations were formulated in this thesis.

Four ponds (two sites of high pesticide usage and two control sites) were visited twice in one summer and damselflies were sampled to address the hypotheses. FA in wing length tended to be higher in animals from sites of high pesticide usage, whereas none of the comparisons of meristic (numerical) traits was significant. No difference in asymmetry levels was found between paired and unpaired males (H1), whereas males were significantly more symmetrical than females (H2). Damselflies caught earlier in the year were more symmetrical than those caught later (H3). These findings suggest that evolutionary and behavioural ecology interferes with the suitability of FA in mature damselflies as a biomonitoring tool. It was concluded that emerging damselflies should best reflect the levels of stress present in the aquatic environment rather than mature adults.