Bekæmpelsesmiddelforskning fra Miljøstyrelsen, 99 – Toksiske effekter af pulseksponering af pesticider - akutte og kroniske effekter hos Daphnia magna

Toksiske effekter af pulseksponering af pesticider - akutte og kroniske effekter hos Daphnia magna

Summary

Drainage from fields, surface runoff after rain events, or spraying of pesticides are all examples of events that may lead to short-term high concentrations of pesticides in surface water. These kinds of events are often referred to as pulses of pesticides. The standardized aquatic toxicity tests used in hazard assessment are performed with a continous exposure of test organims for a specified period of time. Thus, these methods do not take effects of pulse exposure into account. The aim of the present study has been to develop and test a new method to detect toxic effects of pulse exposure to pesticides using the fresh water crustacean Daphnia magna.

Eigth pesticides (dimethoate, chlorpyrifos, pyrazophos, esfenvalerate, pirimicarb, diquat, bromoxynil, and azoxystrobin) and two reference compounds (4-nitrophenol and 3,5-dichlorophenol) were tested. In the pulse exposure tests juvenile Daphnia magna (<24 h old) were exposed to the test compounds for 0.5-6 hours. After these exposures, the animals were transferred to clean water and observed for 48 hours. During this post-exposure period changes in the mobility of the animals were registered immediately after the transfer (0 hours) and after 24 and 48 hours. For the compounds dimethoate, pirimicarb, esfenvalerate, chlorpyrifos, bromoxynil, 4-nitrophenol, and 3,5-dichlorophenol the observation period was prolonged to 21 days to study chronic effects of pulse exposure. In these studies, the following endpoints were included: Time to first offspring, number of offspring, length and weigth of surviving mother animals. Furthermore, the effect on the mobility of Daphnia magna of two consequtive pulse exposures to dimethoate, pirimicarb, esfenvalerate, and pyrazophos was studied.

During and immidiately after pulse exposures to dimethoate, pirimicarb, esfenvalerate, bromoxynil, 4-nitrophenol, and 3,5-dichlorophenol the animals had lost their mobility, but after 24 hours in clean water they regained mobility. After pulse exposure to azoxystrobin no such recovery was observed during the 48-h post-exposure period, and for chlorpyrifos, pyrazophos, and diquat the number of immobile and/or dead animals increased after the pulse exposure ended.

Repeated pulse exposures resulted in more pronounced effect on the mobility of Daphnia magna than single pulse exposures. In groups exposed to short pulses, animals, which apparently were not affected by one pulse, lost their mobility and an increased number of dead animals was observed after a second pulse exposure.

Delays in the time to first offspring and reduced number of offsprings were observed in Daphnia magna after pulse exposure to dimethoate and pirimicarb. Significant reduction in the number of offsprings was observed for pulse durations of 2 hours and the number of offspring decreased with increasing pulse duration. Mother animals exposed to pulses of dimethoate or pirimicarb for 2 hours or more were also significantly smaller than unexposed animals, and a correlation between the size of the animals after 21 days and the number of offspring was observed. This effect may be related to a lower uptake of food in the post-exposure period, but this was not studied in the present project. On the other hand, pulse-exposure tests with esfenvalerate, chlorpyrifos, bromoxynil, 4-nitrophenol, and 3,5-dichlorophenol did not reveal any significan decrease in number of offspring after 21 days, eventhough animals exposed to 3,5-dichlorophenol were smaller than non-exposed animals.