PARAQUAT

Paraquat can cause endocrine disruption. It
decreased testosterone, follicle-stimulating
hormone, luteinizing hormone and prolactin
in male rats. One epidemiological study linked
paraquat exposure to hypothyroidism.
Regulatory assessments generally conclude
paraquat does not cause reproductive effects,
but independent studies show that it can cause
reproductive problems in rodents and hens. It
crosses the placenta and also concentrates in
the placenta. Foetal death in pregnant women
poisoned by paraquat, and neonatal death after
induced delivery, has been reported.
Similarly, regulatory assessments generally
conclude paraquat is not a teratogen, but
independent studies show that it can cause birth
defects in rodents and frogs, prompting some
scientists to state that it should be classifi ed
as a teratogen. An epidemiological study has
linked congenital malformations in children with
paternal exposure to paraquat.
There is some evidence of effects on the immune
system, and it may also be implicated in type II
diabetes.
Environmental effects
Paraquat is described by US Environmental
Protection Agency as “extremely biologically
active and toxic to plants and animals”; and by
the Environmental Risk Management Authority
of New Zealand as “very ecotoxic to the aquatic
environment”. It has caused teratogenic
malformations in fi sh and amphibia, disrupted
hormones in frogs, and is genotoxic in tadpoles.
Amphibia are at risk from paraquat, through
residues in plants, reduction in food sources and
habitat, spray drift from up to 300m away, and
downstream transport of paraquat in sediment.
Aquatic plants can concentrate high levels of
paraquat. Planktonic algae are very sensitive to
paraquat and it can cause significant ecological
disturbances in freshwater ecosystems through
alterations in species composition, potentially
resulting in loss of biodiversity, harmful algal
blooms, disease, and decline in fisheries.
The European Commission‘s Scientifi c
Committee on Plants expressed concern about
the effects of paraquat on hares and birds. They
concluded that it “can be expected to cause
lethal and sublethal effects and this is confi rmed
by fi eld reports”. Freshly sprayed foliage can
induce death in rabbits, and especially the hare.
The US EPA concluded that paraquat is
moderately toxic to birds, and it can affect
reproduction or hatchability of eggs when adult
birds are exposed. It also causes endocrine
disruption in birds.
Paraquat is toxic to some soil fungi and bacteria,
but can also increase populations of some soil
pathogens.
Poisoning incidents include fi sh, dogs, hares,
cattle and sheep; there have also been many
deliberate poisonings of dogs.
Environmental fate
Paraquat binds strongly to soil particles and
tends to remain strongly bound for a long time
in an inactive state, although it can also desorb
again and become biologically active. Half-life in
soil can be up to 20 years.
In water it is adsorbed on to particles and
sediment, with a half-life under mid-European
conditions estimated to be between 2 and 820
years depending on sunlight and depth of water.
It has been found in surface waters, drinking
water, and in groundwater although it is believed
to be immobile in the soil and not to leach to
groundwater.
Herbicide resistance
There are 22 different species of weeds in
13 countries that have become resistant to
paraquat.
Alternatives
There are numerous design, management,
mechanical and cultivational practices, as well
as some plant extracts, that can be used instead
of paraquat, depending on the weed species
and the situation.
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