Fipronil: Chemistry in its Element podcast

Fipronil: Chemistry in its Element podcast


The Chemistry in its Element podcast:
curious tales of chemical compounds This week, Harriet Brewerton on the
household pest killer that may be threatening our friendly pollinators. In the 1990’s, there were alarming reports of healthy bee colonies failing in France. The ground near beehives was said to be
carpeted with dead bees. Similar incidences cropped up around the world,
and in 2006 beekeepers in North America reported losses of up to 90 percent, as
forager bees simply failed to return to their hive, causing the colony to
collapse. Global bee populations are declining each year, with some areas regularly losing 50% of their hives over winter. Aside from producing honey,
bees are vital for the world’s food production, and in the UK alone insect
pollination is worth around $690 million a year. Declines in global bee populations have prompted the formation of organizations such as the
Bee Informed Partnership in the US, who monitor and respond to colony collapse
events. Causes for these colony collapses are complex but scientists are starting
to unravel the mystery, and one of those causes could be related to a product in
your own home right now. A French company first synthesized fipronil in 1987. It is
a unique phenolpyrazole that contains a trifluoromethylsulfinyl
group and is a powerful insecticide. It is used in cockroach traps, and is the
main active ingredient in many spot-on treatments used by cat and dog owners to
control fleas, ticks and ear mites. Spot-on treatments typically comprise only
one-tenth fipronil – the rest is oils that spread the insecticide across the skin.
As it spreads, the oil-fipronil mix forms wells in the sebaceous glands that
exists just below the surface all over the animals’ skin. These glands work to
produce a fatty substance to protect the skin and the oil-fipronil wells get
incorporated into this natural process, replenishing the levels of fipronil
on the skin’s surface for days. This allows the very small dose of fipronil
in the spot-on solution to not only kill all the insect pests on our pets, but
then prevent reinfestation for up to a month. To achieve this level of cover,
fipronil has to be very potent, and this power has made it a promising
insecticide for crop protection. It had been deemed safe to use on seeds to
protect sunflowers and other crops from ants and flying insect pests as
they grew. It was very effective, but the regulatory tests at the time missed a key
fact. Even the tiny amounts of fipronil remaining in the crops could adversely
impact pollinator insects and bioaccumulate to reach toxic levels that
can affect behaviour, including forager bees’ ability to navigate. Research has
linked fipronil use with the 1990’s colony collapses in France, and it has
been implicated in other reported bee losses. The accepted LD50 toxicity value
of fipronil to bees is 4 ng/bee. Ainsley Jones at Fera Science
Ltd explains what this means in real terms: “A single dose of one gram would kill
125 million bees, or 4000 bee hives.” Unfortunately, fipronil affects all
insects, and does not differentiate between insects considered to be pests
by farmers, and insects that are useful pollinators. Fipronil and its oxidation
product, fipronil sulfone, work by binding to chloride channels in an insect
central nervous system, specifically those gated by gamma-aminobutyric acid,
or GABA, and glutamate. The opening and closing of these channels controls nerve
cell firing which in turn controls the insects muscles. With fipronil bound, the
gate inhibitor is blocked and the chloride channel remains open. This leads
to loss of coordination, paralysis and the eventual death of the insect.
Fipronil binds to insect GABA-gated channels in preference to those in
mammals, and glutamate-gated channels do not occur in mammalian central nervous
systems. This means fipronil is over 500 times
more toxic to insects than mammals, which is why small amounts are safe to use on
pets in our homes. The World Health Organisation defines fipronil as only
‘moderately toxic’ to humans, but large doses could cause nausea and dizziness,
with long-term effects on the kidneys and liver. Fipronil in the food chain is
closely monitored and no such side effects have been reported from
household use. After decades of research, the main concern with fipronil use
remains to be its high toxicity to bees. Colony collapse is a complex issue, and
understanding the impact of fipronil, other pesticides and environmental
pressures on colonies is helping us to find ways to reverse the trend. In 2013,
governing bodies started outlining strict guidance on the data for bees
that needs to be gathered before new pesticides are approved for use.
Selwyn Wilkins at Fera explains what information scientists now realize is
important: “Things that were identified were not only the acute contact and oral
tests but the need to look at chronic toxicity, so over a longer period for the
adult bee. Also laboratory testing to look at in vitro larva rearing, so looking
at effects on brood development. And also looking at bumble bees and solitary
bees.” Europe banned the use of fipronil on crops in 2017, with China following
suit last year. The potency of fipronil may be difficult to contain for
large-scale agricultural use, but on a small-scale it remains a useful compound
that I, and thousands of pet owners, use to keep itchy fleas out of our homes. That was Harriet Brewerton with fipronil. Next week, a chemical route to a
lustrous full beard, with Catherine Hodges. With the current beauty trends
for thick eyebrows and full beards, more and more people are using minoxidil not
just to replace what they’ve lost on top, but to enhance and supplement their
facial hair. Join Catherine next week. Until then, you’ll find all of our
previous podcasts at chemistryworld.com/podcasts and you can drop us a
line with any compounds to cover: email [email protected] or
tweet @chemistryworld. I’m Ben Valsler, thank you for joining me.