In parts of Africa the tsetse fly is still a serious pest whose bite
transmits the potentially fatal disease sleeping sickness to humans and
a related infection, nagana, to cattle. One method of controlling the fly,
Glossina morsitans, is to set up cloth traps, or targets on which the flies
alight, which are treated with pyrethroid insecticides.
Scientists from the University of Bristol, together with colleagues
from France and Japan, are now trying to adapt this technique to work with
smaller doses of a chemical which persists for longer so that the traps
need less maintenance.
The substance they are using is pyriproxyfen, or 1-methyl-2-(4-phenoxyphenoxy)ethoxypyridine,
which mimics the action of the insect juvenile hormones effectively sterilising
the flies (Entomologia Experimentalis et Applicata, vol 66, p 153). It also
has very low toxicity for other living things.
If a female tsetse fly takes up some pyriproxyfen through her feet before
she gives birth to a larva (which hatches from the egg inside her), the
chemical prevents the resulting larva from metamorphosing. Pyriproxyfen
does not affect the fertility of male tsetse flies, but contaminated males
can transfer a sterilising dose to females when they mate.
In experiments under field conditions in Zimbabwe, using a preparation
of radioactively ‘labelled’ pyriproxyfen, the scientists checked how long
the substance lasted on the traps. They found that Terylene netting treated
with radioactive pyriproxyfen still retained 20 per cent of its original
radioactivity after 9 months. If the feet of female tsetse flies touched
this netting for only 5 seconds, between half and three-quarters of their
larvae failed to develop satisfactorily. After female flies were exposed
to the chemical for 2 minutes, none of their larvae would ever develop.
Targets and traps impregnated with pyrethroids would have lost all activity
by the end of 9 months.