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The mechanism that comes into play after a female mosquito bites her
victim could be the key to controlling these pests, according to a biochemist
who has been studying the insects’ hormones.

‘When a mosquito bites you, she takes more than twice her body weight
in blood,’ says Timothy Hayes of Texas A&M University in College Station.
Most of this is unwanted water and salt. ‘Within seconds of piercing your
skin, she has begun to excrete water and salt – the result of a rapid chain
of events involving several organs,’ says Hayes.

These events include the filtering of water and salt from the blood
by the Malpighian tubules – the insect kidney – and the contraction of the
hindgut in order to eliminate the water. This sequence is coordinated and
communicated by a group of peptides, small proteins which act as chemical
messengers in all animals.

Hayes and his colleagues at Texas A&M have identified and sequenced
several of these peptides, and discovered their functions. ‘We would like
to understand how peptides mediate behaviour in insects and in higher animals,’
he says. ‘But we also intend to discover synthetic nonpeptide analogues
to mosquito peptides to disrupt their behaviour.’

The simplest way to use peptides to control insects would be to overwhelm
the mosquito with analogues, causing her to dehydrate. ‘Insects, because
they are so small, and have such high surface to volume ratios, are always
in danger of dehydration,’ says Hayes. ‘If we could find a chemical that
the Malpighian tubule recognised, and which stimulated the mosquito to urinate
constantly, she would soon die.’ Alternatively, says Hayes, it might be
possible to find a synthetic peptide that would bind to the receptors in
the tubule, making it impossible for the mosquito to eliminate the water
after sucking blood.

Using peptide mimics for insect control would bring enormous advantages,
Hayes believes. ‘Almost all of the pesticides now in use are toxic to all
species of insects, and to higher animals, including humans,’ he says. ‘If
we can discover peptide analogues for mosquitoes, they will probably be
active only in other closely related insects – perhaps the order Diptera,
which includes mosquitoes and flies.

Hayes admits that such chemicals are years away. ‘After we identify
the peptides we want to mimic, we will have to design analogues that are
stable in nature, yet still effective – then there will be years of testing
to make sure that they do not affect other animals,’ says Hayes. ‘But learning
the general peptide language of biology will also be useful in designing
new pharmaceuticals based on the peptide-mediated behaviour in our own bodies.’