INSECT crop pests may locate their favourite plants by sight rather than
smell, as had been assumed. The researcher who backs the theory says that their
vision is so bad that although they can distinguish green vegetation from brown
earth, they can鈥檛 tell crops from neighbouring weeds鈥攐r any other green
object.
The theory, advanced by Stan Finch of Horticulture Research International in
Wellesbourne, Warwickshire, has been attacked by other researchers, however.
They say that smell and other cues are just as important to insects.
If proven, Finch鈥檚 findings would strengthen arguments for the organic
farming technique of allowing weeds to grow alongside crops. The weeds would
serve as visual decoys, attracting insect pests away from the valuable
crops.
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鈥淪terilising鈥� fields of all weeds may be asking for trouble by denying the
crop plants any camouflage. 鈥淵ou take the cover away by surrounding them with
brown earth,鈥� says Finch.
He exposed cabbages to eight species of insect pest. These included
egg-laying flies and butterflies, aphids that suck sap and beetles that chew
vegetation. 鈥淲e had four orders of insects and we showed that all of them were
behaving exactly the same,鈥� says Finch. First he planted the cabbages alone,
then with clover decoys sown beneath.
The insect can smell that there are cabbages around. 鈥淏ut the smell doesn鈥檛
give a directional cue,鈥� Finch says. So it heads for anything green. If it lands
on a weed such as clover, the insect often then flies away.
Even if the cabbages were surrounded with green paper, the insects were just
as likely to land on the paper as on the cabbages. Finch says this supports his
argument that the insects are guided by vision rather than smell. If they could
smell where the plants were, they wouldn鈥檛 land on paper.
Organic farmers say the results support their methods and highlight the
advantage of biodiversity over monoculture. 鈥淚t backs up what organic farmers
have believed for many years,鈥� says a spokesman for the Soil Association, which
promotes organic farming. 鈥淭olerating a certain amount of weed cover prevents
insects homing in.鈥�
Other researchers have serious misgivings about the findings and accuse Finch
of overstating the importance of sight, however. 鈥淥f course vision鈥檚 important,
but smell鈥檚 important too,鈥� says John Pickett, head of the Institute of Arable
Crops Research at Rothamsted, Hertfordshire. 鈥淲e already have huge amounts of
evidence in field trials that you can manipulate behaviour of insects with
smells,鈥� says Pickett.
Finch is defiant, however. 鈥淚 don鈥檛 think [odours] do a lot,鈥� he says.