A SCRAWNY strain of wild maize from Argentina could help farmers do battle
with the European corn borer, an insect pest which does about $350
million of damage to the US maize crop every year. Unlike commercial maize, the
wild strain makes chemicals in its leaves which discourage female corn borer
moths from laying eggs.
It鈥檚 the larvae that do all the damage. They start on the leaves, and then
nibble their way through the stalk, which they hollow out completely. Nutrients
can no longer reach the parts of the plant above ground, and often the stalk is
so weakened that the plants collapse.
Farmers usually fight off the borers (Ostrinia nubilalis) with
larvicides. Newer genetically modified strains can deter the larvae by making
the Bt toxin derived from the bacterium Bacillus thuringiensis. But a
maize strain which stops females laying their eggs in the first place would nip
an infestation in the bud.
Advertisement
Bradley Binder of the US Agricultural Research Service鈥檚 Corn Insects and
Crop Genetics Research Unit in Iowa State University at Ames has spent five
years studying the Argentinian strain, known as B-96, and has now identified the
chemicals which discourage egg laying. 鈥淚n field tests there was an eight to
ninefold reduction in the number of egg masses on B-96 compared with susceptible
plants,鈥� he says.
Binder filmed females to learn more about their egg-laying tactics. 鈥淲e don鈥檛
really understand what cues they are looking for,鈥� he says. What makes the study
even more difficult, he adds, is that the moths are nocturnal.
The films showed that when a female lands on a leaf she runs through a
鈥渟cratch 鈥榥鈥� sniff鈥� routine. First she scratches the plant surface to stir up
plant chemicals. Next, she sniffs the aromas by fanning her antennae.
Through these and other screening experiments, Binder has now discovered a
family of 20 or so substances in B-96 which appear to discourage egg laying. The
main substance is a hydroxamic acid called 2-hydroxy-7-methoxy-1,4
(2H)-benzoxazin-3(4H)-one, or HMBOA for short. The other is DIMBOA, the
2,4-hydroxy variant of the same substance. Binder is now working out how B-96
makes these odours, and which genes are involved. He says that commercial maize
may already have the genes, but they seem to be switched off except in very
young plants.
He and his colleagues hope to transfer the egg-repelling trait by crossing
elite maizes with their scrawny Argentinian relative. Another option might be to
transfer extra copies of the genes involved, or to engineer maize plants so that
they switch on their own genes for the odour chemicals. Alternatively, it might
be possible to make the repellents and spray them on maize plants to head off
infestations.
Binder would also like to alter the GM maize which already makes the
anti-larval Bt toxin. Armed with the extra trait to discourage egg laying, the
maize could deliver a 鈥渄ouble whammy鈥� to the pests.