Louise Tickle tells the story of the hunt for a natural defence against the virulent Sunn Pest which can cripple wheat production in some of the poorest countries in the world.

For a few thousand odd years, life has been pretty good for the Sunn Pest. Throughout the Middle East, west Asia and north Africa – wherever humans cultivated wheat and barley – there these bugs could be found, contentedly scoffing their way through field after field of valuable grain. As every new generation of young Sunn Pest grew into adults, the voracious hordes would decamp en masse to cooler highland areas, over-wintering deep in the leaf litter beneath trees until the following spring, when the cycle would begin again.

Quite apart from the significant loss of yield through Sunn Pests eating valuable crops, however, is the fact that the insects inject a chemical into the grain that renders it unsuitable for human consumption.

It takes just three to four per cent of grain to be infected for an entire consignment to be rejected, and with more than 10 million hectares of wheat worldwide now affected by Sunn Pest every year, the effects on poor countries trying to become self-sufficient in cereal production can be devastating.

In Afghanistan alone, 200,000 hectares of grain production was lost in 2002, with a value of almost £13 million. This represented enough to feed 300,000 Afghans for 12 months. Widespread Sunn Pest infestation means national governments must import large quantities of expensive wheat, as well as buying chemical pesticides for spraying on an industrial scale. This of course has additional implications for the health of farm workers, build-up in the human food chain, indiscriminate killing of other insects and vertebrates and the pollution of groundwater.

Finding ways to manage the Sunn Pest problem has been exercising the minds of scientists from Iran, Syria, Turkey and the UK for the past three years. Brought together by the International Centre for Agricultural Research in Dry Areas (ICARDA), they examined measures including: changing the culture of mass spraying to more directed methods; moving the timing of crop planting; choosing more resistant strains; physical barriers such as the use of shelter belts of unplanted land around field margins; and last, and by no means least, biological control.

The benefit of biological control at its best is that products can be developed that are easy and cheap to mass produce, environmentally safe and specific to their target. They do not accumulate in food chains and cause no harm to other vertebrates and invertebrates.

Already experienced in this work through developing the acclaimed and highly effective fungus-based insecticide Green Muscle®, Egham-based research organisation CABI Bioscience decided to focus its efforts on finding a natural predator for Sunn Pest. By spring 2004 they had achieved a breakthough.

It was lucky they did, because the project funding was just about to run out and, after two years of painstaking effort, results up to that point had not been encouraging.

In the winter of 2002-03, hibernating Sunn Pests in their thousands had been dug up from their cosy leaf litter and, when they eventually died, their corpses were placed in laboratory conditions that would encourage any fungi that might have caused mortality to keep growing. These various fungi were isolated, identified, and one was chosen for a field trial to test its effectiveness.

When spring arrived in Turkey and the resident Sunn Pests emerged from hibernation to lay their eggs, CABI’s project leader David Moore was joined by insect pathologist Steve Edgington, and the fungus spraying began.

"It turned out we got absolutely no mortality due to the fungus," explains Steve Edgington ruefully. "We did the same in Syria, and got the same result. It was very disappointing, because when we’d sprayed in the lab, the adults died. But these were fungus isolates collected in the cooler highlands and we were spraying in the much hotter lowlands. The isolate might not have been so happy in the heat, or maybe the Sunn Pest was able to hide from the spray."

While in the field, the CABI team collected around six thousand adult Sunn Pests from the hotter climate of the lowlands in Turkey and Syria and brought them back to England, where seven isolates of the fungus Beauvaria bassiana were identified.

Adult Sunn Pests were sprayed repeatedly in the CABI lab and died in gratifyingly large numbers, so when spring 2004 came round, it was with high hopes that Dave Moore and Steve Edgington travelled out once again to Syria, with the best performing isolate carefully prepared and ready to go.

On a single field that had been assigned to them for the trial, the pair used special spraying equipment that allows ultra-small droplets to move easily through an entire crop. Though confident their fungus was hitting the bugs, once again, the Sunn Pest remained stubbornly impervious.

There was only a window of a few weeks left before the juvenile Sunn Pests grew up and flew off, but as well as running out of time, the scientists were also running out of adult bugs, which had already laid their young and were now at the end of their life cycle. Never daunted, they started collecting two-to-three-day-old Sunn Pests from areas of wild, unsprayed wheat. If the fungus isolate didn’t work on the adults, maybe it would on the young.

Some of the juveniles were allowed to settle on ears of wheat in the laboratory, three hours drive away, while others were placed in the wheat crop in the field that had already been treated. The fungus was sprayed again, and this time the scientists observed that 85-90 per cent of the Sunn Pest died, while bugs in the control experiments were only showing 20 per cent mortality.

"It’s very promising, because our spray was a relatively crude formulation of vegetable oil and kerosene, used in 35 degree heat in intense sunshine," says Steve Edgington. "If you made a commercial formulation you’d have to look at ingredients that protect the fungus from the damaging effects of sunlight and we didn’t add anything like this.

"We also sprayed over the top of the crop, but a lot of the juveniles were spending their time at the base of the wheat stalks, and even in the cracks of the soil. So you might want to look at spraying to target exactly where they were. It was a very good result in less than perfect conditions."

The team doesn’t yet know why the fungus worked on the juvenile Sunn Pests and not the adults, but with another three years hard work, Steve Edgington suggests that an international research team could have completed large-scale field trials and be close to getting a commercial formulation onto the market. And at that point, the Sunn Pest, which currently causes such economic damage to many of the world’s poorest countries, could be well on its way to being routed for good.

More information
CABI Bioscience