A Mallee grower in South Australia says locusts have eaten their way through hundreds of hectares of early-sown crops.

Wunkar farmer Ian Cass said he thought sowing crops including rye slightly early was a good idea after good rains, but they are now all gone.

"100 per cent of that, the whole lot's gone, there's not even a blade left out there of any kind," he said.

With sowing to begin soon throughout the region, there is widespread concern about how long locusts will hang around.

South Australian Research and Development Institute entomologist Ken Henry says temperatures need to cool down before the locust problem will ease.

"The locusts [will] probably be around 'til the middle of winter," he said.

Growers are being encouraged to watch for locust eggs which would hatch in spring.