MASSIVE swarms of locusts have completely destroyed tens of thousands of hectares of forage and self-sown pasture crops in the Mallee.

The swarms have been described as the "worst autumn invasion ever" and have turned the best start to a season in years back into a "drought situation" for hundreds of farmers.

The locusts are also thwarting early grain-crop sowing, with the threat of large-scale damage forcing many growers to put their plans on hold.

And there are fears that spring will see mature crops attacked before they can be harvested.

Department of Primary Industries Swan Hill agronomist Rob Sonogan said the locust attacks had caused "stress-levels you wouldn't believe".

"Farmers have gone from the best start to a season in nearly 10 years to being back in a drought situation," Mr Sonogan said.

"The stress-levels in these farmers are extraordinarily high - they are some of the most stressed farmers I have seen in 10 years of dealing with drought."

Mr Sonogan said farmers were now hand-feeding stock when just a week ago they had large tracts of thriving grazing crops.

"It's fair to assume that 50 per cent of the Mallee has had its pastures, if not completely destroyed, certainly severely affected," he said.

Kyalite fat lamb and crop producer Greg Rae said locusts had totally wiped out his 1700ha of vetch, dual-purpose grazing wheat, barley and lucerne.

"It was all for sheep-feed and now we are back to hand-feeding again - it's quite unbelievable the damage they did," Mr Rae said.

"We are about spent here. Everyone is worn out and sick of it."

Piangil sheep and crop producer Peter Nichols said locusts had destroyed 75 per cent of his ryegrass and 85 per cent of his cereals across about 200ha.

"They are attacking everything. It's just devastating out there," Mr Nichols said yesterday morning.

"We sowed everything on the big rain and it was all looking fantastic but now we are back to hand-feeding.

"You don't understand the harshness of it until they land and completely turn your paddocks from green to black in two days."

Speed farmer Peter O'Brien said he had lost about 200ha of sheep-feed and 600-700ha of self-sown pasture.

"We sowed early and it was magnificent ... now it's like we haven't sown anything," Mr O'Brien said.

Mr O'Brien said he had no idea when he should begin sowing his milling crops given the threat of further locust damage.

"It's pretty hard to decide to start sowing if you think it's just going to disappear," he said.

The Australian Plague Locust Commission said farmers delaying sowing plans were right to be concerned about potential damage to crops.

APLC director Chris Adriaansen said farmers needed to make a "risk decision" on whether to sow their crops or not.

"I can understand that farmers want to sow now as the moisture is right and the soil temperature is right, so germination will be quicker than if they waited, but there is the risk of locust damage," Mr Adriaansen said.

"What we can say though is when the cold weather sets in, the locusts will probably only hang around for two weeks and you would see significant falls in populations within a week of the cold weather starting."

Sea Lake farmer Bryan Hanns was contemplating last week whether to continue with his sowing program by planting his wheat and barley crops.

"It's really up in the air (on what we decide to do)," Mr Hanns said. Victorian Agriculture Minister Joe Helper said the Government would step in to help control the pest in spring.

The Government is likely to run its own aerial spraying program but questions remain as to whether it will supply landholders with free chemicals to control locusts, as they did in 2006.

Mr Helper would not be drawn on the issue, arguing it was too early to make such a decision.

Mr Sonogan said in his 35 years in the Mallee, "this is certainly the largest number in an autumn invasion I have ever seen, and farmers 20 years my senior are saying the same thing".