Swarms of locusts have invaded an outback town in Queensland, damaging vegetation and crops.

Locals said the locusts began thronging at Longreach last Thursday and by the end of the weekend, most of the trees were bald.

"They really hit us about on Thursday so by Saturday and Sunday there are adults, really sort of eating up a fair bit of fodder," Mayor John Palmer said.

Grasshoppers are known to change and become locusts to migrate when their colonies face food shortages due to increasing population.

"Tree behind me, that was flourishing on Saturday before we went away for the weekend and we just come home and it was gone," a local resident Chris Ford said.

The plague is said to be the biggest to hit the region in three decades. Farmers east of Longreach who were enjoying a relatively good harvest season are worried that the pests will reach their fields.