A mid-west Western Australian agronomist says mice numbers in the northern agricultural region have reached near plague proportions.

Growers in Yuna, Binnu and Mullewa have reported increased mice activity in their paddocks over recent weeks.

The mice cause significant damage by eating the nodes of flowering and early podding canola and lupin crops.

Paul Norgate from Great Northern Rural Services says the problem is the worst he has seen in almost 30 years in the industry.

"Haven't seen these levels of numbers certainly in my time in this northern agricultural region," he said.

"I mean ... we've got farmers coming in now ... they're attacking into their machinery ... they're checking their headers ... and they've got populations of mice that have already been in there, chewing wires and whatnot."