UNDER cover of moonlight, thieves have been stealing thousands of dollars worth of mangoes from orchards, just as growers were hoping to cash in on bumper prices.

The Courier Mail reports thefts in the Burdekin area, south of Townsville, have put growers on high alert, forcing them to protect their fruit with nightly patrols that could last until after Christmas.

Police and growers say mango thieves are highly opportunistic, striking when the mango market is running hottest.

The crime, known as moonlighting, is virtually impossible to police because there are no tests to trace a mango to the tree it was grown on.

Burdekin grower Sam Scalia has been robbed three separate times in the past fortnight, losing up to $30,000 worth of mangoes.

"We're trying to make a living and they're coming in, stealing the mangoes and we have to cop it on the chin," Mr Scalia said.

Queensland Police Northern Region crime co-ordinator Warren Webber said the thefts happened "periodically" when mangoes were scarcest and thus most expensive.

"In many instances it's in very rural areas so it can be very difficult to police," he told the Courier Mail. 'That's why it's very much dependent on the growers to be alert and take note of anyone who is in the area."

Burdekin District Mango Growers Group spokesman John Morton said police increased patrols about six years ago but thieves, who work in teams of up to six, were still active every year.

"We try to protect every piece of fruit we've got on the tree and some clowns come along and steal it," he said.

"They case the orchards, probably during the day, and then they come in at night, properly organised with vans or utes and manpower.

"It happens particularly under a full moon. I'm on the highway but it still doesn't stop them coming into my place."

Australian Mango Industry Association industry development manager Trevor Dunmall said the Burdekin region was the worst affected because its picking season usually coincided with the highest prices in the cycle.

The association has never estimated the total loss.

Read more on the Courier Mail online.