An ancient enemy looms in legions of biblical scale

This is the third day in a row of horrible heat in the Mallee, perfect conditions to turn hoppers into flyers. And that's the word from here to Mildura - they're taking to the air and any day now they'll start swarming.

''They swarmed last April,'' says Chris Baker. ''It's a frightening thing to see … overnight I saw 450 acres [180 hectares] of young oats vanish. All that remained was bare ground.''

The April swarming carpeted local bowling greens and sports fields, and choked car radiators.

But, as Mr Baker says - and it is a mantra heard throughout the region - it was nothing compared with what's coming.

The scale of what's coming is anyone's guess, but it won't just be those in rural areas affected; the city will not survive this locust plague unscathed.

Mr Baker farms wheat and sheep. For the past 19 days he's been spraying around his 800 hectares of wheat crop and neglecting other work that needs to be done. Some growers haven't bothered. ''They figure they'll lose 10 per cent of their crop and all the work isn't worth the trouble. But it means the rest of us are taking up the slack.''

Frustration has led to the breaking of a serious rural taboo.

''Some blokes have been jumping on to other people's land and spraying it,'' says Mr Baker. ''It's fractured the community. We're going to have to do some mending when all this is finished.''

But no one can say when the plague will be over. Without intervention, the locusts will keep breeding and spreading until they've eaten every bit of greenery they can find, including suburban lawns.

It's also a matter of guesswork as to when the swarming will begin, although it's most likely to occur in the next week if rains that began at the weekend ease off. There are no mathematical models predicting how the locust population will blow out, or even how big it is now.

''Have you heard anyone talk about multiple generations?'' asks Chris Baker. ''Because I dug up some eggs the other day.''

Many farmers believe there have been several breeding events because they continue to find locust eggs and hoppers at various stages of development. But according to the Department of Primary Industries' Robert Walker, incident controller at the Mildura Regional Locust Management Centre, the locusts taking over Victoria are a single generation, bred from eggs laid in autumn. Most of the eggs lay dormant through the winter and started to hatch in spring.

''We believe there will be another breeding event in a month or so,'' says Mr Walker. ''And they'll breed again in autumn. What we're concerned with at the moment is the imminent swarming of this generation, the impact of which is being seriously underestimated in the cities.''

The plague locust is a species of grasshopper that normally occurs in very low numbers throughout Australia. According to the DPI's head entomologist, Mali Malipatil, they usually live a solitary existence. But once numbers multiply, ''they undergo a behavioural change and start to band together''. Once they hatch, the hoppers move through five phases (or instars) of development. In the third and fourth stage they band on the ground, and move like an army, eating vegetation in their path.

Each band contains a thousand or more locusts per square metre. They make an easy target for contact spraying, but high wind or rain makes spraying impossible. In high temperatures, it is thought some chemicals will evaporate once they leave the nozzle, making them ineffective.

The DPI is conducting tests on efficacy of spraying, and Dr Malipatil is assessing dead hoppers to verify the correct species are being killed.

Half an hour south-west of Mildura, the sun is setting as Colin Hunt drives out to look at the patch of stubbled ground he sprayed earlier in the day. The road is red-brick dirt, flanked by a sea of wheat. The crop, almost harvest-ready, barely knee-high, is rich with the promise of bread. As the ute bumps along, a locust jumps through the window and perches on the seat within thumping distance. But Mr Hunt lets the devil ride along with him.

Pulling up at the killing field, a scrubby piece of pasture about 200 metres from the crop, Mr Hunt explains he's been using a contact spray, meaning he's had to hit each grasshopper to kill it. Just about every day for the past five weeks, he's been laying into them. He's not sure how successful this particular day has been. ''They're at a stage where quite a few just flew off. There's not a lot you can do once they start flying.''

Now, as Mr Hunt steps from his ute, several dozen locusts rise to greet him. Every footstep sets off another shower of activity. Some land a few metres away. Others keep on going to God knows where. ''I'd have thought some of those younger ones would be dead by now,'' he says.

The DPI is favouring a fungus-based spray that takes several weeks to kill the hoppers, while farmers are keen on the faster-acting sprays and residual chemicals to control new arrivals - all of which have specified withholding periods, meaning crops can't be harvested for a prescribed number of days after spraying. Little of the spraying occurs on crops because locusts tend to breed in nearby pasture and scrublands. But stock is vulnerable and also requires a withholding period before going to market.

Where previous plagues 40 to 70 years ago were met with heavy-handed and dangerous chemical responses, the DPI's Robert Walker says demand for environmentally safe treatments makes dealing with the plague ''a matter of complexity''.

He says: ''As long as farmers are following labelled instructions as to the use of chemicals, there is no problem.''

But a number of farmers told the The Sunday Age they're aware that other growers are using chemicals at higher concentrations than recommended to ensure killing success - news Mr Walker finds ''highly disturbing''.

Another hard reality is that many areas of the state can't be treated. Shane Southon, a ranger at the Hattah-Kulkyne National Park, says ''we're trying to get as much done as we can before they begin to swarm, but there are plenty of gullies you just can't get into. You have to wonder how much good you're doing. You have to do something to slow them, but that's all you're doing.''