I believe there are Lawn Grub killers that are on the market. Just stick the container on the end of the hose and water the affected area. By accounts I've heard they work. HTH teddy,J

They are actually a member of the moth family and are commonly called lawn armyworm due to their numbers when they attack.
Firstly ensure it is in fact armyworm by laying a wet/damp hessian bag (or carpet) on the lawn near the brown patches and check it later. There should be some under it.
If it is armyworm then go and see a nursery to get good advice on what product. Baythroid, Pennside etc. are great products but all require different methods and therefore the directions should be read properly.
Always change active chemical from season to season to ensure there is not a resistance over time.
The ready made spray packs are not worth it from experience. Firstly, you are dealing with a fairly dangerous chemical which should never be attached to your drinking water supply or sprayed willy nilly over an area and secondly it seems to never get an even coverage which is important.
Anyone who uses spray attachments should have a backflow preventer fitted as you can never be too careful when it comes to chemicals in your drinking water.
BE QUICK TO ACT AS THEY CAN RUIN A LAWN IN NO TIME.

There are stacks of diferent types of lawn grubs and you'll have to identify the grub to use the right chemical. On some of the containers they have little pictures that might help you identify which grub your friends have. A client of mine had lawn grubs in January that wrecked their couch lawns, but everything I tried did'nt seem to be very effective.
The one that seemed to be the most effective was the hose click on type called Lawn Grub Killer. But as nunoftheabovescuds said, fit a backflow prevention device such as a "Hose tap vacuum breaker". It just screws on to the tap. It will prevent chemicals going into your water. You can buy them from irrigation suppliers and I think Bunnings might even sell them too. They're about $30.

If you have an irrigation system permanently intalled, a one way (none return) valve should have been included/used when it was being installed.
Otherwise I would'nt waste my money buying things to prevent a most unlikely backflow. Nothing can run against the flow of the water pressure in a hose while it is running. Simply let the hose run for a couple of minutes after disconnecting the chemical dispenser from it. If you have a very fertile imagination though and are really worried, and to make doubly sure, once the the watering is over you could simply disconnect the hose after use. ATB teddy,J

Teddy,
You can have backflow while the tap is running. The chance of it happening is about 1 Billion to one though. It can happen if the water board flush the mains out, or if Fire fighters connect to the mains. Water can be sucked out of all the pipes in the area(very unlikely though)but it can happen.
When I was doing my irrigation course they showed examples of it happening where people actually died because of the chemicals they accidentally drank.
Sydney Water are very strict about using Backflows on irrigation systems and making sure they are specific heights , but allow people to connect hoses with dangerous chemicals without backflow devices. The only thing that might get sucked through an irrigation system would be a bit of dirt!
Bryan

Hi Bryan, Yes mate the chances are very remote of a feed back into a house from a garden hose. In fact one would have a bigger chance of being trampled to death by an elephant in the middle of Sydney than being poisoned by a chemical sucked from a hose dispenser into the reticulated household system. To the best of my knowledge these 'garden hose' dispensers all work on a venturi system and the chemical has to be sucked out by the vacuum created by water passing over its venturi valve. In the other direction, only air would be sucked back into the hose unless one deliberately turned the applicator bottle upside down.
In the case of my old commercial irrigation system. It had been set up to feed and spray when required to and there was a 50 gal injector tank added, the outlet of which worked on a system similar to a venturi valve but at the bottom of the container. It was therefore (remotely) possible that chemicals could have syphoned back into the main line so there was a none return valve fitted to the main line. ATB teddy,J

When I was doing my course, they re-enacted an accident scene where a guy was using one of these click on things. It was dramatised to scare us! He's out there using it when the water board flush the mains around the corner. His spray stops working so he goes inside to get a drink of water from his kitchen tap, and unknowingly drinks the chemical he's using which has been sucked back through the pipes. Then he goes back outside to see if its working again and it is, so he finishes off spraying then he collapses and they tell us he later dies!
Never know, it could happen.
They showed us another one, where it was at a bowling club. The greenkeepers had to spray for lawn grubs so they put the hose into the spray tank and left it to fill up while they had their morning tea. Same thing happened with the water board flushing mains and the chemicals were sucked back into the kitchen of the bowling club, where the old ladies were making themselves a cuppa, and drank the chemicals. You'd have to be unlucky! But you'd probably have a better chance of getting struck by lightning or watching Souths win a game of footy.
Bryan