Do cabbage moth decoys really work?

Brassicas just don't grow at my place because of cabbage white moth caterpillars.

I won't spend money. So no nets, no purchased sprays. The only options I've found for me are (1) regular chilli-garlic spray just on the brassicas, so I don't kill all beneficial insects (easy), and (2) cabbage moth decoys which are white plastic cut into shapes of the moths which mistake them for a real moth and so avoid competition.

Plenty of people say they're trying but don't follow up with results. Some companies sell decoys but I don't know whether to trust them.

I feel your pain.  I've got butterflies too, and I can't really see how decoys would work.  That said, I've never tried so I can't completely disavow them.  

If you can grow over winter, I would recommend doing a winter/spring brassica crop--sow it late summer/early autumn and harvest in six to nine months time, depending on variety.  I grow cauliflowers this way--sown in late September, overwintered in a sheltered spot, then they crop for me early June before the butterflies are active.  I also grow two kinds of cabbage for overwintering, one for late winter harvest and the other for May-June.  

And I find that some brassicas are vigorous enough and have a long enough growing season, they can shake off the damage anyway;  purple sprouting broccoli does this for me--it's got pretty much a 12 month growing period between sowing and harvest, and I harvest in April/May before those pesky bugs are out.  I've had young plants eaten bare by caterpillars in summer, then continue to grow over winter and give a big harvest in spring.  I have a Savoy cabbage like this too:  I'm harvesting them now (July-August) and even though the outer leaves can carry dozens of caterpillars, the damage to the big heads and inner leaves is minimal, as the plants are already so big.

Finally, as a cheap or free option for netting:  look for sheer curtains at your local charity/thrift/op shop.  Or ask around on freecycle.  I've been collecting them piece by piece and they work just as well as manufactured insect mesh, so long as the weave is close enough (no big gaps in the fabric).

As a fun aside, I've noticed a big increase in parasitic wasps in the last few years.  I have a nasturtium growing up the wall right next to my outdoor tap where I fill my watering can every day, and I've watched the wasps wreak carnage on the caterpillars.  Though there has been a lot of damage to the nasturtium, there are currently only a couple caterpillars left crawling around;  the others are immobilized or already dried out husks.  I can't say for certain my cabbages are getting the same treatment (as I don't give them the same amount of daily scrutiny) but I assume so.

Yes I've seen fewer moths during the colder months. There's no harm in making decoys to see if they have any effect I guess. I'll chilli spray all brassicas every few days too.

You don't need specialized parasitic wasps. We have paper wasps of some variety and mud wasps and I've watched them "harvesting" the cabbage butterfly larvae off my brassicas. So long as their chosen nest site is not in a problem location, they generally leave me alone which is essential because I react *really* badly to wasp stings. So you could look up local wasp varieties and see if there's anything you can do to attract them rather than trying to discourage the far more numerous butterflies that may be doing a certain amount of pollinating.

Jay Angler wrote:You don't need specialized parasitic wasps. We have paper wasps of some variety and mud wasps and I've watched them "harvesting" the cabbage butterfly larvae off my brassicas. So long as their chosen nest site is not in a problem location, they generally leave me alone which is essential because I react *really* badly to wasp stings. So you could look up local wasp varieties and see if there's anything you can do to attract them rather than trying to discourage the far more numerous butterflies that may be doing a certain amount of pollinating.

So true! I had the same experience!

My previous place had wasps all around the yard. I'd water my terracotta pots and they'd be digging into the soil. I must remember to fill some pots with just heavy clay soil to give them a home!

I leave broke pots upside down to provide insect habitat. I'll look into more ways to encourage wasps, and dragonflies.

Just thought I'd share my experience.......last year, I laboriously made realistic looking decoys, put about 12 in a small patch, didn't work.  
This year I made strings of very simple decoys as follows - and it's WORKING!!
I used thin fishing line, opaque white shopping bags cut into rectangles roughly 7 cm x 5 cm or 2" x 2 3/4".  Cut off about 3M of line, find the middle, tie a slipknot, pinch/scrunch the rectangle in the middle of the long side,  put it through the slipknot and secure with another knot, spacing each butterfly about 15- 18 cm or 6-7" apart, continue till you've reached the end and then do the other side of the fishing line. The shorter lengths are easier to pull through when making the second knot and I just tied each run of fishing line together when installing it.  
I strung these up all around the outside of the brassica bed and crisscrossed them over the beds using stick/stakes.  I used about 50 butterflies per square meter.  
The cabbage moths are checking out the bed and then flying off WITHOUT LANDING WOO HOO!!  I think because I used so many they are seeing the patch as already having too much competition.  I've been observing this for over a week - and they are definitely avoiding that bed!  
Cheap, easy and I hope this helps someone else! Cheers