Archie McLaren from the Highlands & Islands Growers passes on the following alarming information:
I just want to warn all members to be on the lookout for Diamond-back Moth. In the past few evenings I've noticed small moths flying up from the Brassicas when I do the watering, and this afternoon I identified the culprit at close quarters. It's definitely Diamond-back or Diamond-backed Moth, Plutella xylostella, the world's most serious pest on Brassica crops. I've seen it many times before, but only once in UK. It's a tiny moth about 1cm in length at most, and you rarely see it until it flies up from amongst the crop, then it's hard to follow them to see where they land again. They have a pretty amazing "R" rate, one life cycle completing in 15-30 days so their numbers can increase exponentially! Within a week or two their larvae are decimating your Brassicas - and this includes Wallflower, Aubrieta and Alyssum, not just the edibles. The adult, if you can get hold of one, is smaller than a clothes moth, and has 4 vaguely diamond-shaped markings on its back, hence the name.
There was a fairly major outbreak about 5 years ago, and certainly in the Nairn area almost all gardeners lost their cabbages, cauliflowers, sprouts etc. The moths come in from the continent, and this lot have probably come in from Scandinavia with its warmer summers, on the easterly winds which have predominated recently. Prior to that outbreak, the internet reports a UK outbreak in the late 1980's when I was abroad, and before that in the 1950's. Notice how the first three were approx. 30 years apart, now we're talking 5 years - this might say something about global warming!
I'm not sure how this affects commercial growers of Brassicas. The moths may only infest the field margins in which case losses will be minimised. But on small plots, gardens and allotments you can expect a total crop loss if these moths continue to spread, which seems likely in the current warm weather. A change to colder or wetter weather might stop them in their tracks, and it may be that they will not penetrate far inland, but certainly all our members should be aware of the problem.
What can you do? Well, if you had already covered your brassicas with fleece or stuff like "enviromesh", all should be well. If not, it may already be too late. Pyrethroids are effective, but are probably no longer available to amateur growers. Yellow sticky traps in the crop may help trap a few hundred adults, but on current evidence there will soon be millions of these moths around. SB Plant Invigorator might have some effect if used weekly, and is organic. Other things like garlic solution, washing powder in dilute mix with water, and other home remedies might help. If you do nothing, and the moths arrive at your garden, you will lose the whole crop.
Be warned, please, and take appropriate action!