Friday, June 26, 2020

Diamond-back moth danger to Brassica plants - warning from Highlands & Islands Growers group

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!



Anonymous said...

Doesn't affect me personally, but good to read such well informed interesting information.

Unknown said...

My pest book said to inspect weekly and squash the eggs and caterpillars. I have been doing..I do not have many (and have also covered rather roughly with fleece) but I have no pictures to see what i am looking for. Are the caterpillars tiny and brown? Thank-you for the warning which was relayed to me by a friend..The search also alerted me to a cabbage white...Many thanks for the info.
Diana Brockbank in Forres

Archie McLaren said...

Almost impossible to get decent photos of the moths because they are so quick-moving. But there are some good photos on the internet if you Google the moth, but they tend to make it look huge, whereas it's smaller than clothes moths! Fortunately the colder, wetter weather seems to have stopped a major outbreak, but I still have some on unprotected kale, watercress and wallflower. The damage shows as large numbers of sort of "windows" through the leaves which eventually become shredded in appearance. The caterpillars are small (c. 0.5cm at present)and yellow, much smaller than those of cabbage white butterflies, but might appear brown at different stages of development.

Archie, Nairn

Unknown said...

Thank-you I am now quite familiar with the wee beasties!. By dilligent inspection of every leaf! I seem to be keeping it under control. I have seen a couple of "rafts" of eggs but guess they must be laid singly as well as they keep coming. Also the caterpillars sometimes
know I am coming and drop off the leaf on a fine thread!