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Who’s been eating MY leeks?

The Leek Moth caterpillar and the mess it creates

Caught in the act!

In addition to the attempted decimation of my tomato crop by the perpetually hungry Tomato Moth, I have been battling a pest which has increased in numbers in my garden over the past few years.

This particular irritant is the Leek Moth (Acrolepiopsis assectella). Well, in truth, it’s not the moth itself, but rather its caterpillars and there are two generations of them every year. The first generation causes damage in May and June, these then become adults lay more eggs and a second generation of caterpillars emerge to cause damage from August to October. Thankfully I wasn’t bothered by it earlier in the year due to the dry spring conditions, but a return to the damp, temperate climate towards the end of August has seen me fighting my nemesis on an almost daily basis.

A Tomato Moth caterpillar amongst some onions

What! Tomato Moth too?

Unfortunately, caterpillars of the Leek Moth don’t just consign themselves to my leeks, but I’ve also suffered from them on my main crop onions, tree onions (RIP), Welsh onions and chives. Apparently, they can also be a pest of garlic and shallots, but I’ve found this to be less of a problem, probably because the plants of these are usually over and out of the ground before the main onslaught in late summer/autumn.

Slugs and slug damage on a leaf

Slugs! Adding insult to injury...

Typical symptoms see the leaves develop whitish patches where young caterpillars have burrowed inside and are feeding within the leaves. Older caterpillars then tunnel down into the stem/bulb and cause extensive damage. You normally see early signs of this as bright green frass (caterpillar poo) and if you catch it too late, secondary rotting turns the inner parts of the plant to sludge and slime (this is particularly icky to touch and sticking your tongue out is a requirement). At this point, you may also get slugs and snails showing interest in the demise of your plants, which just adds insult to injury.

Different stages of the pupating Leek Moth

A silken cocoon contains the pupa

WARNING! SCIENCE! The moth itself is small, inconspicuous and brown (an LBJ -Little Brown Job). The caterpillars are yellow-green in colour with grey-brown patches and a yellowish brown head, with mature caterpillars growing up to 13mm long. Caterpillars pupate as reddish-brown pupae contained within white silk cocoons attached to foliage or debris around the plants. Adult moths also overwinter in plant debris.

Oddly enough, I only seem to get massive damage from this pest on my plants at home and only a small amount (if any) on my plants up the allotment. I think this may be due to the different micro-climates of each. My home garden is confined by fences and hedging, leading to a still, warm and humid environment, whereas the allotment is on the side of a hill, is more open and tends to be fairly breezy and dry. Not to mention that the soil conditions are different too and I have to say, when growing the same variety of leeks in both places, the ones up the allotment are always much taller and fatter than anything I could ever grow at home.

Cutting leeks to remove Leek Moth caterpillars

Drastic action is sometimes required

There isn’t much that can be done to get rid of these little blighters, but there are some things you can do to reduce the numbers of caterpillars, moths and as a result, damage to your crop:

  • Clear away all plant debris and dig over the soil in winter to disturb hibernating adults and pupae
  • Check leek plants for damage in the spring, removing and destroying any caterpillars and pupae visible on the leaves
  • Destroy severely infested plants
  • Protect the crop, from seedling stage onwards, with horticultural fleece or Enviromesh* to prevent adult moths from laying eggs
  • Late plantings after May can avoid the first generation of caterpillars
  • Encourage birds, bats, hedgehogs, frogs and beetles into your garden, as they will happily munch through moths, pupae and caterpillars.
Leeks and Onions recovering and regrowing after being cut back

Amazing onions!

I also manage damaged leeks and onions that aren’t too severely affected, but obviously suffering, by cutting back foliage that is under attack. With both onions and leeks, the cutting looks quite severe, but they both seem to bounce back, even with repeated cutting, putting forth new growth that goes on to produce a decent harvest.

*A note about Environmesh. I’ve used this stuff for a few years now, both as a screen around my carrots to reduce the dreaded carrot-fly and also as a cover for my cabbages to hold off the cabbage whites (I’m seriously considering it for my onion bed next year too). It’s quite expensive to buy, but it’s UV stable and will last for many, many years of happy, pest-free gardening.

Environmesh protecting vegetable crops

Nothing beats Enviromesh!

You can buy it in lengths (like fleece) or in custom-made shapes for going over raised-beds, hoops and the like. I’ve even seen covers with zips! If you’re serious about growing veg and have suffered for years, do yourself a favour and suffer no more. I bought mine from Harrod Horticultural, as I wanted fitted covers for my raised beds, which just happen to be the same as one of their standard raised bed sizes, however, Enviromesh is widely available and I’m sure there are a few bargains to be had if you look around…

Have you suffered from Leek Moth? Do you have any other ways to reduce this annoying pest? Tell me about it by leaving a comment below.

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  1. Caroline Foxhall
    Posted 2 November 2011 at 11:42am | Permalink

    Yes, I think I have had this pest as well – I’m picking masses of little brown seed shaped insects from inside my leeks, which are looking rather sorry for themselves in the ground. I first discovered this pest the year before last. Last year I started my leeks under nets because I also have a problem with birds ‘dust bathing’ in my veg patches and disturbing roots and didn’t get the problem.

    • Posted 22 March 2012 at 1:21pm | Permalink

      What a pain! You saved your crop from bird damage, but then the lack of biological control meant you got leek moth. I’m not sure there’s much way around either of those problems. Hope you managed to salvage some of your leeks.

  2. Sophie Royles
    Posted 22 March 2012 at 1:12pm | Permalink

    I’ve just found one!!!! O.O
    I planted my onions sets out end Feb as the weather was looking good. Its now 3rd week in March and I spotted something wrong with one of my onion leaves. I cut open the leaf and inside was this caterpillar. Its my first year growing veggies and although I’m prepared for a lot of things trying to get to my veggies before me, this was not something I knew anything about.
    Not even April and already my little plants are being attacked, I’m just going to end up covering my whole garden in mesh and be done with it lol 😀

    • Posted 22 March 2012 at 1:20pm | Permalink

      Wow! That’s early. I guess the mild winter and good weather we’ve been having has set things in motion early this year.

      Just keep being vigilant with your crop and take out any leaves that have been attacked to stop the little pests in their tracks. I hope you get some onions out of it all this year!

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