Leek Moth in Britain: spread and chemical-free control

The Leek Moth (Acrolepiopsis assectella) is a native of continental Europe and Siberia and can be a significant pest of various Allium crops, especially leeks. First recorded in Britain in 2003, it has spread around southern and eastern England, and although it is largely found near the coast, it does also occur further inland but is generally rather scarce and localised. The NBN map below shows the approximate current distribution although they are highly likely to have been found on other sites – the dot marked with an arrow is the location I mention below and was added by me (it’ll end up on NBN eventually!); it’s also spreading north and west and has been found in North Wales. I don’t know of any research on this, but it does seem to be a candidate for a species that’s benefiting from the warmer average temperatures associated with climate change – it would be interesting to see if this has been analysed, although a longer dataset would probably be needed than available in Britain – there may be a recorded spread through continental Europe as well... In any case, in Britain at present, although it does have localised impacts, its numbers are too low to really be considered a major pest, at least yet.

NBN map of leek moth distribution in Britain, though it has now moved further north...

However, research in Canada (Mason et al. 2010) has looked at the species biology and development in the Ottawa Valley where it has also recently become established. This found that at a threshold of 7°C, populations required 444.6 day-degrees for development from egg to adult, and noted that there were spring, early-summer and late-summer flight periods with adults overwintering. Depending on the temperature pattern, the life cycle took 3–6 weeks in the field, with up to three generations being produced. Comparing this to the case here in Britain, as temperatures rise in spring, the moths become active with egg-laying in April and May. Larvae hatch after about a week, feeding for about a month before moving back up the leaves and pupating in silken cocoons.

Leek moth larvae in situ showing feeding damage.

Damage from younger larvae higher up the leaves; note the frass to the left.

A fairly mature larva approaching 1cm in length.

A second generation of larvae feed from August to October; as far as I am aware, there is currently no third generation as seen in Canada, but I wonder if climate change may shorten the life cycle and/or extend the egg-laying/feeding season and thus permit a third generation to be produced. I will be checking leeks this summer and will note the occurrence of leek moth larvae to see if I can collect useful data on their life cycle.

The adult is harmless and fairly nondescript in appearance, but the young larvae burrow inside leaves, and as they grow they move down into the stem or bulb – this phase causes more serious damage and also facilitates secondary bacterial or fungal infection. This is when the leaves tend to turn yellow and slimy. On the whole, they are not a major problem for commercial growers as they are controlled by other pest-control activities; however for gardeners, allotment holders, and other small-scale producers, as well as organic farmers, they can cause a great deal of damage. You’ll see from my pages on the right that there is one about Highbridge Farm – this is the location of our local community farm project and as a chemical-free (though not organic-certified) concern, we were not pleased to find leek moth larvae happily burrowing through our leeks; it’s also where the photos of larvae were taken.

An adult leek moth; very similar to other species in the family Yponomeutidae.

However, as a community farm we do have many pairs of hands, so mechanical (but not mechanised) control is available in the form of us picking off the affected parts at an early stage, and pulling up badly affected plants if they were not identified quickly enough. These can then be composted. This does work (leeks are tough and recover well from the damage), although I have heard anecdotal reports of leeks being effectively ungrowable on a small scale in some places. In general, if keeping away from pesticides, any or all of the following measures are ones that I personally like:

• Check plants for damage in the spring; remove and destroy any caterpillars, pupae and damaged parts, and destroy severely infested plants.
• Protect the crop, from seedlings onwards, with covers such as horticultural fleece to prevent adult moths from laying eggs.
• Later planting (after May) may avoid the first generation of larvae.
• Encourage predators - birds, bats, hedgehogs, frogs and beetles will eat adults, pupae and larvae.

Other options include clearing away plant debris at harvest (as adults and pupae can overwinter in this) and dig over the soil to disturb overwintering adults and pupae; however they both involve unwanted toil and I feel are likely to impact on non-pest, even beneficial, invertebrates as well.

So, from a veg-growing point of view I hope they don’t appear this year (if they didn't trash the leeks, my bug-nerd side would consider them an interesting and scarce species), but if they do, it will at least be an opportunity to try to collect useful data – if I do, you’ll be able to read about it here...

Reference


Mason, P.G., Appleby, M., Juneja, S., Allen, J., & Landry, J.-F. (2010). Biology and Development of Acrolepiopsis assectella (Lepidoptera: Acrolepiidae) in Eastern Ontario The Canadian Entomologist 142(4):393-404. 2010 , 142 (4), 393-404 : 10.4039/n10-026