An Invasion of Diamonds

Diamond Back Moth

Diamond Back Moth

It’s not often that moths get on the news but recently there have been scare stories appearing about a ‘mass invasion’ of a tiny micro-moth which is supposedly going to be a threat to the nation’s cabbages! As ever, the likely issues are being exaggerated but it is an interesting phenomenon nonetheless.

The diamond-back moth (Plutella xylostella) is a tiny micro-moth so-called because of the bright line of diamond-shaped markings running along its back (see photo). It is a migrant species that turns up in the UK every year but 2016 has seen huge numbers turning up across the country. It’s impossible to estimate just how many have landed on our shores but thousands have been reported turning up in some moth traps in the south-east of England and a number of moth trappers in Cardiff have had hundreds waiting in their traps in the morning. The total number that have arrived must be in the millions and we have recorded them from a number of our nature reserves across our patch, with a maximum of 42 having been found in our moth trap at Parc Slip up to now though numbers do appear to be dropping off now.

They can easily be seen being disturbed from areas of grassland during the day too but it’s not always easily to get particularly close to them. The species has a worldwide distribution and the caterpillars do feed on members of the brassica family on which it can sometimes be a pest. It is not yet known whether they will cause any problems with our crops this summer – it may be that the birds and other predators make the most of the glut of food and the weather may also limit the breeding success of the moth if we suffer from a period of cool, damp weather (not unheard of in summer sadly).

Instead we should just marvel that such a tiny insect is able to migrate here from the continent every year and enjoy the spectacle as who knows when such a large migration event will happen again!