What Cabbage Pest?

[Monika]

My husband set 19 winter cabbage plants which are now around 4 inches tall with a few leaves.

Their numbers keep dropping (now at 13 plants) due to some pest which chews into the stem just above ground level.  The pest doesn't bother with the leaves and leaves them on the ground.

Until today, he has found the leaves on the ground with no sign of any stem or roots underground but, today, he has found one chopped off as usual but with the roots still intact.

Could anyone please throw light on what is doing the damage?

[Monika]

[ancellsfarmer]

My thoughts equal cockchafer larvae:
https://www.google.co.uk/search?q=cockchafer+larvae&client=firefox-a&hs=xfz&rls=org.mozilla:en-GB:official&channel=sb&tbm=isch&tbo=u&source=univ&sa=X&ei=zMQIVL2pII307AbT7oDgAQ&ved=0CC4QsAQ&biw=1024&bih=611
Dig around the victim plant(5" radius) of stem and you may find the culprit. No available chemical remedy as far as I am aware for domestic use. Especially fond of leafmould, deadly to many plants. Those mandibles on the front are just like bolt cutters! Where you find one, there are likely to be others.

[Tee Gee]

due to some pest which chews into the stem just above ground level. 

The pest doesn't bother with the leaves and leaves them on the ground.

Sound like slugs / snails see here;

http://www.thegardenersalmanac.co.uk/Data/Slugs%20and%20Snails/Slugs%20&%20Snails.htm

Until today, he has found the leaves on the ground with no sign of any stem or roots underground

Sounds like Cabbage root fly see here;

http://www.thegardenersalmanac.co.uk/Data/Cabbage%20root%20fly/Cabbage%20Root%20fly.htm

but, today, he has found one chopped off as usual but with the roots still intact.

Slugs & snails again

Edit; I have just seen Ancell's suggestio and yes that is another suggestion.

[jimc]

Could be cutworms too