I went out to the garden today to weed and discovered large holes in the leaves of my kohlrabi (see photo). What is doing this, and how can I get it to stop, preferably using organic methods so I can eat the kohlrabi leaves myself later? I didn't see any insects when I poked around a bit.
It is damage from a caterpillar. It has eaten a lot, so it may have metamorphosed into a butterfly already. Look under the leaves and along the stem. If you are keen you can also regularly remove the eggs which are often under the leaves about a centimetre from the edge.
That looks more like slug damage than cabbageworm damage, especially since many of the holes are in the middle of the leaf and don't hit an edge (caterpillars feed from the outside in, not from the inside out). Another indication of slug damage is the rounded holes. Slugs have tiny teeth and always leave rounded holes, not jagged ones like those left by caterpillars.
To remove the slugs organically, fill a shallow dish with beer and put in your kohlrabi bed. Partially covering it is not a bad idea if you're expecting rain. The slugs are attracted to the beer and drown in it.
