Slugs & SnailsĀ 

Usually the bane of every gardener, snails and slugs can actually be relatively easily deterred from your windowbox, provided that you start off with slug-free and slug egg-free compost. One of the best deterrants is to run a strip of copper foil (available at garden centres) around the top of your windowbox. This generates a very low electric current, effectively creating an electric fence that your slimy foe won't cross! Around your plants you could also put coffee grounds, bran, crushed mussel or egg shells, all of which slugs & snails hate, but which will need renewing form time to time.

Top Tip 1 : Plant red varieties of lettuce and they'll be untouched by slugs & snails!
Top Tip 2: Plant well-established pot-grown plants whenever possible as these will prove sturdier to any slug or snail attack

Aphids : Greenfly / Blackfly / Whitefly

For small infestations of any of these tiny, but nasty pests the best thing is to just squash the critters with your fingers - It's best to do this early on as they can reproduce themselves exponentially in a very short space of time, at which point they can be really problematic. For larger infestations or to get those hiding out of reach of your fingers you can spray your plants with various organic pesticides - Manufactured ones can be bought from a garden centre or you can make your own, by boiling garlic or rhubarb leaves and using the concentrated liquid (obviously make sure you thorougly wash your crops after harvesting)

Caterpillar of Cabbage White butterfly

Caterpillars

If you're growing any kind of Brassica (eg. Kale, Cabbage or Brussel sprouts) then you're very likely to get butterflies landing on your plants and laying their eggs. Cabbage White butterflies and their caterpillars in particular (see above) can be a right menace and they're best dealt with as soon as you see them. Either squash the eggs or pull off the caterpillars on a regular basis. Bear in mind that other butterfly species may target some of your other other crops so it pays to be alert to the threat and treat them in exactly the same way.