Plant Compatability as Natural Insecticide

Including aromatic plants and herbs around the garden, to act as pest deterrents.
Well, yeah, certainly this can deter insect pests, but 'most' aromatic plants will just sit there minding their own business. In order for these plants to be effective, they need to be in high traffic areas, where either they are being walked on, or brushed against everytime someone passes by. The plants must be disturbed in some fashion - such as crushed leaves and branches, or indeed the breaking of roots- in order to stimulate the release of the aromatic oils.
As an experiment, plant a citronella geranium in your outdoor entertainment area. For that matter, plant 20 of them - unless you are actively bashing these plants every now and then - you are still going to be attacked by mosquitoes.
Similarly, marigolds are said to be good for getting rid of nematodes.
Well now, this is quite true, but - it is a particular variety of marigold that is best at achieving this. Not the lovely heavy-flowering varieties, so attractive in the garden, no such luck. In fact it is Tagetes minuta, or Stinking roger. You would NOT plant this on purpose. It can grow to 12 feet, a magnifying glass is needed to see the flowers, and the miniscule seed is shed anytime the plant is disturbed - by the thousands. Not only will this plant out-compete everything else in the garden - it can and will take over.
Let's have a look at mint. Highly aromatic and hardy, can be used as a living mulch in high traffic areas. However, The only mint I would use is Mentha pulegium, or Pennyroyal, as it grows on the surface of the soil and is relatively 'soft'. Most other mints, notably Mentha suaveolens, or Apple mint (sometimes called woolly mint), and Mentha spicata, or Common mint, are worthy plants in their own right, plant them in high traffic areas, however, and you've got trouble.
These plants can reproduce by seed, but most notable is the speed at which they send out runners - all across your garden, and deep, looking for areas of no traffic and moisture. This means IN your garden. Often travelling UNDER your plants, so once established, is not any easy beastie to get rid of.
It is often propounded that these mints will grow in poor soil areas and don't particularly 'like' rich soil. Well, plant some in your compost-heap and see how that works for you!
Personally, I would only plant it: in a concrete pot, sitting on concrete, preferrably on someone else's property.
Some plants do, however, exude their aromas without any help - carrots and the onion family spring readily to mind, all they need is a bit of sun.
For the most part, this type of 'insect control' is that of a 'deterrent'. Nothing more.

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