Beware of Immunity

by Siggy Franz

Medicine tells us, that many bacteria have become immune against antibiotics. The pharmaceutical industry is continuously working to keep one ahead of the adapting "baddies."

Similar problems have been occurring in the control of plant insects. This is of course of particular concern for agriculture. Many insects and mites have become immune towards some of the active ingredients in insecticides. Apparently it is an ever-increasing problem and obviously is also of importance to us, the Bonsai grower.

Investigations have shown five common causes, which contribute towards the development of resistant strains in insects. As bonsai growers we should be aware of these and ensure that we do not add to the problem.

  1. Make sure that the chosen insecticide is the appropriate one for the insect which causes a problem. Species-specific chemicals are better than "all-round" sprays.
  2. Correct application. Do not mix the insecticide weaker than recommended from the manufacturer, thinking that our bonsai trees do not need full strength. The insect is the same regardless if it eats the leaf of a garden tree or a shohin. If it says to also spray the undersides of leaves, make sure you do it. Increase the effectiveness by adding a wetting agent, such as a little washing-up liquid or a commercial product such as Ludwig's Stay Spray.
  3. It is a common mistake, to ignore the recommended repeat applications. These are necessary to "catch" the bug in its various stage of development. This also applies to systemic insectiicides, since these do not react on insects in the egg or larvae stages hiding in the crevices of the bark. A systemic poison can only deal with them after they start feeding, by which time the poison may have lost part of its effectiveness.
  4. Do not use stronger chemicals than absolutely necessary. There are a number of organic products that are very effective; some can be prepared at home. Also some insecticides are known to be much stronger than others and kill everything that is alive, even carnivorous insects like lady birds and praying mantas. These can play an important role in the control of pests.
  5. It is not recommended to spray "just in case". While it seems a good idea to spray before there is an infection, it has been shown that this can contribute towards immunity in insects.

The above shows, that immunity in pests is most often formed when insects are exposed to poisons which are too weak (wrong mix) or insecticides which were not actually intended for their species (wrong choice). While we should use as few chemical poisons as possible in order to affect the environment as little as possible, if we do use them we must apply them correctly in order not to create new additional problems.



  • Before transplanting or repotting allow the growing medium to dry out for a few days. This will allow the Growing medium to fall away from the roots without clinging to the roots and pulling hair roots off.
  • Always scrub your Bonsai containers before use to get rid of any bacteria from previous trees. Allow the pot to soak for a while in clean water to allow the pot to absorb water.
  • Loosen the root ball when repotting, teasing the roots out from the tangled mass of root ball.
  • Use sharp cutters when cutting roots.
  • If you have potted and have to travel, tie the tree into the container.
  • When repotting and you have taken the soil off the roots, check the soil mass on the table before cutting any roots as the falling away of the growing medium may have pulled off a substantial amount of roots and you may not have to take off as much as you thought.
  • When settling soil into the pot around the roots do not poke your stick roughly into the Growing Medium. Insert stick into soil mass and move from side to side in a jiggling motion. Poking roughly may break off roots.
  • Every year people contact me in Spring asking have I any growing medium to sell. They have had all year to get this together but they wait until now.
  • Do not use too fine a sieve material for the hole in the pot. Fine mesh gets blocked very quickly. If you use a coarse material you can cover with small pebbles.
  • Beginning August (in Cape Town) sort out your trees into two groups - ones that need potting and those that don't. Then sort the trees out in order of repotting. With me it's usually Swamps, Elms, Celtis, Maples, etc.
  • To check what needs repotting you should lift the tree out of the pot and check the root mass. A solid mass of white roots means repot. If not and the tree was vigorous the previous year, don't bother.

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Random Bonsai Tip

Collecting moss - During winter, check your roof gutters for moss - usually a very fine bright green at that time of the year.