Chitosan (Ky-Toe-Zan) Solution is used by some Natural Farmers to strengthen plants against attack and to kill leaf eating pests. Commercially it is chemically extracted from crustacean shells. Gardeners can achieve a similar outcome by using micro-organisms.
Clean crustacean shells are submerged in pond water (with some pond sediment) and micro-organisms extract the chitosan over several months. Some of the liquid can then be used and the container topped up with more pond water and crustacean shells - ensuring a continuous supply. Locating in dappled light and with a permeable lid will help replicate the pond environment. It smells pretty funky and so it’s best to ensure some natural airflow. Dilute 1:500 and water into the soil and spray onto the leaves.
Whilst the Chitosan is non-toxic, treated vegetables should be washed before consumption due to the use of pond water.
The efficacy of Chitosan solution requires further research and scientists are currently doing just that. My own tests on caterpillars of Cabbage White Butterflies (a common pest species in many parts of Australia) were effective. It is important to understand that this isn't a toxic pesticide and that it appears to take some time for the Chitosan to disrupt the caterpillar's digestive tract and kill them.
Chitosan is at the cutting edge of Natural Farming. So if you are a cutting-edge Gardener it may also be of interest to you.