The high Brix (sugars) in the plant sap ferments in the insects guts to alcohol and they are incapable of processing this and die. The insects can detect the Brix levels in plants so can select the low Brix weaker plants to feed on. Insects have been described as the garbage collectors in nature.
What causes low Brix? The most common reason is excess nitrate nitrogen. Excess nitrogen dilutes all other minerals. It is impossible to achieve a Brix above 6-7 with excess nitrate nitrogen.
High Brix is achieved only with a balanced nutrient approach and with the microbial bridge functioning. This is why soil balance principals like Albrecht's Cation Balancing is so important, it's a vital part in the soils biological health. This means that high brix also means its nutrient dense and its pH is balanced making it highly resistant to disease, as its immune system response (ISR) is functioning at the highest possible level.
How do you insure high brix levels? The greatest tool is Foliar Feeding. This allows the plant to thrive even when the soil health is not at its best. The Biologix Horti-Tech Range of Foliar Boost and Foliar Protect are specially designed with this goal in mind.