Dear Fateh, im not sure did i have understood the question properly. Tannins are famous plant secondary methabolite, having many ecophysiological roles in plants onthogenesis. Tannins are mostly knows by their chemical characteristic to precipitate and "collect" proteins within living organisms and also to desicate the environment inside organism of vertebrates. It should be interesting to explore several hypothesis regarding your questions:

1. Are tannins really responsible for mortality of targeted insects? Which coumpounds might be also responsible? What are their chemical properties and influence on the biotic enviroment?

2. What is insects natural defense system regarding toxicity of plants parts with focus of ecophysiologica relationship between the herbivory insect and its target plants for nutrition?

3. Via which part of the body or tissue the possible toxic substances are absorbed into the insects methabolism?

4. What are genetical and biochemical profiles of the targeted insect?

5. Does targeted insect have the ability to digest tannins? In which level?

My really raw answer might be also that tannin might be "to much" for the insect within digestion system and cause hiperprecipitation of essential functional (maybe even structural) proteins like enzymes of the insects (and maybe can be relatied to antioxidative reaction within organism), and at the same time, insect doesnt have proper defense mechanism, both enzymatic and non enzymatic, to cope with tannin relatively large molecule. As a secondary methabolite, tannins also are known as a scavengers of free radical oxigens forms, which also have essential role in animal methabolites as a part of natural defense system (immunity). So, to much tannins might be also toxic for lack of insect natural ability to defense itself from other predators and factors. 

Dear Branislav Trudić · your answer is absolutely right because tannin bind with proteins inside the of organisms.  

Perfectly right. Thanks for answer.

I agree with the answers above. But also, as it is known its effect on the plant after cell breakdown. Also,  it is important to focus on Tannins roles in the environment.

http://www.sciencedirect.com/science/article/pii/S0031942211000690

Sir Fikrat Hassan, why not tannin in cotton leaves effect on jassid

Dear Fateh,

May be the following link explain these effect:

http://link.springer.com/chapter/10.1007/978-3-642-80373-4_23?no-access=true`

may be