What Chemical/Herbicides Effective On Bugs & Insects On Farm Crops?

You can use caterpillar force mix with water and spray. Or try the organic way, just pour ashes on the leaves.

Alternative, spray Laraforce or Kombat or hallakat if you are a synthetic farmer, but if you are an organic farmer, spray neem oil.

As for the already affected leaves, you should cut them off and apply DD FORCE ON stump.

Although, most farmers actually believes that ashes na myth, however, in most cases it works, but not guaranteed. 

Just use insecticide on it, But it must be done in d evening.....say 6:30-7pm. The effect is caused by bugs. They will come feeding at night.