Q.Nuturing Orange trees
1.What fertiliser is require for orange plants of a bout 3/4 years of age.
2.What medicine or insecticide/pesticides is use to prevent orange plant of 3/4 years from insects?
Use a fertilizer formulation for citrus trees.
Neem Oil is a good insecticide for citrus trees.
Here are some links with more information for you.
https://www.gardeningknowhow.com/edible/fruits/oranges/growing-an-orange-tree.htm
https://www.gardeningknowhow.com/edible/fruits/oranges/leaf-curl-in-orange-trees.htm
https://www.gardeningknowhow.com/edible/fruits/oranges/pruning-orange-trees.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm