Grub worm is becoming one of the most root pest issues in my hazelnut plants. Therefore, is there any solution other than nematodes?
Root maggots are not a common pest of hazelnut trees, so there isn't much information. This article might be of some use: https://www.gardeningknowhow.com/plant-problems/pests/root-eating-insects-identifying-vegetable-root-maggots-and-root-maggot-control.htm
A bT product might work for you: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/using-bacillus-thuringiensis.htm
If you have any horticultural resources available to you, through a nearby university, botanical garden, or government dept of agriculture, you might look there for information on this pest and its control.