Question
what do i use to kill this pest and will the tree die thank you

Answer
The sawfly usually eats the older needles and the treee does not die from this defoliation. These can be controlled by removing the sawfly caterpillar from the tree or by spraying insecticides.

Cultural/Mechanical Control - Larval Destruction - Colonies of larvae can be easily removed by clipping off the infested branch. Place these branches in a plastic bag and destroy. Colonies can also be knocked off by sharply striking the infested branch. Crush the larvae or knock into a pail of soapy water. If few colonies are present, they can be controlled using these methods but large infestations are better controlled by general spraying.

Insecticides with the active ingredients acephate, malathion, carbaryl, esfenvalerate, rotenone and pyrethrin have longer residual properties, providing some extended control against pine sawfly larvae. The foliage will have to be sprayed to kill the caterpillars.