Wendy,
I mixup the following: Elmers wood glue (Yellow, not the white), water and a drop of dish detergent. You can go 1 to 1 even 2(glue) to 1 just make sure the mixture isn't thick. Put this into a generic spray bottle and spray on the area where you want to put the pine needles. Sprinkle the needles, wait a min or two to let it set then blow off the excess. I would then seal this with dullcoat and proceed with painting weathering etc. Hope this helps!
Best Regards,
John Stonaker