Pour a bit of rubbing alcohol over the spot, covering the entire area stained by the tree sap. Let sit for at least 30 minutes before washing. Then, run though the washing machine with regular laundry soap to remove the sap.
You can use regular bug repellent on tree sap the same way you would use stain remover. Spray the repellent over the sap and let it sit for about 30 minutes. Then wash the clothing in the washing machine to fully remove the stain.
You can make a paste out of laundry detergent to apply to the tree sap stain. Mix a few tablespoons of powdered laundry detergent with a spoonful of water to make a paste. Rub the paste into the stain and let sit for 30 minutes before washing in the washing machine.
You can use ammonia if the clothes come out of the wash and still show signs of the sap stain. Test the hem of the clothes with a drop of ammonia to ensure it's resistant to color change, then pour a dab of ammonia over the stain. Let sit for 30 minutes and wash again in the washing machine.