Abstract: The disclosure relates to a slow-release biodegradable composite pesticide plug that can be used for trunk injection delivery of pesticides to protect woody plants such as trees (e.g., fruit trees) against pests. The composite pesticide plug generally includes a thermoplastic polymer matrix with a water-soluble thermoplastic polymer component, a cellulosic reinforcement, and a pesticide or other active ingredient. After injection into the tree trunk, the composite pesticide plug provides a uniform, relatively consistent dose of pesticide or other active ingredient to all parts of the tree throughout the growing season, thus reducing waste of material and cost. The biodegradable nature of the composite plug also enhances the healing of the tree after injection.