All pesticides, including those designed to kill hornets, yellowjackets, wasps or their nests, need to be used in accordance with the label instructions. There are a lot of different pesticide products that are designed to kill hornets and yellowjackets, all with various active ingredients. The label would need to state that it can be applied to a crop (ex. plums or fruit trees) for you to be able to apply it to your plum tree. With regards to pesticide residues, every active ingredient behaves differently. Some break down very quickly and leave behind little residues, whereas others have longer half lives. Without knowing what product or active ingredient you are using, it is hard to answer your question. The National Pesticide Information Center has a list of active ingredients where you can look up the environmental and health effects of chemicals that are used as pesticides: http://npic.orst.edu/ingred/specchem.html. One other thing I will note, all chemicals designed to kill hornets or yellowjackets, will also kill honeybees, bumble bees, and native bees. In order to protect pollinators, it is important that you don’t spray trees or plants that are in bloom (or trees that have flowers growing beneath them that the bees may be foraging on). If you must spray a tree or plant in bloom, then we recommend chemicals with lower residual levels, and that you spray late evening or early morning when the bees are less active. For more on controlling hornets and yellowjackets, refer to Solve Pest Problems: https://solvepestproblems.oregonstate.edu/insects-spiders/yellowjackets-wasps. For more on reducing bee poisoning from pesticides, refer to: https://extension.oregonstate.edu/sites/default/files/documents/pnw591.pdf