This was located on our fruit tree. What is it? And is it harmful or helpful?
Harmful and helpful are so relative. Are Preying Mantids harmful or helpful? They probably eat more pollinating insects than they do insects that are injurious to plants, yet they are considered beneficial in the garden. Honey Bees and butterflies might tend to disagree with that classification. This is an Orange Dog, the caterpillar of the Giant Swallowtail. In the scheme of things, the caterpillars eat leaves from citrus trees, but they do not harm the fruit. The caterpillars are rarely so numerous as to do harm to the tree. Eventually, the Orange Dog will metamorphose into a lovely butterfly. Our vote is on the helpful side, but we have a long standing record of voting for political candidates who lose the election. The horns that are visible in your image are a retractable scent gland known as the osmeterium. This caterpillar uses that scent gland as well as mimicing bird droppings to avoid being eaten.