You can see in the photo that Rhonda sent us that there are yellow leaves all over the ground. There is an insect known as the maple petiole borer that can often cause this damage in May and June here in Michigan. This native insect lays its eggs in the petioles – the stems that connect the leaf to the branch – of maple trees. They hatch out, eat the tissue in the petiole, and when they are ready to emerge as an adult wasp (a harmless adult wasp!), they eat a hole through the outer layer, causing the leaf to drop. This won’t harm the plant but can be distressing to see. You can tell if maple petiole borers are the cause if you examine one of the fallen leaves and find that it has broken off in the middle of the petiole rather than making a clean break at the point where the petiole meets the branch. If you find that the leaves have fallen at that point, it’s most certainly due to drought or other stress – by shedding leaves, a tree reduces the amount of water it loses during photosynthesis, conserving its resources to weather any stress that may be occurring. If the fallen leaves are attached to small, thin branches, the culprit is almost certainly squirrels.