Answer: Unfortunately, based on your description I am unable to make a definite identification of the insect. It is possible that Japanese beetles might be munching on them, however these take on a green metallic look rather than black. They can be hand picked early or late in the day when they are slower moving, or they can be deterred with a neem based spray, or the damage can simply be tolerated. Before spraying any thing more, I would strongly suggest you take a sample of the pest to your county extension for a definite identification of it and their suggestions as to how best to control it, if indeed control is necessary.
Mid to late summer is a very stressful season for planting, and watering is really critical to success -- especially when temperatures are in the 90's as they have been recently. I am seeing established cherries dropping a few yellow leaves due to our heat and drought, conditions are really rought this summer and especially so for a newly planted tree. The yellowing can also be a sign of transplant shock, in which case careful watering and patience from here on out are all you can do.
With a newly planted tree yellowing, I would look at the watering practices first. Over and under watering can both cause yellowing, oddly enough.
Your goal is to supplement natural rain to keep the soil evenly moist but not sopping wet. You should use your finger to dig down into the soil to see if and when you need to water. Check both the potting mix and the surrounding soil because they can dry out at different rates.
It is best to water slowly and deeply less often to encourage deep roots, instead of sprinkling daily. The frequency will depend on the heat, wind, and of course whether or not it has rained and how much rain actually fell. Unfortunately there is no formula amount I can give you.
To begin to understand how much to water at a time, water and then wait a few hours, then dig down and see how effective your watering was. It can be surprising.
Finally, using several inches of mulch over the root area (do not place it up against the trunk though) can help keep the soil more evenly moist and also keeps down weeds.
If you suspect watering is not the cause of the yellowing, you may want to consult with the nursery where you purchased the tree and/or with your county extension for a more detailed analysis of what is happening. Good luck with your tree!
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