I found dozens of these on my tomato leaves. They shake off easily and don't seem to be attached to the leaves. They are dark green and fairly soft. There is no damage to the fruit or leaves but I want to know what they might be. Thank you!
Allegheny County Pennsylvania tomato hornworms
In the picture, those look more like droppings than an insect. I would look at it under magnification and see if there are any insect parts. I couldn't see any when I magnified the picture.
After stopping at the local nursery they thought it could be seed droppings. While there are no overhanging trees there is wild grapevine and I found this image that looks similar:
Thanks again for your help! Perhaps you can solve this one. In the past few weeks most of the tomatoes I have left are doing this - please see photos. What could be wrong?
It looks like insect damage. The only thing even close is stink bug damage. However, stink bugs don't usually produce that yellow mass on the outside of their probe. They are known to cause a hard yellow tissue to form under the skin, and for the skin in the area they probe to look yellow.
I would hunt over the plant looking for any insect. Here is a link to a publication on scouting tomato pests.
One thing I do to exclude insects is cover my tomato fruit with net bags and tie them shut. It's possible to use net paint strainer bags sold at home improvement stores. Also I pick my tomatoes when they start turning color. The tomato vine is finished with the fruit when it gets to that stage, and the fruit has everything it needs by then. I ripen the tomatoes on my kitchen counter. So far, this has helped me avoid insect damage.
Finally, I recommend you take samples of the fruit to your local Penn state Extension and get them to either send photographs or samples to Penn State for identification. I'm particularly interested in the yellow residue on the outside of the skin. I can't tell anything about the texture or other attributes, so anything I say is a guess. If it turns out to be stink bugs, I would love to hear the results.