Thank you for your question and pictures which are very helpful to diagnose what's wrong with your crabapple. I believe your tree is infected with fire blight, which causes the dieback and weeping branches that your pictures demonstrate. It is caused by a bacteria which enters your tree in hot, humid weather. The bacteria oozes out of the bark with a sticky liquid, which attracts insects similar to what you are noticing. Since it is now the growing season, it's recommended to prune out the diseased sections, making sure you sterilize your pruning tools to avoid the spread of the bacteria. If your city has a forester on staff, you might want to call them to report that others in your area are possibly infected. Here is an article from U of MN Extension which describes exactly what you need to do:

https://extension.umn.edu/plant-diseases/fire-blight

Good luck with your crabapple tree.