Thanks for your question.

There are two pests commonly found on chokecherries that are often confused with each other. One is called a Chokecherry Gall mite (Eriophyes emarginatae). It is very similar to spiders. It causes finger-like protrusions (galls) on the leaves of chokecherries. See:

https://bugguide.net/node/view/434978/bgimage

This mite is often confused with a Chokecherry Gall Midge (Contarinia virginianiae), which is an insect. The adult form is a very small fly that lays eggs on the developing chokecherry fruit. These eggs hatch inside the fruit to produce maggots in the fruit. These maggots cause misshaped fruit that are wart-like or blister-like, often discolored. Your pictures depict this nicely. See the following for an image of these maggots inside the fruit; perhaps like what you observed:

https://bugguide.net/node/view/1092554

Sorry to go into the weeds so much but when trying to figure out control measures, it’s important to decide whether mites are present, or midge flies are present. They are two very different organisms. In your case, midge flies are guilty critters.

The primary control method for the chokecherry gall midge is hand-picking and destroying the damaged fruit in early summer, before the maggots mature. As far as I was able to determine, there are no insecticides registered specifically for this pest on chokecherry grown for edible fruit. See the following for further information on these and related points:

https://wiki.bugwood.org/HPIPM:Contarinia_virginianae

https://spencer-hort-solutions.ca/chokecherry-fruit-gall-midge-2/

Good luck. Thanks for consulting us. Please get back to us with any additional, related questions.