Good day! For the first time I encountered such a pest on cucumbers - like a fly! The green abdomen, brownish wings, on the upper stalk after it sits, a defeat appears in the form of a yellowish coating, the stalk gradually becomes thinner, as if juice was sucked out of it, I cut off these stems and anointed everything with green, as the chemical treatment did not help. I will try to take a picture and send a photo. Maybe someone met such a pest on cucumbers?
I know such a problem. This is a fly miner. Its adult individuals are not as dangerous as the larvae; they are the biggest trouble for cucumbers (I had them on tomatoes). Most often they can be found on the leaves, in the recesses. They have a greenish little body. Then from this state they turn into a chrysalis and fall to the ground. Larvae eat everything in their path, all the juices are sucked out of miserable plants. Plus, the larvae increase the already existing rot on the leaves, secondary rot develops.