A. From your description, and the pictures you sent, it looks like it might be one of the scale insects.
|Yes, the picture is blurred but those are frequently the kind of pics i get. Just have to use your imangination a bit but I think i can see brown scale on the branches.|
I have never seen scale insects on quince in the Las Vegas valley but they do infest trees in other climates. Scale insects, particularly the soft scales, will exude a sugary liquid that drips on limbs and leaves and attracts ants.
They are easy to squish and act just like you are describing it. There is an insect under the harder outer shell. Young are produced by the female and they leave the protection of the mother’s shell, move about and find a new home. Here they can suck plant juices and also build their own protective shell on the outside of their body.
The usual method of control is using horticultural oils and spraying the oil on the plant to suffocate the insect. This is done in winter or early spring.
If you are lucky enough to see the young emerge and start looking for a home, most any spray will kill them including soap and water sprays like Safers insecticidal soap. But you will have to spray oils and soap and water sprays through the year to get them under control.
Once under control an oil spray in the dormant season such as winter months when it is warm should help keep the populations reduced.
A wild guess from the picture, it might be brown soft scale particularly because you mentioned the ants.