The culprit for the honeydew (the sticky residue) is probably Crapemyrtle Aphid, a very common insect that feeds on only this plant. They don't harm the tree, though the honeydew buildup can be a nuisance, as well as attract wasps and flies. If the weather was rainier, it would rinse off periodically and not be as prominent. You can hose some of it off, though it won't wear off immediately (but it will eventually on its own). Treatment for the aphids is not necessary, but if you can reach the canopy, most can be blasted off the foliage using plain water from a garden hose on a strong spray setting.

Crapemyrtle Bark Scale is another recently-widespread pest on this plant that can create honeydew, though we don't see any in the photos. You can use the information on the linked page to look for indications of their presence on the bark; they will not be on foliage, whereas the aphids will (on the leaf undersides).

You mention treatment, but not what pest it was being treated for, and with what pesticide. Some work well on aphids and scale, and others do not, so knowing what was used can help us determine if it was the best choice (and timing of application). Some pesticide applications, if they are applied to the root zone as a "drench," will take longer to take effect than those sprayed directly on the pests or which are sprayed onto foliage. Be aware that many such options will potentially harm pollinators and beneficial insects, so verifying scale or aphids are present is the first step to determining if treatment is warranted.

Miri