|The leaves on the right, are last year's. On these leaves, you can see the pupae coating sheath which the adult fly shed. Once the fly cracks through this, it emerges and sits a while, until its wings have dried enough for flight.
|The flies time it perfectly. The tender new leaves have soft tissue, easy to poke their eggs into.
|Interestingly, when it's windy, the flies have little success attaching their egg laying bottoms to the leaf. You can easily swat them away at this point.
Best to prune the tips of growth a week or two after this process is complete. This removes the eggs and then the boxwood will respond with putting on new growth without any adults left to lay more eggs.
Neat process to witness entirely. Have a look at my previous post on Boxwood Leaf Miners - the worm stage.