last night I collected some Polyphaenis sericata larvae in a locality in southwestern Germany, at a low altitude (180 m a.s.l.), on a westerly exposed partly wooded slope overlooking the Rhine plain in the vicinity of Karlsruhe.
This is a fairly typical spot to find Polyphaenis sericata in the wild: the border of a mixed/deciduous forest (Quercus, Fagus, Pinus) with a fringe of shrubs, mostly Ligustrum vulgare, Crataegus sp., Cornus sanguinea and some Prunus spinosa. There is no Lonicera in this locality.
Larvalhabitat: Deutschland, Baden-Württemberg, Kraichgau (mit Blick über die Oberrheinebene im Hintergrund), Umg. Grötzingen, 180 m. 2. April 2021. Foto Axel Steiner.
I saw 8 Polyphaenis sericata, all in the last but one instar (2-2,5 cm) and most on Ligustrum vulgare though one was sitting on a dry bramble twig reaching over to feed on a privet leaf. They usually climb quite high to reach and feed on the terminal shoots of the bushes. In the past I have seen them in more than 2 m height. They frequently sit in the midst of tangled bushes where they are difficult to get at. They are very sensitive and drop down immediately if you touch and shake the twigs. In the past I used to collect them by tying a kitchen sieve to a long handle and holding it underneath the larva when I came nearer so I could catch them. Last night it was easier because the bushes were not standing very densely. Most larvae were found a few meters inside the forest, on the extreme right and also slightly outside of the photograph (which had been taken a few days earlier when we prospected the area), only one was seen on a more convenient spot right beside the footpath. It seems the females like to enter the woods to lay their eggs on privet that stands in somewhat shaded positions, especially in warm and dry localities.
Of course the species also occurs in cities and villages where it is supposed to use the garden privets (and Lonicera?).
When feeding most larvae seem to prefer sitting on the underside of privet twigs. That is why you can see quite well those that are sitting high up too. For this photo I turned the twig a bit around (very carefully...).
Deutschland, Baden-Württemberg, Kraichgau, Umg. Grötzingen, 180 m. 9. April 2021. Manipuliertes Freilandfoto Axel Steiner.
By the way, have you noticed that the larvae like to rest communally during the day? In my cage this looks like this; in the leaf under which these two larvae were hiding another two were resting, all with the head turned around.
Deutschland, Baden-Württemberg, Kraichgau, Umg. Grötzingen, 180 m. 10. April 2021. Zuchtfoto Axel Steiner.
As far as hostplants go Ligustrum vulgare is the one most commonly recorded in Central Europe, but there are some records from Lonicera sp., Lonicera xylosteum, Cornus sp., Cornus sanguinea, Prunus spinosa, Crataegus monogyna and even Syringa vulgaris.
In the larval habitats of Polyphaenis sericata privet often grows entangled with these other bushes, so it is necessary to take a really close look to be sure the larva is actually feeding and not just sitting on a certain plant. I mentioned the one I saw sitting on bramble twig feeding on a privet leaf. I never noticed Polyphaenis sericata larvae on Cornus sanguinea or Lonicera but from your breeding experience it is clear that at least Lonicera must be taken into account.