Grubs in sticky mass.

Yesterday I took down my nestbox to clean it out for the season. It has a hinged lid which is then covered over the top & down the sides with pond liner to help keep out wind & rain. After pulling out the panel pins which hold this cover down I couldn't work out why I couldn't lift the lid & on investigation I found both sides of the lid were stuck to the body of the box with a white, sticky "glue". At first I thought it might be a wasp nest but there was obviously no wasp activity & on prising the lid off I found yellow grubs in the sticky mass. I'm afraid they all got squashed or otherwise mangled in the process of opening up and clearing out but I wondered what they could have been? There were the usual Earwigs hiding under the covering but that's what I find every year. I was disappointed to find that although full of moss & soft dog hair from my Japanese Chin, the Blue Tits that I had seen entering the box had obviously not raised a family in there. I have cleared all their nesting material out and the box is about to be disinfected ready to put back up, hoping next year will be more successful.