One of the less common types of nest plugs we get in our bee blocks are the Silk plugs made by the yellow-faced or masked bees, Hylaeus species. They are a real treat for those who do see them, often blindingly shiny due to their cellophane-like texture. These filmy plugs are essentially dried bee saliva. They come in a several colors (white, clear, or yellow) and depths (recessed, flush, or plastered to the outside of the bee block), but are all made by one of our 16 species of Hylaeus that occur here in Colorado.
The other plug types that they are most commonly confused with are the resin plugs and spider webs. The easiest way to separate silk plugs from resin plugs is to poke the plug with the point of your pencil. If your pencil point tears a hole in the now-obviously filmy plug, you have silk. If it gets all gummy or if it barely scratches the surface of the super hard plug, you have resin. As for the spider webs, OK, they’re silk too, but we couldn’t really refer to the Hylaeus plugs as “dried bee-spit” plugs, now could we? I suppose we could call them “cellophane” plugs, but it seems like cellophane has decreased in popularity despite Lady GaGa using it as clothing. But back to spiders– their “plugs” are a compilation of threadlike filaments, where as the Hylaeus silk is more sheetlike. If you do the pencil point poke test to a spider web, you’ll feel a definite spring-like or cushiony bounce. Hylaeus nests are made in the small tunnels, 1/4 inch in diameter or less. Some will even nests in tunnels as small as 1/16 inch in diameter. Now that’s a small bee!
Hylaeus are wildly different from the leaf-cutting and mason bees in the family Megachilidae. In fact, they are in a whole different bee family – the Colletidae. Other cool facts about Hylaeus are that they carry pollen in their crop (stomach) rather than on their legs or “bellies”, the larvae do not spin cocoons before pupating, and the adults produce a chemical that makes them smell like lemons if you handle them. (I don’t recommend it though– their potent, yet short-lived stings feel like hot needles.)
Hylaeus adults superficially resemble wasps more than bees. They are small, black and, since they carry pollen internally, are rather bald. The yellow face markings on the females are on the inner edges of the eyes (sometimes with an additional central spot about their mandibles). Of course our largest species lack any yellow markings at all, and one of our most common (introduced) yellow-faced bees actually has white markings, not yellow. The easiest way to know you had Hylaeus nesting is to identify the plug! So get out your pencil points and poke ’em.