Ask Mr. Smarty Plants

Mr. Smarty Plants called on a couple of entomologists to help with this question—Val Bugh and Alex Wild.  They both thought that the honeybees were after honeydew that was occurring on or near the galls on the oak.  There are two possible sources for the honeydew—1) the galls themselves or 2) sapsucking insects that may be feeding on the tree.  The galls are a result of a parasitic wasp (there are several varieties) that lays its eggs in a leaves or twigs of a tree.  In response to this act the tree grows tissue around each egg and encloses the developing larva within the gall.  The larva of the wasp feeds and grows within this tissue until it is mature and emerges as an adult.  The larva is protected within the gall, but for even more protection the larvae of some wasp species turn the plant starches it is feeding on into sugars that exude from the gall to form honeydew on the surface of the gall.  The honeydew attracts ants, beetles and other insects that serve as predators to parasitic insects that might attack the enclosed larva. Here is a information about bees collecting honeydew extruded from galls on the California, Quercus lobata (Valley oak).  You can read more about the galls occurring on an elderly Quercus douglasii (Blue oak) in California from BayNature. 

Honeydew is also produced by several sapsucking insects such as aphids, scale insects, leafhoppers, mealy bugs and other psyllids.  You can read more about these insects and the honeydew that they produce in Honeydew: a Mixed Blessing. 

Bees do make honey out of honeydew—called Honeydew or Forest Honey—and it is prized in many countries.  It has a stronger flavor than honey produced from flower nectar and the flavors differ depending on the trees that the sapsucking insects are feeding on.