This spring we visited delightful mason bees as they made an early debut and foraged for nectar and pollen to provision their nests inside cardboard tubes and galleried firewood in a mason bee colony. Tiny bee larvae will spend the next several months completing development before pupating in their chambers come fall, but right now all is not well in the realm of mason bees. Enemies are afoot. Within the next week or two, gangs of noisy yellow and black insects, leucospid wasps, will be carefully inspecting the modular condominiums of my mason bees. Leucospids are generally considered rare insects, but each year the tubular homes of the mason bees attract scads of these parasites.
Leucospids are rather unique in the wasp world. Unlike most of their kin with rear facing or under-slung egg-laying tubes called ovipositors, leucospids carry their ovipositor arched up and over their back. The small yellow and black wasps move back and forth across the surface of the mason bee’s tubes tapping them gently with their antennae. This behavior has been noted in other species of leucospids and is likely how the female wasp evaluates the suitability of the mason bee within the tube as a meal for her young. If the female leucospid likes what she finds, she uses her remarkable ovipositor to drill through the cardboard tube or my tough oak logs and deposits an egg inside the cell of the developing mason bee. It is fascinating to watch the female wasps drill into mason bee galleries in search of bee larvae to serve as food for her young. After a few days, the wasp’s eggs hatch into voracious larvae that consume their mason bee victims. Larvae of the parasitic wasps complete development and emerge as adults to find a mate and search for more victims.
With antennae a twitter, a leucospid wasp searches a mason bee log for clues to locate potential victims. Once the bee larvae are located, she unsheathes the ovipositor and drills into the log to lay her eggs.
I pondered the peril of my mason bee colony and soon realized that the majority of my hard working bees would be spared from the treacherous leucospid wasp. When it comes to attacking bee larvae hidden in tubes, size does matter. The ovipositor of the leucospid wasp is only long enough to penetrate the outermost tubes of my modular mason bee condominium. Likewise, bee babies occupying nests in holes drilled in the center of my mason bee logs will remain unscathed. The vast majority of mason bees sheltered therein are well beyond the reach of leucospid’s dangerous egg-laying appendage. While some mason bee aficionados might cover their bee condominiums with netting to prevent parasitism, I let nature take its course. Rare leucospids are magnificent in their own way and part of the circle of life in the realm of insects.
Two interesting articles, “Parasitic Behavior of Leucospis cayennensis Westwood (Hymenoptera: Leucospidae) and Rates of Parasitism in Populations of Centris (Heterocentris) analis (Fabricius) (Hymenoptera: Apidae: Centridini)” by Ana Lúcia Gazola and Carlos Alberto Garófalo, and “Osmia ribifloris, a Native Bee Species Developed as a Commercially Managed Pollinator of Highbush Blueberry (Hymenoptera:Megachilidae)” by P. F. Torchio, were used as references for this episode.