Houdini fly poses new threat to native pollinators

Karla Salp
Communications

Light-colored Houdini fly grubs found in mason bee nests.
Photo credit: Crown Bees

As if the Asian giant hornet was not enough, the Washington State Department of Agriculture (WSDA) is warning that bees and other pollinators face another invasive threat – the Houdini fly.

Unlike the Asian giant hornet, which attacks and kills honeybees, the Houdini fly threatens native mason bees. Mason bees are small, solitary bees that are one of the first pollinators to emerge in the spring and are excellent pollinators in Washington’s cool climate. They are one of the few reliable early spring pollinators and are increasingly used in orchards to pollinate fruit trees in the spring.

The Houdini fly does not attack mason bees directly. The Houdini fly is a “kleptoparasite” -- it lays its eggs on the pollen meant to be the food for mason bee larvae. When the Houdini fly’s eggs hatch, the fly maggots consume the pollen, leaving the mason bee young to starve. Fully grown, the adult Houdini fly makes an amazing escape that gives it its name: it inflates its head to break through the mud cell walls.

Adult Houdini fly on mason bee nesting tube.
Photo credit: Flickr user gbohne

WSDA received reports from mason bee producers who had been detecting the Houdini fly maggots in their mason bee nests. WSDA first received reports in 2019, but only after mason beekeepers had been finding the maggots for repeated years. Because of this, WSDA believes the Houdini fly has established in Washington State and possibly in other states and can no longer be eradicated.

Managing mason bees to limit Houdini fly

While eradication is not an option, mason bee producers and enthusiasts can take several steps to limit the spread of this pest.

• Harvest mason bee cocoons – Open mason bee nests before they emerge in the spring and destroy Houdini fly maggots.
• Control adult mason bee emergence – If you cannot open nests, place the nesting materials in a fine mesh bag and close it tightly. As the bees emerge, release the mason bees daily and kill any Houdini flies.
• Only use nesting materials that allow you to open, inspect, and harvest cocoons. Visual inspections can greatly reduce Houdini fly populations.
• Before purchasing mason bees, ask the provider how they harvested the bees and whether they inspected the cocoons for Houdini fly. Only purchase pest-free mason bee cocoons.

While eradication is not possible, using these best management practices will not only help managed mason bees survive but will help wild mason bees as well.

Visit agr.wa.gov/pollinators to learn more about the Houdini fly and how to look for it in your own mason bees.