8. Pests & Diseases

USDA Alert: Houdini Fly

8.7 Invasive Pest Alert: Houdini Fly Pest

8.7 Invasive Pest Alert: Houdini Fly

Industry professionals warn of an invasive parasitic fly species spreading across North America. Known in Europe as the Houdini fly, this pest threatens the mason bee industry. Learn how to reduce this pest's impact and protect your mason bee population.

Houdini Fly - A new invasive pest threatens Mason Bees

How the Houdini Fly Invades Mason Bee Nests

The Houdini fly, Cacoxenus indagator is a kleptoparasitic pest that is well-known in Europe and found in the nests of red mason bees (Osmia bicornis). A member of the fruit fly family, this small fly is named after its ability to erupt out of the mud walls of mason bee nests.

Kleptoparasites are animals that steal another animal's food, a good example is the cuckoo bird. The Houdini fly waits for the mother mason bee to gather sufficient pollen and nectar and crawls into the nest to lay eggs before the nesting chamber is sealed with mud.

The fly eggs hatch to become maggots who consume the pollen and nectar loaf, starving the mason bee larva. Houdini flies hibernate as maggots throughout the summer, fall, and winter. Evidence of a Houdini fly infestation includes a sticky mass of white maggots and their frass (waste) within a nesting chamber. The maggot frass is curly lines of orange or yellow.

The fly maggots develop into adults around the same time that mason bees emerge from their cocoons in the spring. As a juvenile form without wings, the fly can expand its head using body fluids like a hydraulic jack. In the video below by George Pilkington of the UK, a fly inflates its head to move aside pieces of dried mud to escape and the rest of the flies trapped in the nesting chamber will share this exit.

If Left Unchecked, Flies Make A Daring Escape

Kleptoparasites are a natural but unfortunate part of raising bees. During mason bee cocoon harvest, we have observed that each infected mason bee nesting chamber appears to have about 20 maggots. If nesting materials are left alone or unopened, the Houdini fly will either escape and disperse, infecting other nearby mason bee nests, or stay and reinfect the original mason bee house. We predict that the Houdini fly infestation can overtake a mason bee nest within a few years. Each year fewer bees will survive, with less spring-blooming fruit and nuts pollinated by the amazing mason bee.

Houdini Fly on a Bee House - photo: Bryan P.
Houdini Fly on a Bee House - photo: Bryan P.
Adult Houdini Fly
Adult Houdini Fly - photo: Bryan P.
Adult Houdini Fly
Adult Houdini Fly - photo: Bryan P.
Houdini Fly Maggots Escape

Why Mason Bees Need Our Help

The mother mason bee does as much as she can to protect her offspring before her short life as an adult bee is over. Unfortunately, all man-made nesting habitats are attractive to a variety of pests and the density of nesting holes means that one pest can infect several nests easily. Man-made nesting habitats require our care and maintenance to keep the bee population healthy. If we do nothing to control the Houdini fly, our actions will not only reduce the mason bees in our yards but other nearby wild bee nests as well.

Managed pollinators like the well-known honey bee help us grow food. Flying in cooler weather, mason bees are much more effective pollinators than honey bees of spring-blooming fruits, nuts, and berries. We need bee diversity in our gardens and farms and the mason bee industry is still in its early stages. The Houdini fly is a new pest that threatens both our managed and wild mason bee populations.

Houdini Fly Larvae
Houdini Fly Larvae

History of Discovery

Cacoxenus indagator is a well-known pest of red mason bees in Europe. Although not as widespread in North America, evidence of the arrival or emergence of this pest causes serious concern for the growing mason bee raising industry.

In Crown Bees' local bee raising program, the occurrence of the fly was first seen in 2017 and more prevalent in 2018. By 2019, we saw an infestation in about 1-2% of our bee raising programs and customers found the pest in British Columbia and in Portland, OR. In May 2019, Crown Bees worked to identify the Houdini fly with a gardener in NY along with the Cornell Danforth lab.

At the Orchard Bee Association meeting in December 2019, Dave Hunter presented the pest to the group as a cause for concern. A visiting Swiss professional identified the pest sample as Cacoxenus indagator.

Teaming with Government Agencies

Crown Bees has provided the USDA with fly maggot samples and the USDA is currently investigating pheromones or lures for the adult fly. Crown Bees has also met with the Washington State Invasive Pest task force and together we have created mason bee management actions. The goal of this outreach is to prevent further spread of the insect and will include press releases and documents that alert garden stores, online mason bee companies, master gardeners, and garden clubs.

How to Control Houdini Fly Infestations

  1. Harvest Mason Bee Cocoons: Open your nesting materials before your mason bees emerge in the spring and destroy the maggots. Harvesting cocoons is the best way to reduce this pest and you will also reduce other harmful mason bee pests and diseases.
  2. Control the Emergence of Adult Mason Bees: If you cannot open your nesting materials, purchase a BeeGuard Bag and place your nesting holes inside the bag and close tightly. When mason bees emerge in the spring, release the adult bees daily and kill any parasitic flies.
  3. Raise Awareness: Share this page with your friends and family who raise mason bees. Help our spring pollinators by learning healthy bee management practices!
  4. Track Observations: Please submit your observations of the Houdini fly on the iNaturalist website. Your observations can help us understand the prevalence of this new invasive mason bee pest.

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