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Both commercially sold and wild native Mason bees are at risk from a European parasitic fly that was introduced in Seattle a few years ago. Steps should be taken urgently to "bag" occupied bee tubes and bee hotels to trap any flies that may emerge this spring.

One of the risks of importing Mason bee tubes from the mainland has been the unwitting introduction of pathogens and parasites that could impact the islandsâ€™ native Mason bee species, which are important for spring pollination of wildflower meadows and early-flowering shrubs and trees. Commercially-available Mason bees in the Pacific Northwest are mainly Osmia lignaria, the so-called Blue Orchard Mason Bee; but there are thirty other native species of Mason bees in the Salish Sea region, at least eight of which have been identified in the San Juan Islands.

Houdini fly (Cacoxenus indagator), a European parasite of Mason bees, was discovered in New York State in 2011, and first confirmed in the Seattle area in 2020. It presumably has been spread by trade in Mason bees. A distant cousin of the familiar laboratory â€œfruit fly,â€ Drosophila melanogaster, Houdini flies lay their eggs in Mason beesâ€™ nests. When they hatch, the fly maggots feast on nectar and pollen meant for larval bees. In the spring, adult flies rather than bees emerge from the Mason beesâ€™ nests. The newly emerged adult flies then search for fresh Mason bee nests to parasitize. In areas where Houdini flies are well established, they can affect a majority of Mason bee nests or nest-tubes, sharply reducing Mason bee survival and pollination services.

The Houdini fly gets its name from the way it manages to escape from Mason bee nests that were sealed with mud the previous summer. Unlike bees, these flies lack the ability to chew. But astonishingly, they can inflate their heads and use them as battering rams. The process may take hours.

What can island gardeners and farmers do?

The first line of defense is to ensure, if you already have Mason bee tubes or â€œbee hotels,â€ that you are not raising Houdini flies along with your bees. The simplest way to achieve this is to bag your Mason bee tubes and bee hotels in mosquito netting with fine mesh, 1/16 inch or less, to detain whatever insects may emerge in spring. If there are flies, their prominent red eyes make it easy to distinguish them from Mason bees. Squash the flies, and release the bees. Then dispose of the empty bee tubes; do not reuse them. Wash and bleach the bee hotel.

The second line of defense is to avoid importing any more Mason bees from the mainland, in tubes or bee hotels. Instead, you can support native Mason bees by providing them with more nesting habitatâ€"including empty, previously unused bee hotels, as well as stumps and snags that have beetle and woodpecker borings. You can also provide habitat in your landscaping for some of the other wild native bee species that live in the islands, including Bumblebees and Miner bees that alsoâ€"like Mason beesâ€"emerge relatively early for spring pollination.

Contact [email protected] for additional information on attracting and protecting wild native bees in the islands.

The Houdini fly looks like a common red-eyed "fruit fly" but has a very different lifestyle (Dick Belgers photo, Wikipedia Commons).

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