Cacoxenus indagator

Small brown fly with wing length approximately 2.5 mm. Clear wings and large red eyes. Light gray thoracic coloration provides camouflage when resting near bee nest entrances. Distinguished from other small flies by its kleptoparasitic association with sealed bee nest cells and its documented ability to escape through mud partitions.

Associated with nests of cavity-nesting solitary bees, particularly in artificial breeding conditions such as reed stem nests, wooden blocks with longitudinal channels, and bee hotels. Occupies urban and agricultural landscapes where host bees are managed.

Native to central and southern Europe; recorded in Belgium, Denmark, and other European countries. Introduced to North America, with established populations in Washington, Oregon, and California in the United States.

Seasonal activity coincides with nest-building activity of host bees. Maximum activity recorded between late April and early May, overlapping with the end of the host bee's flight period. Total adult flight period at least 30 days.

Larvae feed on pollen stores accumulated by female mason bees for their own offspring. Larvae possess sticky secretions allowing attachment to cell walls while feeding.

• Osmia cornuta - primary hostCharacteristic inquiline; causes 78% larval mortality or 22% dwarfism in infected cells
• Osmia lignaria - hostBlue orchard bee; parasitism rates increased significantly over three years in Pacific Northwest monitoring
• Osmia bicornis - hostCited in literature

Eggs laid in host nest cells during host absence, most commonly 1 or 3 eggs per cell. Larvae feed on pollen stores, then gnaw through nest partitions using moistened secretions to aggregate in cells near the nest exit or vestibule before diapause. Adults emerge after making exit holes in nest plugs. Maximum recorded fecundity 46 eggs per female.

Exhibits high maneuverability on solid substrates, moving forward, backward, and sideways with equal facility. Females wait 1–2 cm from nest entrances for opportunities to enter during host absence. Escapes sideways when approached rather than flying. One female typically occupies a single nest channel until the host seals the nest with an earthen plug. A single fly usually infects multiple bee nests.

Kleptoparasite and inquiline that significantly reduces populations of managed solitary bees. Infection rates reach 56% of nests and 13% of cells in artificial breeding conditions. Male cells located closer to nest exits are most susceptible (70.7% of infected cells). Reduces effectiveness of Osmia species as pollinators in commercial operations.

Major pest of managed solitary bee populations used for orchard pollination, particularly almonds and stone fruits. Poses economic threat to solitary bee industry and orchard managers. Recommended control measures include autumn nest opening to remove larvae and installation of traps during nest-searching periods.

• Other Cacoxenus speciesSame genus, but C. indagator distinguished by documented kleptoparasitism of Osmia bees and specific escape behavior from sealed cells
• Other small drosophilid fliesSimilar size and general morphology, but C. indagator specifically associated with bee nest cells and exhibits unique locomotory behavior on solid substrates

• kleptoparasite
• inquiline
• mason bee parasite
• Houdini fly
• pollinator pest
• invasive species
• Drosophilidae
• Osmia parasite
• managed bee pest
• cavity-nesting bee associate

• BugGuide
• Wikipedia
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
• Sharing Research About BOB | Bug Squad
• Outstanding Group of UC Davis Graduate Students at ESA Meeting | Bug Squad
• THE NATURE OF DAMAGE TO THE NESTS OF THE OSMIA CORNUTA BEE (LATREILLE, 1805) (HYMENOPTERA: MEGACHILIDAE) BY THE KLEPTOPARASITIC FLY CACOXENUS INDAGATOR LOEW, 1858 (DIPTERA: DROSOPHILIDAE)