Cacoxenus

Small flies with wing length approximately 2.5 mm; brown body with clear wings and large red eyes. Distinguished from other Drosophilidae by kleptoparasitic association with bee nests and cryptic gray coloration blending with nesting substrates. Larvae possess sticky secretions enabling adherence to cell walls. Specific identification to species level requires examination of genitalia and other microscopic features; C. indagator is the most frequently encountered species in bee nesting contexts.

Associated with cavities and artificial nesting structures used by solitary bees, including reed stems, wooden blocks with drilled holes, and "bee hotels." In natural settings, occurs in hollow stems and beetle borings in wood. Thrives in synanthropic landscapes where mason bees are abundant, particularly in agricultural operations using managed Osmia populations for orchard pollination.

Native to Europe (central and southern Europe documented for C. indagator). Introduced to North America, with established populations in the Pacific Northwest (Washington, Oregon) and California. GBIF records indicate presence in Denmark, Norway, and Sweden.

Adult activity coincides with host bee nesting periods. For C. indagator, maximum activity occurs late April to early May, overlapping with the end of Osmia cornuta flight period. Adults remain active for at least 30 days.

Larvae feed on pollen and nectar provisions accumulated by host bees in nest cells. In C. indagator, larvae may also consume host bee larvae directly, though primarily subsist on stored pollen.

• Osmia cornuta - primary hostMason bee; 56% nest infection rate observed in artificial breeding conditions
• Osmia lignaria - hostBlue orchard bee; parasitism rates increasing over three years in Pacific Northwest
• Osmia bicornis - hostDocumented in literature

Females oviposit in host nest cells during bee absence, depositing 1–3 eggs per cell (range 1–14, mean 2.6). Eggs hatch into larvae that feed on pollen provisions. Larvae possess sticky secretions for adhering to cell walls. After feeding, larvae gnaw through nest partitions to concentrate in cells closer to the nest exit, typically the vestibule region, before entering diapause. Pupation occurs after larvae create exit holes in the nest plug. Imagoes emerge through these openings. Maximum fecundity approximately 46 eggs per female.

Adults exhibit exceptional maneuverability on solid substrates, moving forward, backward, and sideways with ease. Females wait 1–2 cm from nest entrances for entry opportunities, occupying a single nest channel until sealed by the host. When disturbed, adults evade detection by moving sideways rather than flying. Cryptic gray coloration provides camouflage against nesting substrates. No observed pursuit or aggressive interaction by host bees. Single females typically infect multiple nests.

Kleptoparasite and significant mortality agent for solitary bee larvae, particularly in artificial breeding systems. Acts as a biological constraint on managed pollinator populations. Infection causes death of bee larvae or occasionally production of dwarf imagoes. Represents a potential threat to native bee population persistence through spillover from managed systems.

Pest of commercially managed solitary bees used for orchard pollination, particularly Osmia lignaria in almond and stone fruit production. Introduction to North America poses economic threat to the solitary bee industry and orchard management. Management recommendations include autumn nest opening to remove larvae and installation of traps during fly searching periods. Subject of research on bee-parasite dynamics and invasive species impacts.

• Other SteganinaeShare family Drosophilidae but lack kleptoparasitic association with bees; not found in bee nest contexts
• Melittobia spp.Parasitoid wasps of bees; distinguished by Hymenoptera morphology, different life history, and wasp-like appearance

• kleptoparasite
• mason bee parasite
• invasive species
• pollinator pest
• Houdini fly
• solitary bee enemy

• 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)