Megarhyssa macrurus

Distinguished from the similar Megarhyssa atrata by smaller overall size, more extensive brown coloration on the body, and clearer wings with a well-developed spot on the costal margin (M. atrata has smoky wings with only a narrow costal spot and more black on the body). The ovipositor of M. macrurus is shorter than that of M. atrata, reflecting its parasitism of horntail larvae at shallower wood depths. Males of M. macrurus are smaller than M. atrata males and show the same wing and body color differences.

Body reddish-brown with black and yellow-orange transverse stripes; wings transparent. Females reach up to 51 mm in body length, with the ovipositor adding substantial additional length; total length including ovipositor can exceed 130 mm. Males are smaller and lack an ovipositor entirely. Body form elongated and slender.

Deciduous forests and woodlands with standing or fallen dead trees, particularly those infested by wood-boring insects. Associated with declining or dead hardwoods such as sugar maple (Acer saccharum). Requires sufficient wood depth to harbor host larvae but occupies shallower tunnel depths than sympatric M. atrata.

Eastern half of the United States, from the Great Plains eastward; extends into extreme southern Canada near the Great Lakes region.

Adults active primarily in summer; emergence timed to coincide with availability of host horntail larvae. Following oviposition, Megarhyssa larvae consume the host and pupate, with adults emerging the subsequent summer.

• Tremex columba - parasitoidPigeon horntail; females parasitize larvae tunneling in dead wood, reaching them through bark via long ovipositor

Female locates host larva through bark using unknown sensory mechanisms, paralyzes it, and deposits egg on the living but immobilized host. Megarhyssa larva consumes the horntail larva over approximately two weeks, then pupates within the wood tunnel. Adult emerges the following summer. Univoltine (one generation per year).

Females engage in prolonged oviposition bouts, drilling through bark and wood with the ovipositor to reach host larvae. Males aggregate on host trees and perform 'tergal stroking'—bending the abdomen forward to rub the tip against bark and inserting it into cracks; function appears related to detecting or intercepting emerging virgin females rather than early insemination. Multiple Megarhyssa species may co-occur at the same host tree.

Population control agent for pigeon horntail, a wood-boring sawfly. Coexistence with larger congener M. atrata is facilitated by niche partitioning: M. macrurus parasitizes larvae in shallower wood depths, while M. atrata accesses deeper tunnels with its longer ovipositor.

Harmless to humans despite formidable appearance; ovipositor cannot sting. Occasionally encountered by hikers and naturalists at dead or dying trees. Subject of ecological interest due to remarkable ovipositor biomechanics and male aggregation behavior.

• Megarhyssa atrataLarger size, more black on body, smoky wings with narrow costal spot, longer ovipositor; occupies deeper wood layers
• Megarhyssa greeneiAnother large congener with different color pattern and distribution

The long ovipositor is frequently mistaken for a stinger; the species is incapable of stinging humans. Male aggregations were formerly thought to function in early insemination of unemerged females, but this has been disproven.

• parasitoid
• wood-borer parasitoid
• ichneumonid
• giant wasp
• ovipositor
• summer active
• deciduous forest

• BugGuide
• Wikipedia
• iNaturalist taxon
• Multiple Megarhyssa males | Beetles In The Bush