Males 23-38 mm, females 35-75 mm (body), 50-110 mm with ovipositor(2)
Forested areas of North America; all 4 spp. widely distributed in the east, two also in w. US(1)(3)
atrata: Nova Scotia south to Georgia; west to Wyoming and Texas
greenei: Nova Scotia south to Florida; west to Minnesota, eastern Kansas, and Mississippi
macrurus icterosticta: Colorado, New Mexico, Utah, and Arizona
macrurus macrurus: Maine south to Florida; west to western South Dakota and western Texas (also into northern Mexico)
nortoni: Newfoundland south to northern Georgia, west to Alaska & California (also introduced in Australia and New Zealand)
Coniferous forests of North America
The larvae of M. atrata, M. greenei
, and M. macrurus
are parasitoids of grubs of the horntail Tremex columba
All these horntail larvae feed within the wood of conifers.
Horntail adult females introduce wood-digesting fungi (e.g. Amylostereum
) when ovipositing, which helps their grubs extract food value while feeding on the wood. Adult female Megarhyssa
are able to detect the odor of these fungi, and once they land on the bark of an infected tree the Megarhyssa
will walk along tapping the surface with their antennae (or "antennating") to further pinpoint the location of horntail grubs within the wood.
Once grubs are located, the female Megarhyssa
positions herself with back legs extended and ovipositor perpendicular to the bark, and drills into the tree to deposit an egg on or near a horntail grub within its burrow (see videos here
). While drilling the female wasp is immobilized and vulnerable to predation by birds.
When the Megarhyssa
egg hatches it behaves as an ectoparasitic idiobiont
, completely consuming the grub. It pupates within the burrow and emerges in the summer.
Males emerge first, and usually inseminate females within their chambers before they emerge (by inserting their long thin abdomens into the escape tube dug by the female (see Crankshaw & Matthews 1981
There is a very detailed and fascinating account of the oviposition process in the 1999 paper by Le Lannic & Nénon (see "Print References" below) as captured in the posts below:
Their article focuses on M. atrata...but most of it presumably applies to other Megarhyssa species as well.
The paper Gibbons 1979
explores how the three species M. atrata, M. greenei
, and M. macrurus
exploit different niches, allowing them to coexist in the same locations and using the same host.
Joseph Le Lannic and J.-P. Nénon, "Functional morphology of the ovipositor in Megarhyssa atrata ( Hymenoptera , Ichneumonidae ) and its penetration into wood", Zoomorphology (1999), Volume: 119, Issue: 2, Pages: 73-79. (First Page
Videos and description
pf the oviposition process in M. macrurus
Web page on horntails & Megarhyssa
from the Univ. of Wisconsin at Milwaukee.