Females try to discern where the eggs of Horntail Wasps were laid, a shorter, robust wasp that also injects its fertilized eggs into tree trunks. It is unknown if the female follows existing cracks and holes to locate the host, or if she creates her own. The Megarhyssa atrata female places her own eggs near the Horntail eggs. The Giant Ichneumon Wasp larvae will parasitize the Horntail larvae, feeding off them as they grow. They pupate inside the tree and chew their way out to the surface. Adult males are believed to be attracted to the chewing noises and may be seen waiting for females on a tree trunk as they emerge in order to mate with her. Not all females are successful in transplanting eggs. Their large size and the egg-laying procedure make them vulnerable to predators like birds and raccoons. Sometimes, all that is left of a snatched female is her long ovipositor sticking out of the tree trunk like a whisker.
Look for M. atrata in woodlands and forests on tree trunks, especially those of dead or dying deciduous trees. They are active from spring through summer. While the larvae are known to be parasites of other wood-boring wasps, it is not known if adults feed.