Megarhyssa atrata

Megarhyssa atrata are found in the northeastern and central United States as well as southeastern Canada. (Gibbons, 1979)

The entire life cycle of Megarhyssa atrata occurs within the confines of the deciduous forests of central and northeastern North America. Megarhyssa atrata is a parasitic species that requires a host which provides nutrients to developing larvae. Deciduous forests include several hardwood tree species that are home to the preferred host: pigeon horntail larvae. Pigeon horntail larvae develop inside decaying hardwoods during the winter, and Megarhyssa atrata seeks out these locations to oviposit. The Megarhyssa atrata cycle begins when the females oviposit their eggs in either decayed logs, fallen trees, or stumps where the larvae of pigeon horntails have burrowed into the wood. Burrows chosen as egg-laying locations have often been invaded by fungus and rot, making it easier to insert the ovipositor. (Gibbons, 1979; Heatwole and Davis, 1965; Vamosi, 2005)

Megarhyssa atrata are large, thin-bodied wasps with lengthy ovipositors. The rounded-triangular head is yellow with with a darker colored band that runs between the large compound eyes. This band encompasses the ocelli. There is also a second dark band that runs laterally across the head. A black spot can be located on the head just above the clypeus. The filiform antennae are sexually dimorphic and are yellow on females but dark brown on males. The mouthparts include a labrum with a long row of thick spines along the outer edge, maxiallae, and mandibles with two teeth. The antennae are yellow in color, and measure 13 mm in length for males and 24 mm for females.

The shiny, black thorax features single yellow spots located on the prothorax, under each fore wing, and on either side of the propodeum. The metascutellum bears a yellow dash in its center. The brownish black abdomen is a distinguishing character for this species within the genus Megarhyssa. The thorax also contains spiracles that run along its entire length. Body length for this species averages 35 mm for males and 38 mm for females.

The long, slender legs are predominantly yellow but the coxae, middle trochanter, hind trochanter, and the incurved tarsal claws are black. In addition, the fore trochanters are both yellow and black as are the middle and posterior femora except on the tips. Each leg is covered with tiny hairs or spines.

The fusco-hyaline wings are long and narrow. This includes a pair of fore wings and hind wings that lock together during flight, as well as a pair of tiny basal wings. Each pair of wings is visibly covered by numerous veins and cells.

Female Megarhyssa atrata feature an extremely long ovipositor, measuring 12.7 to 15.24 cm in length, and is dark brown in color.

Males and females may be differentiated in numerous ways. First, females are much larger than the males. Second, females have long ovipositors. Third, the sternal plates are divided on the females yet entire on the males. Finally, males and females have obvious differences in coloration. Males tend to have more yellow than females and also have a raised yellow spot beneath each fore wing. (Merrill, 1915)

The life cycle of Megarhyssa atrata begins when a female lays her eggs in the burrow of a pigeon horntail larvae between the months of June and September. Eggs act as ectoparasitoids and are oviposited directly on the host larvae, one egg per host burrow. While still inside the eggs the Megarhyssa atrata develop into larvae. Upon hatching, the larvae emerge and begin feeding on the host larva from which they obtain all of the nutrition they will ever need during their short lifetimes.

These wasps remain in their larval form throughout the winter, continuing to feed on their host meal. In the spring they pupate within the horntail larva burrow, undergo a complete metamorphosis, and emerge from their burrows as adults between the months of June and September. Once the wasp emerges it will live for up to 27 days, just long enough to mate and continue the cycle. (Heatwole and Davis, 1965; Le Lannic and Nenon, 1999)

Males and females emerge from their burrows at the same time in the late spring to early summer months. Immediately after emergence both males and females are ready to mate. For poorly understood reasons, males will mate only with the recently emerging females. (Heatwole and Davis, 1965)

Females still embedded in the wood are detected by the characteristic chewing noises emitted as they eat through the rotten wood. This sound acts as a generalized stimulus that attracts males of all three species of Megarhyssa and as a result large mixed-species aggregations composed of up to 28 males form around these potential breeding sites. Males spend large amounts of time circling logs and stumps searching for this auditory signal. (Crankshaw and Matthews, 1981; Heatwole, et al., 1964)

Once such a site has been located, males will alternate between behaviors such as feeling the area with their antennae, flexing their abdomens, tergal stroking, and remaining motionless. With no obvious social hierarchy, males swarm the female emergence site in an attempt to mate with her as soon as she breaks through the wooden barrier. (Crankshaw and Matthews, 1981)

When a female emerges, only the males of her same species will attempt to mate with her while males of other species will disperse. How males discern the species of the emerging female remains unknown although there is speculation that it is an olfactory or chemoreception signal.

Following copulation, females fly off to locate a host larvae and oviposit their eggs while males will continue to search for emerging females in their home range. Females will oviposit multiple times, even within the same day. (Heatwole, et al., 1964)

The most fascinating aspect of the reproductive cycle in Megarhyssa atrata is the ovipositing behavior of the adult female wasps. Once a female has successfully mated she must first locate a host pigeon horntail larva, which burrows inside logs, fallen trees, or stumps at various depths. The preferred host larvae range in size from 7.1 to 31.7 mm in length. Exactly how females locate these larvae in their burrows is currently unknown but experts speculate that it is either via olfaction or auditory cues beyond a 60,000 to 10,000 cps range. (Heatwole and Davis, 1965; Heatwole, et al., 1964)

Female giant ichneumon wasps have the largest ovipositors of the three sympatric species, 12.7 to 15.24 cm long, and can parasitize larvae burrowed up to 140 mm in hardwood. A membranous pouch at the abdomen's tip holds the looped ovipositor while the female lays eggs. This provides support and stability while the stylus tip penetrates the wood. (Merrill, 1915; Sivinski and Aluja, 2003; Vamosi, 2005)

The ovipositor is inserted straight down into the wooden substrate so that it will eventually enter the host larva's burrow at an exact right angle. Females choose appropriate burrows for ovipositing so that the tip of the ovipositor will just reach the surface of the host larva. (Heatwole and Davis, 1965)

Once the female is ready to oviposit her eggs she rotates segments 8 and 9 of her abdomen and unfolds her intersegmentary membranes so that they form a disc 2 cm in diameter. The surface of this disc produces a lytic secretion that disintegrates the wooden substrate and facilitates her ability to insert her ovipositor. After she lays her ectoparasitoid eggs on the surface of the host larva she completes the same rotational movements to remove her ovipositor from the wood and the stylus returns to its resting position. (Le Lannic and Nenon, 1999)

Due to the long length of the ovipositor in female Megarhyssa atrata, completion of this cycle takes over one hour. During this period the females are particularly vulnerable to predators, hence the risk of predation is positively correlated with ovipositor length. (Vamosi, 2005)

Parental care in this species is minimal. Nutrition for the hatching larvae is provided in the form of the host larvae, Tremex columba. No parental care is allocated once the female has oviposited in a host burrow. (Vamosi, 2005)

After emerging from pupae, adult Megarhyssa atrata live an average of 27 days. However, many Megarhyssa atrata will never leave their burrows. Although it is a short time span, there is a high mortality rate in this species between metamorphosis and emergence. (Heatwole and Davis, 1965)

Megarhyssa atrata is a parasitic wasp that requires a host larvae to feed from during development. During their short lifetimes, they spend much of their time as sedentary larvae but become highly motile as winged adults. When adult wasps are not mating or surveying their territories, they rest on the underside of tree leaves for shelter. In Michigan this species has been observed sheltering under the leaves of beech and maple trees. There is no intra-species competition among Megarhyssa atrata adults outside the context of mating. In the forest, shelter is plentiful and adults are non-feeding, thus there is no reason for competition. (Heatwole and Davis, 1965)

Giant ichneumon wasps maintain relatively small home ranges that incorporate multiple logs, stumps, or fallen trees, essentially breeding grounds, which they visit repeatedly. Maximum known range size is 200 m squared. (Heatwole and Davis, 1965)

Various studies have suggested that these wasps communicate with one another and interpret the environment using a variety of sensory cues ranging from auditory, to olfactory, to chemoreception, to vibration detection. However, researchers have yet to determine the precise senses used. Most of what is known about the sensory system in Megarhyssa atrata has been gained from observing their reproductive cycle.

Both males and females are known to use their antennae vigorously during mate or host detection respectively. The antennae have sensory organs near the tip composed of two distinct types of hairs and plate organs. It has been suggested that the hairs perceive olfactory cues and the plates detect sound or vibrations.

During the search for mates males also use an auditory cue, the chewing sound emitted as a female eats her way through wood, to detect emerging females. (Heatwole, et al., 1964)

Heatwole hypothesizes that after a female emerges from a burrow into a male aggregation, males determine the species of the female using olfactory cues. (Heatwole and Davis, 1965)

The larval stage is the only feeding stage in the life cycle of giant ichneumon wasps. After the female oviposits a single egg on a developing pigeon horntail larva, the Megarhyssa atrata larva consumes the host. Adults are non-feeding. (Gibbons, 1979; Heatwole and Davis, 1965)

Females in the process of ovipositing incur the highest risk of predation. During this hour long process females suffer restricted movement that makes them vulnerable to predators, particularly birds. Lone ovipositors are often found in logs where the female was eaten by a predator during the egg laying process. Other predators include chipmunks and robber flies. (Heatwole and Davis, 1965; Vamosi, 2005)

Megarhyssa atrata occurs sympatrically with Megarhyssa greenei and Megarhyssa macrurus. All species occupy nearly identical ecological niches and exhibit nearly identical life cycles, parasitizing the same host species. The primary difference between the species is ovipositor length which in turn influences which Tremex columba larvae burrows they oviposit in, as well as the time which sexually mature adults emerge from parasitized burrows. (Gibbons, 1979; Heatwole and Davis, 1965; Vamosi, 2005)

These small morphological differences between the species enable them to inhabit the same area with minimal competition for resources. Together they decrease the potential population number of Tremex columba. It has been hypothesized that all three species share a common ancestor that was also a parasitoid of Tremex. (Gibbons, 1979)

Tremex columba, the larval host for giant ichneumon wasps, burrows into the trees of the United States and southern Canada, inflicting damage and eventually causing tree death. Although Tremex columba tends to attack injured trees (by fire, flooding, insects, etc.), it can result in economic loss for the American lumber industry.

Megarhyssa atrata have been used as biological control agents since they attack this species while it is still in its larval form. (Solomon, 1995)

There are no known adverse effects of Megarhyssa atrata on humans.

Currently, Megarhyssa atrata is of no conservation concern. Population size likely fluctuates with the prevalence of pigeon horntail larvae. With three closely related species competing for the same host species, there is a chance that Megarhyssa atrata may be outcompeted and could suffer future population declines.