Sunday, 6 July 2014

New Species of Spider Wasp Uses Ant Corpses to Protect Its Nests

A newly discovered wasp species uses the corpses of dead ants as scarecrows, to protect its nest from predators, according to a new study by researchers at the University of Freiburg. Scientifically described as Deuteragenia ossarium and dubbed as the Bone-house wasp, the species uses the chemical cues from ant corpses to ward off predators, by stuffing them into the crevices of its home. The species was discovered in Jiangxi Province in south east China.

Wasps use a wide range of nest protection strategies, including digging holes or occupying pre-existing cavities such as in wood.  Previous studies have shown that the nests of cavity-nesting wasps contain several brood cells separated by thin walls of plant debris, resin, or soil. Once the females have finished the construction of the nest, they lay their eggs, store food and finally construct an outermost vestibular cell to seal the nest. Next, they abandon the brood and leave the offsprings to deal with the hardships of life all by themselves. This means that nest protection strategies play a detrimental role in the survival of the brood.

In this study, the researchers collected 829 nests of cavity-nesting wasps with 1929 brood cells belonging to 18 different species in South-East China. To their surprise, 73 of these nests had a vestibular cell filled with dead ants and belonged to an unknown species.

Image depicting the Bone-house wasp (Deuteragenia ossarium) and its nest protection system
The bone house' wasp and its nest-protection system

As far as scientists know, this strange practice is unique not only in wasps but also in the entire animal kingdom.
"It was a totally unexpected discovery." said Dr. Michael Staab, researcher at the University of Freiburg in Germany and leading author of the study.

The species was described in the same study as Deuteragenia ossarium and was nicknamed the "Bone-house Wasp". The Latin word "ossarium" means bone-house, an attribution to the historical ossuaries piled high with human skeletons found in monasteries or graveyards.

"An ‘ossarium’ is a covered site, where human remains are deposited. The species name is an allusion to the unusual nesting strategy of the new species, which closes the nest with a vestibular cell filled with dead ants. This reminds us of historical bone-houses in monasteries and graveyards, which over time were filled with piles of human bones." an excerpt from study.

Smelly Scarecrows
The researchers were initially puzzled by the presence of dead ants, until they considered the location and the species of the dead ants. The corpses were always placed in the outer vestibular cell, a chamber built by the female wasp to seal the nest after laying her eggs. The ants most commonly found in the crevices belonged to the Pachycondyla astuta species, an aggressive and large-bodied ant. This ant is quite common in the region and as a result, potential predators may have had previous contact with the species and therefore avoid the scent of the species.

In simple words, the wasps possibly use the dead ants as "smelly scarecrows", camouflaging their nests against scent-based predators.

"However, due to the very good condition of all ant specimens in the ant chambers, we assume that the wasp must actively hunt the ants and not collect dead ants from the refuse piles of ant colonies," commented Staab to Live Science, in an email on how the wasps collect the ants.

The researchers reported lower parasitism rates in the nests of the Bone-House wasp, compared to the nests of other cavity-nesting wasps, further supporting the aforementioned assumptions.

"Our discovery demonstrates in an impressive way, what fascinating strategies of offspring-protection have evolved in the animal kingdom." said Staab.

Other species of wasps also resort to other macabre practices to protect their youngs. For example, the parasitic Dinocampus coccinellae wasp turns ladybird bugs into temporary "zombies" that guard its youngs! 

Dinocampus coccinellae larva cocoon and zombified ladybug
Dinocampus coccinellae larva forming cocoon next to zombified ladybug

The paper is open and free to read by all, I highly suggest you to check it out. It is a very interesting read!

- Staab, M., Ohl, M., Zhu, C., & Klein, A. (2014). A Unique Nest-Protection Strategy in a New Species of Spider Wasp PLoS ONE, 9 (7) DOI: 10.1371/journal.pone.0101592

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