ENDPAPER BY SEAN O’DONNELL

Wasp trees mostly with and/or Caci- Fortresses of cus. All taken in Guyana on the Rewa River in tropical forests 2015 by biologist Sean McCann. ca in response to sounds. Given n tropical forests—with their high that this drumming is an indicator of I density and diversity of life—ev- colony alarm and is often a precursor erything seems to be under near- to stinging attack, we retreated to the constant attack from competitors, Tiputini River. predators, and parasites. A salient A fascinating but as yet unanswered feature of the lowland rain forests question is, what factors drive the as- of South America is the prevalence sembly of trees? Arboreal Azteca of venom. Venomous are ants—with colonies populous and highly diverse and abundant, such aggressive enough to stave off as rear-fanged colubrids, candy- swarms—may be the striped coral snakes, pit vipers, initiators and may provide paper tree-dwelling “fire” ants, army ants, wasps protection from army bullet ants, and several species of ant raids. And the stinging paper wasps, to name a few. paper wasps may protect the Why so much venom? In some ants from such predatory cases, it is partly offensive, used to mammals as anteaters. subdue and, possibly, digest prey. Another possibility is Army ant venom contains proteases, Tiputini that wasp trees develop enzymes that break down proteins. But Biodiver- as a result of positive in most cases, if not all, it is deployed sity Station feedback. If paper defensively. For social , such in eastern Ecuador, I got an in- wasps perceive as paper wasps, the painfully venom- timate look at a multi-species as- the presence of ous stings by adults are a key means sociation that may represent a nearly other wasp nests as well as Azteca ants, of defending the large number of nu- impenetrable cooperative defensive or if offspring wasps settle near their tritious offspring housed inside their array: a fortress centered on venom— parents, wasp trees may become active nests. Presumably, pres- the wasp tree. aggregations of diverse wasp colonies. sure is especially intense in tropical I was in a small boat with my Wasps deploy chemical pheromone forests and favors effective and readily daughter and a guide, motoring up signals to communicate alarm and wielded stings. Many relatively harm- a blackwater stream off the Tiputini incite defensive stinging. Perhaps par- less insects and other have River. In an area of flooded forest was ticipant wasp species respond to each evolved to closely resemble stinging a moderately sized tree with paper other’s alarm pheromones. social insects: some spiders and beetles wasp nests in the canopy. In the upper Whatever the mechanism of assem- resemble ants; flies, moths, and even branches was an impressively large bly, wasp trees appear largely immune katydids can closely resemble stinging nest of rejecta, a paper wasp to predator attack. Although monkeys tropical wasps. Defensive stings are noted for its bellicosity. Nearby was and red-throated caracaras are willing deployed mainly against vertebrates, a nest of the large-bodied, painfully to tolerate stings to obtain wasp brood such as primates and birds. stinging P. striata. Each of these nests as food, neither is immune to venom Tropical paper wasps are targets easily contained many thousands of and both have been observed fleeing for two distinct and highly effective workers. An exceptionally large carton large well-defended wasp colonies. predators. From below, army ants (ge- nest of arboreal Azteca ants was on Any vertebrate that approaches a wasp nus Eciton) form cooperative swarm- the trunk a few meters above the wa- tree is likely to evoke a rapid, massive, raids that climb from the ground and ter. On the trunk above the ants were multi-species stinging response. When subdue wasp nests. Tropical paper at least seven active paper wasp nests, I think back on the ominous drumming wasps show specialized responses to including three of Synoeca virginea, that was audible from at least five me- army ant odors, including pronounced one of a different Polybia species, ters away, I am grateful we did not ex- alarms and rapid nest abandonment. and three of an unidentified species plore further the wasp tree’s defenses. From above, forest-dwelling falcons, of Parachartergus. Although no bird red-throated caracaras, patrol in nests were observed in the tree, local Sean O’Donnell, professor of Biodiversity family groups and attack wasp nests birds often nest in association with Earth & Environmental Science and Bi- ology at Drexel University, is a frequent at high rates: a group of five can con- wasps. As we scanned the tree in awe, contributor to Natural History. His most sume as many as fifty nests daily. we noted a low and ominous rhythmic recent article, “Neuroeconomics,” ap- On a recent research trip to the drumming sound, produced by Synoe- peared in the March 2018 issue.

48 n a t u r a l h i s t o r y December 2018/January 2019