Tree of Heaven (TOH) was imported to North America by an avid gardener in Philadelphia, PA in 1784, a time when importing exotic plants from around the world to grace gardens and landscapes was the thing to do. Human-assisted spread of TOH continued into the 1850s, when TOH were transported to Western states. While originally planted for its uncanny ability to thrive in highly disturbed locations and poor soils, its ability to displace native vegetation and to serve as a factory for invasive pests, including spotted lanternflies, has earned it a place on the “trees I don’t want in my landscape” list.
This Tree of Heaven succumbed to the combined attack of lanternflies, wood-boring insects, pathogens, and, perhaps, other non-living stressors of the environment.
In a previous episode, we met the ailanthus webworm, a native moth whose caterpillars feed on the foliage of TOH and contribute to its demise. We also learned that chronic, high-density infestations by lanternflies rob TOH of vital nutrients, and this combined with other factors may lead to the death of TOH. Last week while visiting a Forest Retention Area heavily infested with TOH and spotted lanternflies, I stumbled across a TOH tumbling down the death spiral with hordes of lanternflies sucking its vital sap and a thinning canopy dropping dead branches. Newly arrived at the bole of the tree was a spectacularly large, colorful wasp called the pigeon tremex. She was thoroughly engaged in depositing her eggs into the dying TOH. The pigeon tremex, a.k.a. pigeon horntail, is part of the wood wasp clan known as Siricidae. Adults of this species have a hornlike appendage at the tip of their abdomen giving rise to the moniker “horntail”. The female wasp seeks dying or freshly dead trees as a nursery for her young. The female uses an exquisitely long egg-laying tube called an ovipositor to drill into the bole of the tree and deposit her eggs.
It’s easy to see why the pigeon tremex is called a horntail. The female’s powerful ovipositor drills deep into a tree to lay eggs. After hatching, her wood-boring larvae hasten the demise of the tree.
Along with the eggs, wood wasps introduce fungi into the wound. These fungi help digest recalcitrant woody tissues that serve as the food for developing wasp larvae. Larvae devour the woody tissues and are easily recognized by a characteristic spiny appendage on their rear end. Feeding by wasp larvae reduces the structural integrity of the tree, further hastening its collapse. Reports abound that the pigeon tremex often dies with her ovipositor still inserted into the bark of the tree. However, an inspection of the tree the following day provided no evidence that this wasp perished following the act of laying her eggs. Perhaps, she lived to lay another day or disappeared down the gullet of a hungry bird. Who knows?
Chronically high densities of spotted lanternflies like these weaken invasive Tree of Heaven (TOH) and may usher them along a spiral of decline leading to death. Dying and dead deciduous trees including TOH attract a spectacularly large, colorful wasp called the pigeon tremex. Adults of this species have a hornlike appendage at the tip of their abdomen giving rise to the name “horntail”. Watch as the female uses a long egg-laying tube called an ovipositor to drill into the bole of the tree, where she deposits eggs. When eggs hatch, with assistance from fungi introduced by the wasp, her larvae devour the woody tissues of the tree, reducing its structural integrity and hastening its collapse. Wood wasp larvae are important recyclers of dead and dying trees.
Sadly, not all is well for the helpful larvae of the pigeon tremex. They are hunted by a colorful parasitic wasp called the long-tailed giant ichneumonid wasp, Megarhyssa macrurus. This clever parasitoid lands on the surface of a tree wherein larvae of the pigeon tremex develop. Using a combination of cues, she tracks down the precise location of the tremex larva, drills though the wood, stings and paralyses the tremex larvae, and deposits an egg which will hatch into a hungry larva that devours the offspring of the wood wasp. In a bug eats bug world the twists of fate blur the distinction between which bugs are helpful and those that are not.
Beautiful female long-tailed Megarhyssa wasps use their supper long ovipositors to deposit eggs deep inside a tree where larvae of the pigeon tremex feed. Larvae of the tremex will become food for developing Megarhyssa larvae.
Acknowledgements
These fascinating articles were used to prepare this weeks episode: “Tree-of-Heaven” by David R. Jackson, Art Gover, and Sarah Wurzbacher, “Spotted Lanternfly Frequently Asked Questions” by Heather Leach, “Pigeon Tremex” by James Baker, “Symbiotic relationship between Cerrena unicolor and the horntail Tremex fuscicornis recorded in the Czech Republic” by SYLVIE PAŽOUTOVÁ and PETR ŠRŮTKA, and “Megarhyssa spp., The Giant Ichneumons (Hymenoptera: Ichneumonidae)” by Ilgoo Kang, and Christopher Carlton.