The pigeon tremex horntail (Tremex columba) is a fascinating insect species belonging to the horntail family (Siricidae), which are non-stinging wasp relatives. These insects are commonly associated with dying branches and trunks of common hardwood species, playing a unique role in forest ecosystems. While they may appear intimidating due to their size and ovipositors, they are harmless to humans. This article delves into the diet and nutritional needs of the pigeon tremex, its life cycle, and its interactions with other organisms.
Identification and Habitat
Adult pigeon tremex are large insects, ranging from 1 to 2 inches in length, with a cylindrical, reddish-brown body marked with yellow bands and a black pattern on the abdomen. Their wings are tinted dark brown to black. A key characteristic of horntails is the presence of a "horntail," a projection on the end of the abdomen, present in both males and females. Females possess a longer projection called an ovipositor, used for depositing eggs into tree trunks.
These horntails are primarily found in the northern US and Missouri, particularly in forests with a significant presence of deciduous trees. Unlike some other horntail species that prefer conifers, the pigeon tremex is most commonly encountered on hardwoods like silver maple, ash, cottonwood, beech, elm, and oak. They are secondary invaders, meaning they target trees that are already weakened or dying, and are not the primary cause of tree death or dieback.
Life Cycle and Diet
Adult pigeon tremex are active from June to late September or early October. Female horntails preferentially lay eggs in unhealthy or dying trees, using their sharp ovipositors to insert eggs deep into the wood. On occasion, when egg laying, females become stuck and perish. They can insert their sharp ovipositors (egg laying devices/stingers) up to 13 mm into solid wood. She does this to deposit a single egg.
Along with the eggs, female horntails also vector a species of tree rotting fungus (Cerrena unicolor), also known as Daedalea unicolor. This fungus is a requirement for the successful development of the insects and may accelerate the already begun decay of the tree. The fungus grows within the wood ahead of the horntail larvae and is essential for their successful development. This symbiotic relationship is crucial for the larvae's nutrition, as they cannot digest wood on their own.
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The eggs hatch into whitish, segmented larvae with fleshy, poorly formed thoracic legs. These grub-like larvae bore into the heartwood of trees, creating round tunnels approximately the width of a pencil. As wood borers, the young of horntails tunnel into the sapwood and heartwood of trees as they eat. They feed on the wood, with the help of the wood-rotting fungus, for nearly a year, reaching approximately 50 mm in length before pupating just under the bark.
Pupation occurs, and the life cycle is completed, roughly in one year. Adults emerge through round exit holes in the tree. The most noticeable damage may be the irregularly round exit holes formed by the emerging adults.
Nutritional Symbiosis: The Role of Wood-Rotting Fungus
The pigeon tremex larvae's diet consists primarily of wood, but they rely on a symbiotic relationship with the white rot fungus Cerrena unicolor to digest it. The mother wasp injects the fungus along with her eggs into the wood. The fungus breaks down the wood, making it digestible for the larvae. This is why pigeon tremex prefer dead or dying wood, as the fungus has already begun to colonize and decay the wood, making it easier for the larvae to feed.
Impact on Trees and Forest Ecosystems
While pigeon tremex larvae do consume wood, they are not considered a significant threat to healthy trees. They much prefer dead, rotting wood, and so they aren’t any hazard to healthy trees. Their presence in a tree typically indicates that the tree is already weakened, diseased, or dying. The larvae are only found in previously dead or dying wood. The presence of this insect in live trees indicates dead wood and that the tree is a potential hazard. Damage may be seen on the trunks and main branches of maples and beech especially, other hardwoods as well. The most noticeable damage may be the irregularly round exit holes formed by the emerging adults.
By feeding on decaying wood, pigeon tremex larvae contribute to the decomposition process, helping to recycle nutrients back into the ecosystem. Additionally, the white rot fungus they introduce further accelerates the decay of the wood, promoting decomposition.
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Natural Enemies: The Giant Ichneumon Wasp
The pigeon tremex has its own natural enemy: the giant ichneumon wasp (Megarhyssa macrurus macrurus). This parasitoid wasp specializes in laying its eggs in horntail larval galleries. The females of this species have an even more impressive ovipositor than that of their hosts - it is approximately 8 cm in length, allowing her to drill deep into infested wood to paralyze and lay her egg on horntail larvae. The giant ichneumon wasp is a parasitic wasp (Hymenoptera: Ichneumonidae) specific to the pigeon tremex. It also is generally brown in color with yellow and orange markings. It has a very elongated body form and most notably long ‘tails’ that may extend a couple of inches. These ‘tails’ also are the ovipositor and supporting structures, used to insert eggs into wood onto developing pigeon tremex horntail larvae. Altogether the body and ovipositor of this insect may extend more than 5 inches (males are smaller, lack the ovipositor, and have a blunt tip of the abdomen). The ovipositor of the female consists of three filaments. The central part is the actual ovipositor, capable of drilling through wood. Although appearing as a single filament, it is actually made of two parts, that interlock, slide against each other, and are tipped with the cutting edge. Although very thin, it is a tube and the egg moves down the minute channel in its center during egg laying.
The ichneumon wasp's larvae then feed on the pigeon tremex larvae, ultimately killing them. This parasitic relationship helps to regulate pigeon tremex populations and maintain balance in the forest ecosystem.
Management and Control
Because pigeon tremex is not considered to be a primary pest, controls have not been developed, although it likely can be temporarily managed by use of insecticides in a manner similar to that of other wood borers. Given that pigeon tremex typically attack already weakened or dying trees, the best approach to management is to focus on maintaining tree health. Keep trees growing vigorously and healthy. Plant trees on the right sites and prioritize proper planting. This includes:
- Proper Planting: Ensuring trees are planted correctly to promote healthy root development.
- Adequate Watering: Providing sufficient water, especially during dry periods.
- Fertilization: Applying fertilizer as needed to provide essential nutrients.
- Pruning: Removing dead, damaged, or diseased branches to prevent further decline.
In cases where pigeon tremex infestations are a concern, insecticides may be used to temporarily manage populations. However, it is crucial to read and follow all label instructions for safety and proper use. Always confirm that the site you wish to treat and the pest you wish to manage are on the label before using any pesticide. Read the full disclaimer.
Distinguishing Pigeon Tremex from Other Horntails and Pests
It is important to distinguish pigeon tremex from other horntail species and pests that may cause similar damage to trees. Several other species of horntails also occur in the state, but these are limited to conifers, usually those growing in forested areas. Some notable look-alikes include:
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- Urocerus taxodii: This species has yellow-tipped antennae and a pale spot on each side of the head. Females are black or dark-bodied with dark wings and a yellowish horntail, while males are orange- or yellowish-bodied with paler to clear wings.
- Urocerus albicornis: This is a black horntail with 2 white or yellowish bands on each leg, white or yellowish antennae with black tips and bases, a white or yellow spot on either side of the head, and brown wings.
- Sirex noctilio (European woodwasp): This invasive species is known to cause the death of up to 80 percent of the pine trees in an area. Adults have metallic bluish-black bodies with reddish-yellow legs, black feet, and black antennae.
- Eriotremex formosanus (Asian horntail): Native to southeast Asia, this species has been accidentally introduced to North America. It is a dark brown and yellow-striped wasp with short, black, blunt antennae and long yellow hairs on the top side of the abdomen.
Additionally, it is crucial to differentiate pigeon tremex exit holes from those caused by the Asian longhorned beetle (ALB), especially on maple trees.