Monochamus is a
genus of
longhorn beetles found throughout the world. They are commonly known as sawyer beetles or sawyers, as their
larvae bore into dead or dying trees, especially
conifers[1] such as
pines. They are the
type genus of the Monochamini, a
tribe in the huge long-horned beetle
subfamilyLamiinae, but typically included in the
Lamiini today.
If sawyer beetles infect freshly cut pine logs, they can cause a 30–40% loss in value due to the tunnels their larvae bore. It is important to process logs within a few weeks of cutting or store them in water to minimize damage.[2] Some species are known to transport
phoreticBursaphelenchusnematodes, including B. xylophilus which causes pine
wilt disease.[3]
Description
Beetles in this genus are black or mottled gray in colour. Like other Lamiinae, the head is oriented vertically with ventral mouthparts. The scape (first
antennal segment) has a circatrix, a carinate ring or scar-like area near the tip. Antennae of females are roughly as long as the body, while antennae of males are twice as long. The tarsal claws are divergent.[4][5]
Life cycle
Adults feed within the
crowns of healthy trees. Adult females oviposit (lay eggs) in slits in the bark of dying or dead trees.
Larvae hatch from eggs and develop in wood, passing through several instars. Next is a
pupal stage. In spring, new adults emerge, starting the life cycle again.[6]
Association with nematodes
The pine wood nematode Bursaphalenchus xylophilous is transmitted by several Monochamus species, and its life cycle is tied with that of its vector.[6]
When nematode-infested adult Monochamus feed on healthy trees, they create wounds that allow nematodes to enter. In a susceptible host tree, nematodes breed in the
xylem and eventually kill the tree. In a resistant host, the nematodes die instead.[6]
When nematode-infested adult Monochamus oviposit in dying or dead trees, the slits they make in the bark for their eggs also allow nematodes to enter. Nematodes reproduce and feed on wood cells or fungi. Eventually, new Monochamus adults emerge from pupae, and while these are still
callow, nematodes enter them via the thoracic spiracles.[6]
Associations with other organisms
Bark beetles oviposit on trees at around the same time as Monochamus, and they transmit
blue stain fungi. Bursaphalenchus nematodes feed on this fungi, and the combination of fungi and nematodes may help in overcoming host tree defences, creating a more suitable habitat for bark beetles and Monochamus.[7] Additionally, Monochamuscompete with bark beetles for resources, prey on them (
intraguild predation) and use their
semiochemicals as
kairomones.[8]
The genus is very large, and its boundaries have varied considerably over time, with many species placed in this genus that have long since been removed, and species placed in other genera that have been added, and there are at least 20 subgenera presently recognized by most authorities.[11]