Dendroctonus adjunctus, the roundheaded pine beetle, is a species of bark beetle in the family Curculionidae found in North America. A parasite, the roundheaded pine beetle feeds on and eventually kills pine trees of several species in Guatemala, Mexico, and the Southern United States (New Mexico, Arizona, Nevada, Colorado, Utah).
Roundheaded pine beetles are a serious threat to wild and commercial pine stocks, wiping out up to half of pine trees in a region in outbreak years. To prevent and mitigate the beetle's epidemic outbreaks, loggers have developed a variety of biological, cultural, and chemical controls.
Description
Like many insect species, the roundheaded pine beetle goes through several stages in its life cycle, changing dramatically in appearance between each one. Adults are 5-6 mm long and 3 mm wide, with females slightly larger than males. Adult exoskeletons are shiny, reflective, dark brown or black, and covered with hairs. Their eyes are flat, don't protrude from the exoskeleton, and are generally elongated. The adult beetles have antennae and wings and can fly.
The roundheaded pine beetle's eggs, less than 1.5 mm in length and width, are much smaller than adults. They are oblong and pearly white.
Larvae are C-shaped, legless, and mostly translucent, though their abdomens are sometimes visible as tinges of reddish-brown. Depending on the instar stage, the larvae's size can vary from slightly larger than an egg to slightly smaller than an adult.
When the larvae become pupae, they become an opaque white. The pupae also have antennae, legs, and wing covers, reflecting characteristics of the species' adult form.
Life cycle
The lifespan of most roundheaded pine beetle is one year. In October and November, female adults gain flying abilities and colonize weakened pine trees by attacking the tree bole's basal portion. The beetles often capitalize on trees already attacked by species in the Ips and Dendroctonus genera. Once they locate a desirable pine tree, they release aggregation pheromones, which attract other beetles to the tree. The beetles then bore through the pine's bark, constructing chambers where mating occurs. After mating, each male-female beetle pair works together to build egg galleries, carving grid-like structures of tunnels where females lay and store their eggs. Though egg galleries of different mating pairs often cross, it appears roundheaded pine beetles do not actively cooperate in infesting trees beyond male-female mating partnerships.
The roundheaded pine beetle's egg galleries can grow to be pretty extensive, as each horizontal tunnel is on average separated by 2.5-5 cm and each longitudinal tunnel by 30 cm. Each gallery overall is usually 30.5 cm long, but large galleries can extend to up to 1.2 meters. Female roundheaded pine beetles fill their galleries with egg, depositing one egg per grid niche on alternating sides of the gallery. The beetles initially keep the egg gallery free of dust, but as the gallery deepens into the tree, male beetles packs the first few inches of tunnels closest to the pine bark with boring dust, probably to protect the eggs.
Beneath the bark, these egg galleries mostly exist in the tree's cambium, the layer separating the pine's xylem, transport tissue carrying water from tree roots to leaves, from its phloem, tissue ferrying sugar and other nutrients around the plant. However, the egg galleries extend into the tree's phloem and often its xylem as well extend beyond; the roundheaded pine beetles coopt phloem into their habitat, severing the tree's internal circulation and drastically reducing its ability to get sugars and nutrients to where they're needed. Roundheaded pine beetle pairs also feed on phloem throughout, further sapping the host tree of nutrients.
Roundheaded pine beetle pairs that begin boring into trees in October complete their galleries by December; those that start in November can only partially complete their dwellings by December. For these beetles, cold bark temperatures between December and February limits their gallery tunneling to the few days when the bark is warm enough for activity. Adult roundheaded beetles continue a minor amount of construction in March and April.