Imported leaf-eating beetles slowly adapting to local ecosystems
By Summit Voice
SUMMIT COUNTY — Efforts to control invasive tamarisk plants along the Arkansas River are looking up, thanks to a boost from some unexpected evolutionary adaptations. A small imported but that eats and kills the water-sucking plants has been expanding its range and reproducing more efficiently after adapting to regional cycles of darkness and light.
“This is one of the clearest cases of rapid evolution,” said Tom Dudley, who has been involved in the tamarisk control efforts at UC Santa Barbara’s Marine Science Institute Riparian Invasive Research Laboratory.
The tamarisk leaf beetle has managed to delay its entry into hibernation to adapt to the shorter days of the southern region of the United States. That adaptation enables the beetle to survive until spring and prolongs the time it has to reproduce.
Tamarisk, also known as saltcedar, is a shrub that was introduced to the United States from Eurasia in the 19th century as an ornamental plant. it established itself in riparian areas, consuming valuable water supplies, and competing with native vegetation.
“It makes a lousy habitat for wildlife,” said Dudley, adding that the plant is also a fire hazard, as it is highly flammable even when green and healthy.
Along with intensive manual treatments, scientists introduced the tamarisk leaf beetle after a decade of testing. The Eurasian species eats only the foliage of tamarisk and hibernates in its leaf litter.
The beetle successfully established in the northern regions of the United States, where the day lengths matched those of the beetle’s native Kazakhstan and western China. But in the south, shorter daytime hours threw a wrench into the bug’s biological clock. The beetles started entering hibernation too early. Instead of feeding and reproducing, the beetles used up their metabolic reserves before the arrival of spring.
But about seven years after they were introduced, the beetles started to delay their diapause by about two weeks, adjusting their hibernation time so that the next generation was better suited to the season. Between 2003 and 2008, for instance, tamarisk leaf beetles at eastern Colorado’s Arkansas River went from going dormant in late July to remaining active until mid-August.
The same genetic mechanism also made it possible for the insect’s continued southern dispersal, and further biocontrol of the invasive tamarisk.
“It’s a very cost-effective way of containing the weed,” said Dudley. In some places, the natural vegetation will be able to reestablish itself, while in places where the shrub was particularly pervasive, there will be restoration efforts, he added.
The beetles won’t completely eradicate tamarisk. After almost two centuries of establishing itself in the United States, the plant has become part of the ecosystem. In some areas tamarisk even provides habitat to the willow flycatcher, a federally listed bird that nests in the shrub where the preferred native plants have been displaced, although only in places where some native vegetation still exists.
There is concern that tamarisk defoliation will disturb sensitive bird species in the short term. But promoting and restoring the native species of vegetation is preferable for the long-term sustainability of bird and other wildlife species that depend on riparian systems.
Dudley said it’s unlikely that the beetles will jump to another species of plant to consume, citing many years of lab testing that show they will only feed and reproduce on tamarisk. It has been observed that the beetle population experiences a gradual dieback as its food source decreases. In fact, the beetle is becoming a part of the ecosystem as well, an additional source of food for birds and other predators.
“The neat part is that it’s creating more complexity in filling the herbivore trophic level, thereby sustaining other trophic levels above,” said Dudley.