By Sandy Masuo, Los Angeles Zoo
Characteristic holes in pine bark that was removed from a dead tree.
In November, many Los Angeles Zoo staffers arrived at work to find tree crews removing dead pines from the hillside around Zoo Grill—casualties of the ongoing bark beetle epidemic in Southern California. In addition to some 200 native species of bark beetle, 20 invasive species have been documented in California. Prolonged drought stresses trees, which, like animals, have immune systems that normally defend against pathogens and parasites. In their weakened state, trees are more susceptible to disease and infestation. So although these tiny beetles (only about 1/8th inch long) are normal denizens of our California ecosystem, under the drought conditions of recent years, their populations have swelled, contributing (along with other factors) to mass die-offs of trees, which in turn contribute to catastrophic wildfire and erosion. Between 2012 and 2016, California lost some 100 million trees.
Bark beetles leave telltale holes in the external bark where adult insects have bored through to the phloem, or inner bark, where they lay their eggs. Both adults and larvae feed on the nutrients that flow there and disrupt circulation. A healthy tree can withstand a limited infestation—but too many beetles can kill even a robust tree. The beetles emit an aggregating pheromone that acts as a beacon, drawing more beetles to the site. Many also carry fungi that can infect the tree, hastening its death. In some cases, symbiotic fungi are part of the beetles’ life cycle. According to the Los Angeles Times, a 2014 survey at the Huntington Library, Art Collections, and Botanical Gardens in San Marino (about ten miles from Los Angeles) found that 207 of 335 species had been attacked and more than half contained beetle-vectored fungi.
Among the invasive bark beetle species, one of the most destructive is the polyphagous shot hole borer (Euwallacea fornicatus), aka PSHB. First discovered in California in 2003, the tiny insects are thought to have been accidentally imported from Asia in packing materials or wooden shipping pallets. These beetles are similar in appearance to the bark beetle and females carry the spores of a symbiotic fungus that they plant in the bark of host trees to provide a food source for their larvae. PSHB attacks more than 200 species including commercially valuable trees such as avocado and important natives such as California sycamore and coast live oak.
The widespread nature of these infestations is such that we must rely on scientists to find a large-scale control. However, poor pruning practices, improperly cleaned garden tools, and the transportation of cut wood contribute to the spread of these insects.
Learn more about bark beetles at http://tinyurl.com/y935swh9 and PSHBs at http://tinyurl.com/y8pnjtwv.
New York Times, December 12, 2017, “Widespread Tree Die-Offs Feared” by Jim Robbins