A. There are normally hundreds of different types of beetles that attack trees here in Southern California, in fact beetle attack is a normal stage in the life cycle of most trees in the wild. Without beetles attacking trees that are stressed and dying they would take far longer to decompose after the trees have died, and therefore it would take longer for the new trees replacing them to establish.
This normal beetle activity accelerates when stress on the trees accelerates, which is what's happening now because of the current severe drought. When most of the trees in a forest are stressed, the beetles numbers increase to a point where they are so large that even a healthy tree can't repel them.
But there's more. The current level of trade from around the world has increased the number of new beetles that have no natural checks and balances (most of our native beetles are normally kept in check by parasites and predators that feed on them). One example is the Polyphagous Shot Hole Borer (PSHB). The pest was only discovered in 2012 and since then has caused the loss of thousands of trees from the urban canopy here. Since this pest also threatens the $Billion+ avocado industry, there has been significant resources channeled to its control. Still, there does not seem to be an effective control against it, although several compounds are being looked at and an effort is being made to identify predators and/or parasites that can naturally control the pest. Currently it attacks over 100 types of trees and large shrubs, and its host list (the plants it can reproduce in and spread the infestation) has swollen from 22 in 2012 to over 40 now.
What's the answer then? Short term finding controls is important, but it does not solve the problem. The biggest problem and one of the big reasons that the shot hole borer and other pests are getting such a foothold here is the urban forest canopy itself.
Many of our trees are clonal, they are basically the same tree. That is what ornamental horticulture is about is reproducing clones of plants that have the most desirable traits you are looking for. This make for easy pickings for pests because there is no difference in the plants ability (or lack of it) to resist the pest that is attacking. This makes for large numbers of pests and favors the those offspring that are particularly good at attacking and reproducing.
Also our own urban canopy is made up of trees that have evolved to take advantage of climates that are totally unlike ours. For example the beautiful fall color turning tree Liquidambar styraciflua originated from the South eastern coast of the United States. That area has copious amounts of rainfall throughout the summer, unlike Southern California where summers are hot and dry. Why does it do so well here? Two reasons; 1. Irrigation which allows us to grow species of trees that otherwise would not establish here 2. Lack of pests due to the fact that the plant is grown in an area where its native pests are absent. Both of these conditions are unsustainable in the long run and the only cure is to select trees that actually do well with the amount of water that is naturally available here. We will still get new pests but the trees will not be stressed and therefore huge plague-like infestations like the current PSHB infestation are less likely to happen. That does leave us with a much smaller selection of trees to plant, but if we do that we can enjoy a good looking and long lasting landscape dotted with trees.