The Arroyo, Arundo, and an Increasing Presence of Beetles

Fall 2016 Ecesis, Volume 26, Issue 3

From top: Improvements on site with the removal of Arundo is clear with decreased competitive species and increased sunlight reaching more areas. The photos show the change between one month. 

From top: Improvements on site with the removal of Arundo is clear with decreased competitive species and increased sunlight reaching more areas. The photos show the change between one month. 

Throughout the span of a restoration project there are various obstacles that may arise. As restoration ecologists, our adaptive approach to these routine challenges is based on experience and knowledge of the subject habitats. However, when an unknown stress is brought upon a site, the original restoration plan must be flexible to allow for more adaptive management and collaboration among other scientific communities examined. With the shot hole borer rise within Orange County and, on a larger scale, Southern California, these species are presenting new complications that affect not only the health of the system but also budget constraints associated with the restoration plan. The original restoration design is clear and based on existing conditions within a system; however, with the introduction of shot hole borer species, functional lift may be slowed and vegetation may be threatened, either of which would require immediate attention. One example of this is a restoration site within Orange County, in Arroyo Trabuco Creek. 

Site Background

Prior to restoration activities (enhancement and re-establishment) commencing onsite, Arroyo Trabuco Creek was dominated by Arundo donax. Arundo was present from the invert to the widest parts of the floodplain and acted as a wall for wildlife movement and sediment transport. The existing native habitat within the Arroyo consists of sycamore–alder riparian forest and willow forest within the low-flow channel, and sycamore and coast live oak woodland on the floodplain and banks. The intent of the Arundo removal was to re-establish the historic function of the Arroyo and remove the exotic species from the upstream reach. Arundo removal has been completed for a few years, with the final removal of Arundo this summer. The changes within the Arroyo have been well noted. Degraded trees, in particular arroyo willow (Salix lasiolepis), are showing increased signs of new growth and overall health improvements. Volunteer species are also present within the floodplain where Arundo had been present. The invert has begun to meander and again interact with the floodplain. Wildlife movement throughout the corridor has also increased, and is tracked through volunteer support and wildlife cameras. The riparian habitat and emerging understory continues to improve as future phases of the restoration project are implemented.

Initial Appearance

Entering the second phase of the project (rhizome removal, recontouring, and planting), a new issue arose of shot hole borer beetles onsite. The beetle’s existence became evident during the month of August, with multiple white alders (Alnus rhombifolia) showing signs of staining along the trunks. With the rise of the infestation becoming a concern countywide, previous monitoring reports and photos were sorted through for any indication of the beetle’s introduction onsite — good record-keeping and photograph documentation showed evidence of symptoms in the lower downstream reaches.

Generally, the beetles are more active in the late spring with their flight patterns; however, the drought and warmer, humid weather has allowed high activity to continue through late summer into fall. Research indicates the male and female beetles leave their host trees in search of new trees during this time, which coincides with the spread of beetles monitored onsite. Symptoms along the large spans of alders were not noted until July of this year, once Arundo was removed.

Identifying the Severity of the Problem

To categorize the severity of the infestation, ethyl alcohol bait traps were placed onsite to monitor the population,initially in areas showing symptoms but exhibiting no entry or exit holes. To minimize the unforeseen expenses, traps were created using plastic bottles and string. Ethyl alcohol was used to mimic the ethanol released by trees in response to abiotic stressors, in this case stressors included drought, competition with Arundo, and/or higher temperatures. Different lures were tested with different strengths of ethyl alcohol, with the trap containing Purell (75% ethyl alcohol) having the most success — catching 25 beetles in one area. As of now, categorization of host trees is underway within a two-acre area. 

The area with the highest beetle activity is located at the northern end of the project boundary. The alders are almost dead, with hundreds of entry/exit holes observed up and down the tree trunks. With further monitoring, traps were placed at the base of the high activity area and placed every 40 feet leading up to the area originally tested, totaling an estimated 40 traps. Different beetles were found present and species identification is underway.

Research, Collaborate and Adapt

Since majority of the site appears to be in the early stages, containment and further prevention is a possibility. The continued use of bait traps is ideal for capturing, studying and ultimately decreasing the beetle population. Though few biological controls can be explored, some treatments may not be an option due to the proximity of open water within the Arroyo. Best management practices have been implemented with the occurrence of the beetles onsite. These include cleaning of vehicles and tools before leaving the site and hauling away cut wood to dump sites rather than green waste services. Though few trees have been trimmed or removed during the restoration process, any showing symptoms of the beetles and corresponding fungi will be cut and solarized onsite before ultimate removal.

At this stage, maintaining the health of other desired host species is the most important. With continued restoration of the site, in addition to long-term maintenance and monitoring, the habitat will continue to increase. Because the beetles require trees large enough in diameter to support their galleries, new plantings may not be affected at their younger stages, allowing for the restoration to continue and the beetle population to decrease over time. 

The beetle infestation is generally new to Orange County and information must be shared as data is collected and distribution known. Collaborating with different scientific communities is key in approaching restoration with troubled sites. As more research is completed on shot hole borers and other ambrosia beetles, there are many different groups that can provide guidance for the restoration site, as the site also offers an opportunity for studying these specimens. Our proactive collaboration has included coordination with the University of California Riverside and University of California Agriculture and Natural Resources, which conduct research on a specific species of shot hole borers and polyphagous shot hole borers. Stressors and challenges will always be part of ecosystem restoration; however, we can reduce the threat of such issues through collaboration and proper documentation so that watersheds benefit—not just single sites. — Anisha Malik, Restoration Ecologist, Michael Baker International. Anisha.malik@mbakerintl.com