- Author: Frank Byrne, Akif Eskalen and Joe Morse
Two closely related Ambrosia beetles (Euwallacea sp.) have been identified in commercial avocado groves in California. The polyphagous shot hole borer (PSHB), detected in Los Angeles, Orange counties and recently in Ventura county, and the Kuroshio shot hole borer (KSHB), detected in San Diego and recently in Orange and Santa Barbara counties, are morphologically indistinguishable, but genetically distinct. Already widespread in a variety of reproductive host trees common in the urban landscape (including box elder, willow, several maples, oak and sycamore species), the beetles represent a significant threat to trees in both landscape and agricultural settings. Adult females construct galleries in the xylem system of host trees, where they cultivate symbiotic fungi (Fusarium, Paracremonium and Graphium spp.) as a food source for their developing young. The fungi are taken up by progeny females in specialized organs within their mouthparts, and transported to other sites within the same tree, where new colonies are established, or to newly colonized hosts. The galleries compromise the structural integrity of infested trees, which can represent a serious safety hazard in urban environments, and disrupt the flow of water and essential nutrients within the xylem. In addition to the physical damage, the fungi extract nutrients from the xylem system, further depriving the tree of nutrients essential for healthy growth and fruit production.
An effective biological control agent is not yet available to manage the SHB in California, and so management for now must rely on the use of chemical pesticides. The control of Ambrosia beetles and their associated fungi using
chemical pesticides is complicated because of their location inside the host trees. The application of insecticides to the external surfaces of trees, where the beetles must first alight prior to boring, has the potential to kill beetles by contact activity, and they may also have the potential to control emerging young adults before they can re-infest the trees.
The drawback of surface treatments is that multiple applications are often required because of the relatively short duration of efficacy. In addition, once the beetle burrows inside the tree, surface treatments are become ineffective. One possible solution to this problem may be the use of systemic pesticides, and scientists at UC Riverside are evaluating the use of both systemic insecticides and fungicides in a 2-pronged attack against the symbiotic system.
Systemic pesticides are mobile within the xylem system of plants, and the fungicides could potentially target the fungi growing in the xylem and deprive the beetle larvae of a food source. The insecticides would prevent the beetle from establishing galleries within susceptible tree hosts, and prevent the survival of beetles and their offspring already present within trees. The big problem with systemic pesticides is getting sufficient concentrations of chemicals to the areas within the trees where the beetle and fungus occur. Although there are exceptions, most systemic treatments are administered to the soil for uptake through the roots. However, in mature avocado groves, the high organic matter content of the soil can prevent effective absorption by roots because the pesticide becomes bound to organic components within the soil. Trunk injection of pesticides directly into the vascular system of trees eliminates the potential for binding of pesticides within the soil, and increases the amount of active ingredient inside the tree available to impact the beetle/fungal system. Systemic pesticides must be formulated for trunk injection and so careful evaluation is needed to ensure optimal efficacy. Trials are being conducted with the assistance of avocado industry and grower collaborators in areas where the SHB has been recorded. The chemicals are injected into the trees using commercially available equipment, and the movement of the active ingredients is then monitored over time in wood core samples taken at different heights of the trees. Two methods are being used to confirm the presence of the chemicals. Insecticides are being quantified using ELISAs that are specific for the active ingredients under investigation. Wood cores taken from trees treated with fungicides are placed in direct proximity to the fungal pathogens growing on agar plates to determine if growth of the fungus is inhibited.
The investigations are still at an early stage, but the researchers are optimistic that they will develop effective control strategies for the SHB that growers can incorporate into their overall pest management programs. Laboratory based bioassays have been used to identify several pesticides that are toxic to the beetle and fungi. The objective of the field trials is to determine whether these chemicals can be utilized as trunk injection agents for the protection of avocado trees. Anyone interested in finding out more about the SHB should go to the web site maintained by Dr. Akif Eskalen at:
http://eskalenlab.ucr.edu/avocado.html
- Author: Ben Faber
At a recent conference in Florida, University of Florida entomologist Daniel Carrillo reported some very disturbing news. There is a fungus/pest complex in Florida avocado and related native laurel species that is similar to a complex found in the California - Shot Hole Borer/Fusarium fungus complex. There it is called Laurel Wilt Disease and is a complex of the ambrosia beetle Xyleborus and the fungus Rafaelea lauricola. It is a fungus/insect complex that causes death in avocado and the relatives of avocado trees. The California complex can cause the death of many tree species, such as sycamore, coast live oak and willow, as well as the decline of avocado. The complex and disease are called Fusarium Dieback here, caused by Euwallacea ambrosia beetle and Fusarium fungus.
What Carrillo and other colleagues have found is that there are similar species to their introduced Rafaelea species of ambrosia beetle that are now attacking live avocado trees. These so-called cryptic species are members of a group of beetles that normally do not attack live trees. These beetles are typically some of the first group of decomposers that go after dead trees. These newly identified insects are morphologically very similar to the original beetle, but are native members of the Florida environment. They too are now attacking live trees. There are now ten potential species of ambrosia beetle that can introduce pathogenic fungi.
To exacerbate the situation, there are other fungi now that have been associated with these beetles that may be similarly as pathogenic as the original fungus. These fungi are genetically distinct from the species causing damage in California. However, this ability of different fungi to adapt to a new invasive beetle species and the ability of other beetle species to pick up the pathogenic fungal species is a scenario that might appear in California.
As the world becomes smaller and more living materials are moved around and they mix, this may be the new reality we are facing.
To read more about the Florida findings check out the article:
http://www.mdpi.com/2075-4450/7/4/55
http://ucanr.edu/sites/socaloakpests/Polyphagous_Shot_Hole_Borer/
- Author: Ben Faber
There's a lot of work being done in Florida on a pest/disease complex like we have here with Shot Hole Borer and Fusarium fungus – Fusarium Dieback. This hits avocados and a lot of other native trees like sycamore, willow and coast live oak. Some of the success in Florida may be applicable and we are working with their researchers there to adapt some of the techniques here.
Redbay ambrosia beetle, Xyleborus glabratus, is a wood-boring pest that has now invaded nine states in the southeastern United States. The beetle's dominant fungal symbiont (Raffaelea lauricola) is phytopathogenic, inducing laurel wilt in trees within the family Lauraceae. Members of the genus Persea are particularly susceptible to the lethal disease, including native redbay (P. borbonia) and swampbay (P. palustris), as well as commercial avocado. Cubeb oil lures are the current standard for detection of X. glabratus, but recently eucalyptol and a 50% α-copaene oil have been identified as additional attractants. This study used a combination of choice bioassays, field cage release-and-recapture assays, and a 12-week field trial to compare efficacy of eucalyptol and copaene lures relative to commercial cubeb lures. In addition, gas chromatography (analyzer for volatiles) was used to quantify emissions from lures field-aged for 12 wk. In field cage assays, copaene lures recaptured a higher percentage of released beetles than cubeb lures. In the field test, cubeb lures captured fewer beetles than copaene lures, and lowest captures were obtained with eucalyptol lures. Both copaene and cubeb lures were effective in attracting X. glabratus for 12 weeks, but field life of eucalyptol lures was only 4 weeks, consistent with the quantification of lure emissions. Results suggest that the 50% α-copaene lure provides the best pest detection currently available for X. glabratus.
So why is this important? For one, it will serve as a tool for more effectively monitoring the presents of the beetle. But more importantly, it might be used as a lure to attract them away from trees. Fool them into going somewhere else, like to die.
- Author: Richard Stouthamer
The recent find of the Kuroshio shot-hole borer in Santa Barbara shows that the beetle is expanding up the coast and it comes on top of the finding earlier this year of a single Kuroshio shot-hole borer in San Luis Obispo. Earlier yet in 2014 a single beetle identified as Euwallacea fornicatus was found by the CDFA monitoring in Santa Cruz county, unfortunately this specimen was only identified using morphological characters and therefore we do not know which of the three cryptic species of the Euwallacea fornicatus species complex we are dealing with for that particular find. After the single find (2016) in San Luis Obispo a several additional traps were placed in the vicinity of the first find but no additional beetles have been caught. In a single location in Irvine KSHB has also been detected last year (2015). Recently, the Kuroshio Shot-hole borer has also been reported in Tijuana Mexico, which is not surprising since the heavily infested Tijuana river valley park in San Diego county is less than 0.6 miles from the border with Tijuana. It is clear from these detections that the KSHB is on the move, just like the PSHB. These long distance moves by the beetles are most likely caused by human transport, and the most likely culprit is wood transported after trees have been cut down or trimmed. Both in San Luis Obispo and the location in Santa Cruz no additional finds have been reported, often the density of insects following an invasion of a new area remains low while the population is expanding and followed by it reaching such levels that they are “suddenly” detected in many locations.
- Author: Sonia Rios
Through DNA sequencing, UCR entomologist have determined that we have two separate invasions of the Polyphagous Shot Hole Borer (PSHB) here in southern California attacking avocados. One invasion in San Diego County and the second in Los Angeles County. UCR entomologist, Richard Stouthamer states the two are different species genetically, but morphologically they are indistinguishable, luckily they both respond to the same lure. They do however carry different fungal symbionts with them.
The species that has invaded San Diego, is now known as the Kuroshio shot hole borer, and is originally found only in two places in the presumed native range: highlands of Taiwan and in Okinawa. The species of beetle that has invaded Los Angeles County will now be known solely as the Polyphagous, its native range is in Vietnam, South China, North Thailand, low lands of Taiwan and Okinawa, this species has also invaded Israel and South Africa.
These Shot Hole Bores are Ambrosia beetles, a specialized group belonging to the family Scolytidae. The shot hole borer was first found in Whittier Narrows in Los Angeles County in 2003. From 2003-2010 the beetle was found on a few ornamental trees, then in 2010 it was the presumptive cause of the death of a large number of box elder street trees in Long Beach. In 2012 the beetle was collected from a backyard avocado tree in South Gate, and from several tree species at local botanical gardens, since then it has been causing havoc in the avocado industry. It now appears to be established in Los Angeles, Orange, San Bernardino and Riverside Counties, and is expanding its range in San Diego County. A single beetle was caught in Santa Cruz County in 2014 and has now been found less than 2 miles from Ventura County line.
The beetle is dark brown to black and tiny, with females between 0.07 and 0.1 inches long, and males even smaller, usually about 0.05 inches long. They differ from the bark beetles in this family in several ways. While bark beetles burrow in the phloem layer or at the juncture of the bark and sapwood, ambrosia beetles bore through the bark and into the sapwood. The shot hole bores are highly specialized and feed on fungi that they cultivate on the walls of the tunnels. In the case of the shot hole bores, they transport the pathogenic fungus, Fusarium euwallacea and both the adults and larvae feed on the fungus.
Shot hole borers attacks hundreds of tree species, but it can only successfully lay its eggs and/or grow the fungi in certain hosts. These include: Avocado, Box elder, California sycamore, Coast live oak, White alder, Japanese maple, and Red willow. Visit http://eskalenlab.ucr.edu/ for the full list. Infection with the fungus can cause a dry or wet and oily dark stain surrounding the entry holes, discolored wood, leaf discoloration and wilting, and dieback of entire branches. In box elders and avocados, a while crusty ring of sugar, also called a “sugar volcano” can be produced. Frass (wood dust left behind from boring) may be produced, but because this can quickly dissolve if it rains it can be missed sometimes. If the barked is scraped away, dark dead tissue may be found around the galleries.
Currently there is no cure for the fungus once the tree has become infected. Chipping and solarizing/tarping infested wood can help to limit the spread of the beetle/fungus. Wood should be chipped to pieces smaller than 1". Don't forget to sterilize pruning tools between uses to avoid spreading the fungus.
If you suspect you may have the shot hole bore in your grove please feel free to contact your local CE Farm Advisor/Specialist.