- 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:
- Author: Mary Lu Arpaia
- Author: David Obenland
Over the last two weeks Mary Lu Arpaia (Extension Specialist, UC Riverside) and David Obenland (Plant Physiologist, USDA-ARS) had the opportunity to visit Israel and spend several days looking at avocados including a visit with Zvi Mendel and Stanley Freeman, the lead researchers in Israel on the Polyphagous Shot Hole Borer (PSHB) and its Fusarium fungal symbiont. This is their report.
So far the beetle is still largely confined to the central coastal region of Israel and the northern Negev. The beetle has also been found in the Upper Galilee at Kibbutz Hagoshrim in avocado and on ornamental trees in other locations in this region which is quite far from the primary infested area. The infested avocado trees have been destroyed but the beetle population already spread outside of the site of the initial infestation. Interestingly, this find was with a grower who packs their avocados in the coastal area. It is assumed that the beetles arrived in the bins originating from the infested area. This is a reminder to California growers and packers that to minimize the spread of pests clean bins are essential. The spread of avocado thrips and persea mite in California is also assumed to have been by bins containing vegetative material.
The Israeli researchers have continued searching for materials that will either control the beetle or the fungus. They have had reasonable success in the lab when they test materials under controlled conditions but application out in the field is not effective. There are no chemical treatments on the horizon that growers can use.
We visited infested avocado orchards in the Hefer Valley and the region southwest of the Carmel mountains (south of Haifa). We visited a Reed orchard which is believed to have been infested approximately 5 to 6 years. Three years ago this grove showed heavy infestation in the entire grove. What we saw on our visit was severe limb dieback, many broken branches scattered on the orchard floor, dropped mature fruit and smaller than normal fruit size for the fruit remaining on the trees. Signs of the beetle boring as evidenced by sugar exudates were easy to find wherever we looked. Dr. Mendel told us that the grower is giving up on this orchard and plans to bulldoze the orchard after harvest. We went on to see several other groves; in all except a 2-year-old orchard it was easy to find limb dieback, fallen fruit and sugar exudate up and down branches. We were told that they do not often find infestation of young groves but when they do, it is usually on the base of the trunk (either rootstock or scion).
We visited a plot with some growers along with Leo Winer (an extension officer) and Udi Gafni (head of the research and development unit of GRANOT) where insecticide applications to infested trees had been made last fall (2012). Unfortunately, signs of continued beetle activity were relatively easy to find. The growers told us about seeing fruit shriveling as the branch dies back. Since substantial fruit drop occurs of both mature and developing there is also an overall drop in productivity as an infestation spreads throughout a grove. Growers are extremely concerned and frustrated that there are no control measures for the beetle. Similar to California, Israel has historically used minimal pesticide sprays. The growers know that in areas already infested that spray applications are key to their continued orchard viability.
Avocado growers in Israel are also seeing problems with Botryosphaeria fungal infections. We visited the northern Negev Desert area where avocados are grown. We went to a large Hass orchard the grower is attempting to control this problem using phosphite injections. We were able to see upper limb dieback and staining on the upper branches.
In the meantime, Drs. Mendel and Freeman are continuing to study beetle biology and the behavior of the Fusarium fungal component. Dr. Mendel is developing a method to raise the beetle in the laboratory. This will be a important breakthrough since it will allow for a better understanding of the beetle life cycle. They know that the female beetle once it flies and seeks a place to burrow has about a 48 hour window to successfully establish itself in the host plant since this is the time period it can survive without feeding. We also learned that the beetle carries the Fusarium spores in its mycangium (a specialized structure at the back of its jaw), rather than hyphae. The larvae and pupa do not have mycangium, only the adults. While the larvae pupate, the Fusarium in the galleries sporulates and the emerging adult as it feeds picks up the Fusarium spores. On a side note, Drs. Mendel and Freeman do not necessarily agree with our use of the name PHSB. They argue that the beetle is monophagous (eats only one kind of food, Fusarium) but uses several tree species as hosts. We will have to see how the final name for the beetle is ultimately settled among the insect taxonomists.
Finally, Dr. Mendel is very worried about the spread of the beetle to native tree species in Israel, especially oaks which are found in many areas throughout the country. The box elder (which also occurs as a landscape tree in California) has been decimated by the PHSB. This is a warning flag for all California residents to take action to safeguard our native oaks, other native species and landscape trees.
These visits reinforced the extreme importance that the California industry must be diligent looking for tree infestation. The industry must work with the landscape industry and forestry service in southern California to understand how fast the infestation is spreading and what hosts are most susceptible. The industry needs to continue funding both applied (surveys, control measures and understanding of the beetle and fungal biology) as well as more basic work such as the origin of the beetle. This latter piece of information may lead to better ideas concerning control measures. Finally California researchers and industry leaders would be well advised to collaborate closely with our Israeli colleagues. They are facing this crisis head on since the PSHB has already spread into commercial groves. From their advanced experience, the California industry can learn much.
A conference on the PSHB and its Fusarium fungal symbiont was held in August 2012 in Riverside. The talks from this meeting are available for review on the website www.avocadosource.com,
. Video of the conference presentations from the public meeting are available for viewing on YouTube. The science portion of the meeting will be posted on YouTube in the forthcoming weeks.
Figures 1,2. ‘Reed’ avocado tree in the Hefer Valley, Israel. Note previous limb breakage and branch dieback due to infestation by the PSHB and its fungal symbiont, Fusarium. (Pictures taken 3/9/2013 by M.L. Arpaia).
Figure 3. Infested ‘Reed’ branch in later stages of decline. The white larvae are termites. Once the branch begins to dieback, termite infestation usually follows. Note the staining of the wood and evidence of the PHSB galleries. (Picture taken 3/9/2013 by M.L. Arpaia)
Figure 4. External symptoms of infestation including sugar exudate and bark darkening reveals wood staining due to Fusarium infection of the woody tissue. (Picture taken 3/9/2013 by M. L. Arpaia)
Figure 5. Small branch infestation and dieback in ‘Hass’. Picture taken at Kibbutz Ma’agan Michael along the central coast of Israel. (Picture taken 3/9/2013 by M. L. Arpaia)
Figure 6. ‘Hass’ avocado tree in the northern Negev affected with Botryosphaeria. Note the dieback and the fact that previously infested wood has been removed. (Picture taken 3/9/2013 by M. L. Arpaia)/h3>