The Shothole Borer:  An Ambrosia Beetle of Concern for
Chestnut Orcharding in the Pacific Northwest

H. Omroa Bhagwandin, Jr.
Northwest Chestnuts, 183 Shady Grove, Onalaska, WA 98570

(Originally published in the 93^rd Annual Report of the Northern Nut Growers’ Assn., p. 168-177,
and published here with  permission of the author) 

Bhagwandin, H.O. 1992.  The Shothole Borer:  An Ambrosia Beetle of Concern for Chestnut Orcharding in the Pacific Northwest. 

           The shot hole borer[i], Xyleborus dispar (F.) (Coleoptera:  Scolytidae), is a European ambrosia beetle that was most likely introduced into the U.S. several years prior to 1816, (59) but was first officially reported by Peck in the Massachusetts Agricultural Journal in 1817 as Anisandrus pryi.  It was found in the Northwest in Clarke County, Washington in 1901 and first reported as utilizing chestnut as a host species in the Northwest as early as 1912 (59).  Shothole borer was reported in British Columbia in the early 1920’s (28, 33) and was recorded in California as attacking a variety or deciduous trees in 1942 (31).

      Shothole borer has now been found to be a pest throughout the northwestern states and western Canada (6).  New chestnut plantings are extremely susceptible to attack by this beetle, representing a critical factor in successful orchard establishment.  The purpose of this paper is to highlight important observations and conclusions made by previous research on ambrosia and bark beetles in order to gain a better understanding of how to effectively prevent and/or control infestations of Xyleborus dispar in new and existing chestnut orchards. 

TAXONOMY

       As an insect pest, the bark and timber beetles, Scolytidae, have been extensively studied due to their significant economic destructive capabilities in both forest and agro ecosystems.  The original description of Xyleborus dispar was done in 1792 by Fabricius who most likely gave it the species name based on the disparity between the two sexes.  There have been four noteworthy attempts to classify the Scolytidae in North and Central America (4, 9, 54, 60).  The most recent taxonomic monograph puts the shothole borer in the genus Xyloborus of the tribe Xyloborini and establishes genus synonymy with Anisandrus, Anaeretus and possibly Cyclorhipidion (60).

 DESCRIPTION

       Female:  Length 2.8-3.7 mm, twice as long as wide; Color dark brown to black; Elytra roughly 62.5% of total length, 1.3x longer than wide, striated longitudinally parallel, interstrial space shining, smooth.

      Male:  Length 1.5-2.1 mm; dwarfed, thorax disk and elytra strongly convex; wingless.

Egg:  Length 1mm by 0.06 mm diameter; oblong or oval and clear to white in color.

Larvae:  Length 5mm x 1.45 mm at widest part, white, grublike, legless; slightly sclerotized heads with brown mouth parts; Three instars (60).

Pupae:  Length 4mm x 1.5mm at widest part; Color white.  Males easily distinguishable by their small size relative to females (16, 59).

 BIOLOGY

       The shothole borer, Xyleborus dispar, is within an ecological group of the Scolytidae known as the ambrosia beetles.  Ambrosia beetles are characterized by their utilization of the sapwood of physiologically stressed and recently dead trees for colonization and by the use of symbiotic fungi as their sole food source.  Xyleborus dispar forms an ectosymbiotic relationship with the fungus Ambrosiella hartigii Batra (Fingi Imperfectii) which they transport into the host tree in specialized glands and cultivate on the tunnel walls of their galleries (3, 19, 48).  When growing in the galleries of the host trees, the mycelial form of the fungus converts to a yeast like derivative in response to borer excrements and continued cropping (3, 21, 27, 41).  This yeast like form of the fungus is what is known as ambrosia.  Scolytidae beetles are the primary invaders in dying plant material and thus function as vectors of the decay process in forest ecology.  After the insect has moved on in search of a new host the fungus remains and accelerates the breakdown and degradation of the host tree.

 LIFE HISTORY

       The life cycle of X. dispar is described in several papers (7, 9, 16, 20, 29, 52, 59, 60).  The one distinct feature, which separates it from all the other bark and timber beetles, is that it is the only species in the Scolytidae to exhibit a true diapause (60).  Adult beetles overwinter within the sapwood tunnels they excavate in the limbs and trunks of infested trees.  Males are flightless and spend their entire lives within the galleries or near tunnel entrances where mating occurs.  Female emergence is timed with the beginning months of spring. Once the female has made contact with a suitable host, she crawls around on the trunk searching for a site to begin boring an entrance hole.  The entrance hole is about the diameter of a pencil lead and in small trunks and branches is usually found at the base of leaf and bud nodes.

      Immediately after tunneling into the xylem, fungal gardens are inoculated by the female and a small clutch of eggs is laid.  Clutch size is reported as varying in number being between 1-10 eggs for the primary clutch and up to 45 eggs for subsequent clutches.  Eggs are typically laid at the end of main or lateral galleries and take from two to three weeks to hatch (59).  Larvae take about four weeks for full development and do not contribute to gallery excavation, but rather, move freely about while feeding upon the ambrosia.  The pupal stage lasts another four weeks, making the total life cycle from egg to adult about 10-11 weeks (59).  Tunnels are continually expanded by the parents as the brood size increases or until the moisture content of the host deteriorates.  Host moisture must be available in the tunnels to be able to support the growth of the ambrosia.  Adult females will leave the colony to find new hosts or will re-work an existing host.

      Brood development is a temperature dependent function with development time being proportional to temperature.  Brood development time from larval to adult was accelerated by 52% when temperature was maintained at a constant 21 C (69.8 F) versus a constant 13 C (55.4 F) (1).  Cool temperature appears to impede the growth of newly hatched larvae.  Current flight period and patterns of flight are also influenced by the success of the beetle developmental period of the previous year.  If the previous season was warm it would have been conducive to the accelerated development of the brood and conceivably more fully developed adults will be ready to emerge after the seasonal diapause the following spring (34).  Typically there are two generations per year in Oregon (9).

      Signs of early spring attack include a wilting of newly emerged bud growth and frass tailings from borer excavations.  Frass will usually be evident on infected limbs and alongside, or at the base of trunks.  In late summer galleries usually become considerably honeycombed throughout an infection site to such an extent that trunks or branches become weakened and brittle, breaking easily when bent over, thereby exposing numerous larvae and adults. 

HOST TREES

       The host trees in association with X. dispar include potentially all deciduous fruit and forest trees in its range (6, 59).  Those that have been reported include Acer, Aesculus, Alnus, Betula, Castanea, Crataegus, Corylus, Cydonia, Fagus, Fraxinus, Juglans, Leriodendron, Malus, Platanus, Populus, Prunus, Punica, Pyrus, Quercus, Salix, Vitis (6, 9, 16, 17, 31, 60).  In addition, Xyleborus sp. are reported throughout the world as pests of Carya (22), Camillia (8, 15, 37, 51), Cocos (32) and yam tubers (58).

      The condition of the host tree is a major factor in susceptibility to attack.  It has been consistently reported by several sources (25, 30, 35, 43, 45, 46, 56) that ambrosia beetle attacks are limited primarily to hosts that are physiologically compromised due to some recent injury or stress.  Under epidemic or outbreak proportions, these borers will attack perfectly healthy trees found in the vicinity of suitable hosts – if preferred host material is of limited availability. 

HOST SELECTION AND FLIGHT RESPONSE

       Olfaction is the dominant response factor in flight orientation to a suitable host (23, 24, 25).  Field observations (10, 23, 34, 43) indicate that ambrosia beetles are stimulated to fly upwind and in the direction of the source in response to odor attractions.  In the absence of olfactory stimulus, pioneering beetles are distributed uniformly throughout the stand (45).  Beetles are guided to a chemical stimulus by a type of klinokinesis, (47, 61) in which the attractant source is located by positive orientation to successive pulses from a volatile cloud within its atmospheric range.

      There are two phases of olfactory-guided dispersal:  primary attraction (allocation) and secondary attraction (concentration) (24).  Primary attraction involves the pioneering flight of adult be3etles accurately pinpointing a specific target host within a given stand (7, 10, 11, 18, 46).  The pioneer flight in X. dispar is done by the female since males lack the ability to fly.  The chemical stimuli in this stage of olfactory guidance are volatile breakdown products of anaerobic host tissue metabolism (7, 26), the main component of which is ethanol (7, 36, 52).  Single trees are selected in this early phase of attack based on their current physiological state, with host stress being positively correlated with the production of ethanol.

      An important factor in a host’s resistance to the primary phase of infection in conifers is oleoresin exudation pressure, (OEP), which is directly related to water stress (45).  Low epithelial cell turgor pressure in resin duct lining results in decreased flow in response to a disturbance.  If a beetle encounters heavy resin flow in a pioneer tunnel it will be mechanically flooded out and/or chemically repelled rendering the borer attack effectively unsuccessful (40, 45, 55).  In the coastal Pacific Northwest there is typically no critical soil moisture deficiency that negatively affects the OEP during the early season flight of ambrosia beetles (45).  This is not necessarily true of the summer flight.

      Should a primary attack be successful, species-specific attractant pheromones will then be produced by the pioneering female (7, 12, 40, 45, 47).  This initiates the concentration phase of olfactory-guided flight.  Attractant pheromones have been isolated from the alimentary system (39, 56) and frass of pioneering female beetles (47).  The intensity of secondary attractant is dependent on the concentration of pioneering beetle’s host infestation (47) and/or entirely on the condition of the host (7, 56).  A synergistic influence, associated possibly with ethanol, has been suggest4ed as being responsible for increasing the activity of this insect produced attractant pheromone (7).  Host logs not undergoing active anaerobic metabolism, experimentally baited with pioneer beetles, produced no attraction (7).

      The secondary attractant pheromone is a far stronger olfactory guide relative to primary attractants. It stimulates female beetles to converge en masse upon a host and begin colonization, following a primary attack.  Some species of ambrosia beetles will mass aggregate on vertical objects near established sources of attraction without any apparent regard for the suitability of the object as host material (45).  This suggests that visual orientation aids olfactory guidance (20).  It ahs been shown in field studies with ambrosia beetles that response to secondary attraction can occur within four hours (43).  There are distinct seasonal and diurnal response patterns to secondary attractants and these patterns are influenced by temperature, light and wind speed (7, 18, 47, 48). 

     FLIGHT CAPACITY

      Differences in the capacity of flight response to attractant stimuli between individuals and populations within a brood have been shown to be based on the physiological condition of the beetles rather than due to inherited ability (23).  A very thorough study (49) on the effects of temperature upon the various activities of the Douglas fir beetle, Dendroctonus pseudotsugae (H.) emphasizes temperature as the most important of several environmental factors that influence a given beetle activity.  It has been shown that ambient temperature determines mode of action once the threshold temperature for that action has been reached.

      Absolute limits of activity were laboratory tested (49) and found to be between 0-1 C (32-33.8 F), where there is a reaction to contact or light; and 49 C (120.2 F), when thermal death occurs.  Heat injury resulted in temperatures over 38-39 C (100.4-102.2 F).  At temperatures over 10 C (50 F) beetles exhibited relatively high boring ability.  An increase in temperature elicited increased boring activity correspondingly.  At higher ambient temperatures beetles are able to bore throughout thicker bark in relatively shorter time periods.

      Activity response showed a 6-hour lag when plotted against daily temperatures due to the insulative effective of the host.  Thus, boring activity was greatest in late evening to midnight, with the least activity occurring in the mornings.  Flight activity occurred between a threshold of 10 C (540 F); to the maximum reported flight temperature of 32 C (89.6 F) depending on the species, its population density and the availability of sources of attraction (24, 34, 43, 48).  Ambrosia beetles will fly at cooler temperatures in the presence of sunlight (48).  Spring peak flights occurred at midday with flight ceasing one hour before sunset (47).  No flight occurred in Trypodendron lineatum at night, at temperatures above 29.7 C (86 F), or at heights above 6 ft. when wind velocities exceed 3 m.p.h. (47).  Directed flight from low light to high light intensities was noted and implications for flight from shaded forests to clear cut areas were mentioned (48).

      Relative humidity increased the tolerance and survivability in test beetles (49).  Since relative humidity is effectively 100% inside beetle galleries, only temperatures above 43 C (109 F) are considered injurious.  Surface temperatures on logs exposed to solar radiation for an afternoon commonly reach these temperatures.  This may explain a preference in beetles for attack concentrations on the cooler surface areas of hosts such as underneath a branch node or on the shaded side of a branch or trunk.

      It is important to note that there are two distinct seasonal flights in the Pacific Northwest:  an early spring flight and an August summer flight.  The summer flight coincides with decreased soil moisture when hosts are potentially more drought stressed (45).  Summer flight ability is enhanced by temperatures between 25-32 C (77-89.6 F) (49).  There are two daily summer peaks:  one before and one after noon (47).  These peaks can be determined by monitoring local microclimatic data.

      Flight distances of bark beetles were reported up to 5000-1000m (10-3000 ft) (23).  Flight mill lab tests showed that beetles were physiologically capable of flight for up to 11.27 km (7 mi) in four hours.  This can amount to 24-32 km (15-20 mi) per day for several days in still air with healthy Douglas-fir beetles, D. pseudotsugae.  This implies the virtual impossibility of controlling population outbreaks by silvicultural methods (1).  Relative velocity was recorded to be 115 m/min (4.29 mph) in D. pseudotsugae (49). 

DISCUSSION

       Castanea spp. Seems to be a preferred host of X. dispar in the Pacific Northwest.  Apparently healthy chestnut trees showing no obvious signs of stress are often attacked.  Upon more thorough examination it can often be shown that some of these trees may actually be under duress due to one or several factors including:  transplant shock, winter freeze or sunburn, high water table, poor soil, mechanical injury, graft incompatibility, or some other disease or insect infestation.  The vascular system of Castanea is ring porous, characterized by large diameter vessels, which seal themselves off in response to injury.  This walling off of portions of the vasculature system can result in cambial dieback due to a girdling effect.  This phenomenon may be responsible for rendering thin barked trunks and limbs in chestnut relatively more susceptible to injury than other hosts under similar environmental influences.  As chestnut bark thickens and becomes more woody, increased resistance to border attack may be affected.

      Positive correlations have been made between epidemic outbreaks and periods of rainfall deficiency, catastrophic winds, snows, floods and fires (13, 30, 53).  Differences in length of dispersal (primary fight phase) and the distance beetles will fly to an attractant source have been reported during normal and epidemic population conditions (24).  Under normal conditions pioneer beetles will fly long periods and cover long distances in search of a suitable host.  Under epidemic outbreaks, beetle populations explode into the surrounding hosts in the immediate vicinity without regard for their suitability as compatible hosts.  In mass aggregation like this, beetle populations can overcome even healthy trees.

      The Pacific Northwest has been experiencing reduced rainfall and increased snow pack deficiencies since 1986.  In addition, timber clearcutting has reached record levels over this same time period. The combined effect has had significant impacts on the overall health of host ecosystems.  Such a cycle would encourage populations levels of X. dispar due to decreased host resistance to borer attacks and may contribute to some of the problems currently experienced with young chestnut orchard establishment. 

CONCLUSION

       Based on this literature review, some control methods are offered as suggestions for combating the shothole borer in potential and existing chestnut orchard blocks.  Prevention is by far the best control, but may be impractical given the limits of control on natural climatic cycles and the proximity of uncontrollable attractant sources to an already established orchard.  When possible, considerations for the location of a potential chestnut orchard should take into account both the health and relative proximity of potential beetle attractant sources adjacent to a proposed orchard site.  Avoid planting near unhealthy forests or abandoned orchards and recent clear cuts.  Orientation of the orchard should be with respect to prevailing winds and sources of primary and secondary attractant volatiles.

      Cultural practices should begin with the monitoring of orchards for determination of seasonal flight periods and time of day at which peak flight occurs on a given site.   An understanding of borer reproduction cycles will enhance the ability to predict beetle activity and can contribute to some of the controlling factors affecting potential host stress.  One of the most important potential controllable factors available to the orchardist for influencing host resistance to borer attack may be to increase cell turgor pressure.  High plant cell turgor pressure can be obtained by maintaining adequate soil moisture in the orchard during peak flight periods.  Another controllable factor is the limiting of point source primary attractants within the orchard.  Non-compatible graft unions, nutritionally or otherwise stressed trees and bark damage caused by winter freezing or sun scalding can initiate cambial fermentation and often become some of the most common point sources of primary attractants in chestnut orchards.  Judicious thinning of these and all other non-vigorous and unhealthy growth in the orchard should be done and, along with any prunings, burned.

      Suggestions for future research include the development of resistant cultivars (8, 42), the use of ethanol based (26, 36, 38), and/or pheromone confusion traps (25, 39, 43, 44, 56), the introduction of predator control organisms (14) and the development of biological pesticides (5, 50, 57).  Research also needs to be done to d3eterine proper trap placement for effective control.  A concern with the use of attractant-chemical traps for control of X. dispar is the possibility of inducing epidemic proportion responses within the orchard. 

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