Utah Pest News Spring 2011

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Tree Killers: Bark Beetles and Their Control

 

  Mountain pine beetle (Dendroctonus ponderosae) parental and larval galleries.
   

Bark beetles are one of the most destructive insect groups in the world.  While most of the damage occurs on forest land, trees in the urban landscape are also at risk of attack by bark beetles.  Bark beetles (subfamily Scolytinae) are a group of small, mostly black to brown beetles that are very difficult to identify.  The largest member of this group is the red turpentine beetle, which reaches a maximum length of 8 mm. Despite their small size, bark beetles can work together to overcome host trees (mass attack), killing them outright, or killing portions of the tree.

The bark beetle life cycle begins when adults emerge from trees infested the previous summer or fall.  “Pioneer” beetles search for new trees by cuing in on host chemicals.  They can detect and select stressed trees which are less able to defend themselves.  They land on the tree and bore a hole through the bark into the phloem of the tree.  If the pioneer beetles survive the host tree’s resin or toxic chemicals, they create a nuptial chamber where mating occurs.  Mated females then tunnel their way through the phloem, creating parental galleries along which they lay their eggs.  Parental galleries vary greatly in shape and size depending on the beetle species, and are often used to aid identification.  As the adult beetles eat and digest phloem tissue, they release pheromones that attract other bark beetles of the same species.  

Eggs hatch, and each individual larva chews its own gallery leading away from the parental gallery.  The larval galleries start out very narrow, and increase in size as the larvae grow, getting larger as they extend out from the parental gallery.  Once the beetles have gone through their immature stages, they create a circular pupal chamber at the end of the larval gallery.  There they pupate into adult beetles.  Eventually, the adults emerge from the tree to start the life cycle again.  Depending on the beetle species and climate, beetles can have from one generation every two years to one to five generations per year.
     
 
 

Trees attacked by bark beetles may have beetles on the bark (top left), fading/yellowing crown (top right), pitch tubes (bottom), and sawdust like material (frass) in the bark crevices or on the ground around the trunk. Any of these signs on backyard trees would warrant an examination by a certified arborist. (All photos courtesy of Bugwood.com)

 
     
In Utah, spruces, pines, elms, and fruit and nut trees are the primary trees affected by bark beetles.  The table on the following page lists the most common Utah bark beetles and their hosts.

Usually, by the time people realize they have bark beetles, it is too late to save the tree.  All control tactics for bark beetles are proactive, and begin by keeping trees stress free.  Trees should be properly planted in the appropriate site, given optimal water and fertilizer, and kept free from injury.  For example, blue spruce should not be planted with drought tolerant plants (xeriscapes) because they will not receive enough water, becoming stressed and more susceptible to attack.  Planting rows of the same species of tree provides easy access to host material for bark beetles as they move from one tree to the next.  Planting a diverse landscape limits pest movement.  Where necessary, use chelated iron, and avoid injury to root systems during construction or other projects.

Bark beetle damage may not be visible until crown dieback occurs, so careful observation of susceptible trees is important.  Dying trees in the vicinity might contain beetles that will emerge and move to susceptible trees.  Trees attacked in late summer or fall may still have green foliage the following spring and early summer.  

Infested trees should be removed and chipped or debarked immediately to prevent emerging adults from killing more trees.  Do not cut down infested trees and keep the firewood near susceptible trees.  A preventive insecticide spray can provide protection to trees that have not been attacked but are near beetle-killed trees.

Preventive insecticides should be applied before beetles emerge in spring or early summer.  Typically, spring applications should occur once temperatures are consistently over 50°F.  At this temperature, many bark beetles continue development under the bark or emerge to find new host trees.  Properly timed and applied insecticides will kill bark beetles as they chew through the insecticide-soaked bark, preventing successful attack.  Once beetles are under the bark, no insecticide treatment will save the tree.  To date, no systemic insecticides have been proven effective at preventing bark beetles from killing trees.   

For pines and spruces, formulations of carbaryl (Sevin SL), bifenthrin (Dragnet, Masterline Plus C, Astro), and permethrin (Onyx) are effective at preventing bark beetle attack.  Carbaryl offers 2 year protection, while the pyrethroid insecticides last 1 year.  Avoid using lawn and garden products with these active ingredients, as they may not be as effective as the products listed above.  For fruit, nut, and other ornamental hardwood trees, active ingredients such as spinosad, endosulfan, chlorpyrifos, carbaryl, and permethrin can effectively prevent successful attack.  Always make sure that the site of application is listed on the insecticide you select.  In most cases, commercial applicators with high pressure sprayers are needed to propel insecticides high up on the main stem and branches.

Beetles collected from infested trees may be sent to the Utah Plant Pest Diagnostic Lab (UPPDL) for identification.  When submitting a bark beetle sample always include host tree information, and a picture of the galleries if possible (remove bark with a hatchet).  Proper identification of bark beetles can lead to precise prevention tactics for other at-risk trees.  

 

-Ryan Davis, Arthropod Diagnostician