Forest Insect & Disease Leaflet 2 - U.S. Forest Service

Forest Insect& DiseaseLeaflet 2Revised May 2009U.S. Department of Agriculture Forest ServiceMountain Pine BeetleKen Gibson1, Sandy Kegley2, and Barbara Bentz3IntroductionThe mountain pine beetle,Dendroctonus ponderosae Hopkins(Coleoptera: Curculionidae,Scolytinae) is a member of a groupof insects known as bark beetles.Its entire life cycle is spent beneaththe bark of host trees, except whenadults emerge from brood trees andfly in search of new host trees.Mountain pine beetles canreproduce in all species of pinewithin their range. Outbreaks oftendevelop in dense stands of largediameter ( 8 inches), older ( 80years-old) lodgepole pine and densestands of mid-sized (8-12 inchesdiameter) ponderosa pine. Whenweather is favorable for severalconsecutive years, severe outbreakscan occur in high-elevation standsof whitebark and limber pine.Widespread outbreaks developMountain pine beetle-killed whitebark pine in Yellowstoneover several years and can resultNational Park, 2004.in millions of dead trees. PeriodicExtensive tree mortality associatedlosses of high-value, mature sugar andwith a mountain pine beetle outbreakwestern white pine are less widespread,can significantly influence successionalbut also serious.pathways and forest communityUSDA Forest Service, Forest Health Protection, Missoula, MT.USDA Forest Service, Forest Health Protection, Coeur d’Alene, ID.3USDA Forest Service, Rocky Mountain Research Station, Logan, UT.12

composition. Many ecosystemshave become dependent onmountain pine beetle disturbanceevents and subsequent wildfirefor forest renewal. However,outbreaks can also depletecommercial pine forests. Latesuccessional tree species that areless commercially valuable, suchas subalpine fir, often replacemore valuable pioneer species,such as lodgepole pine, that havebeen killed by mountain pinebeetle. Many wildlife speciesmay benefit from mountain pinebeetle outbreaks, including birdsthat feed on developing larvaeand nest in dead trees. However,in the short-term, compositionand distribution of others, suchas elk and deer, may be affectedby a reduction in hiding andthermal cover. Other forestprocesses such as water yield andwildfire extent and severity canalso be affected in the long- andFigure 1. The range of mountain pine beetle generally followsshort-term by tree mortalityits major host pine species throughout western North Americaassociated with mountain pine(shown in green). Current range of mountain pine beetlebeetle outbreaks.Range and Hostsextends from northern Baja California, Mexico and southernArizona, US to central British Columbia, Canada. In recentyears, mountain pine beetle outbreak populations (shown inred) have been found further north into British Columbia andeast into Alberta than had been observed in historical records,including an outbreak in 1985.Mountain pine beetles are nativeto pine forests of western NorthAmerica. They are found fromthe Pacific Coast east to the Black Hills ofSouth Dakota, and from central BritishColumbia and western Alberta to northern Baja California, Mexico (Figure 1).Their habitat ranges from near sea level inBritish Columbia, to 11,000 feet in southern California.tack. Douglas-fir, true firs, western larch,incense-cedar and western hemlock areoccasionally attacked, but because theyare not true hosts, broods rarely develop.Recently, mountain pine beetle has beenfound producing viable brood in spruce.Attacks on trees other than pines usuallyonly occur when nearby pines are heavilyinfested and beetle population levels arehigh.Major hosts of mountain pine beetleinclude lodgepole, ponderosa, westernwhite, sugar, limber and whitebark pines.All pine species within in their range,including Coulter, foxtail, pinyon, andbristlecone pines, can also be infestedand killed. Scotch and Austrian pines,introduced into North America as ornamentals, Christmas trees and/or forestplantations, are also susceptible to at-Evidence of InfestationMountain pine beetle typically initiates attacks on the lower 15 feet of a treebole, although the crown of large trees,particularly sugar pine, may be attacked2

first. During outbreaks, a tree is generally“mass-attacked” and killed by girdling ofthe phloem (i.e., layers of cells just insidethe bark that transport photosynthatewithin the tree) by beetles of a single generation. When population levels are low,partial attacks around the bole’s circumference referred to as ”strip attacks,” may result in two or more generations of beetlesbefore the tree is killed. Strip–attackedtrees may survive for many years.during periods of drought when trees maybe stressed and capable of producing littlepitch.Needles on successfully infested treesbegin changing color several months to aSigns of a mountain pine beetle attack include “pitch tubes” which are made whenfemale beetles bore into the tree’s bole. Onsuccessfully attacked trees, pitch tubes arecream to dark-red masses of resin mixedwith phloem boring dust, and are aboutone-fourth to one-half inch in diameter.When beetles are not present in sufficientnumbers to overcome a tree’s defenses,enough resin is produced at the attack siteto “pitch out” beetles as they attempt tobore into the inner bark. Pitch tubes onunsuccessfully attacked trees are larger,three-fourths to one-inch in diameter,generally generally lighter in color, andwidely scattered over the trunk (Figure 2).Figure 3: Pitch tubes and boring dust—indicative of a successful mountain pine beetleattack in lodgepole pine.In addition to pitch tubes, successfullyinfested trees will have dry, reddish-brownboring dust, similar to fine sawdust, inbark crevices along the tree bole andaround the tree base (Figure 3). Infestedtrees may also have boring dust but noapparent pitch tubes. These trees, referredto as “blind attacks,” are more commonFigure 2: Unsuccessful mountain pine beetleattack—commonly called a “pitchout.”3Figure 4. Fading mountain pine beetle-killedlodgepole pines. Trees generally fade 8-10months after being attacked.

whitebark pine. These signs, plus resultingpiles of bark flakes around a tree base, canalso be evidence of mountain pine beetleinfestation.Life HistoryBeetles develop through four stages: egg,larva, pupa, and adult. Except for a fewdays during the summer, when adultsemerge from brood trees and fly to attacknew host trees, all life stages are spent beneath the bark.Figure 5. Blue-stained sapwood of a mountainpine beetle-killed tree.year after attack. In unusually dry years,foliage may begin to fade within a fewweeks. Needles change from green to yellowish green, then sorrel, red, and finallyrusty brown (Figure 4, see page 3). Fadingoften begins in the lower crown and progresses upward in lodgepole pine, but mayvary somewhat with other hosts. In largediameter, tall sugar pines, for example,upper crown fading may be the first signof an infestation.Temperature has a significant influence onmountain pine beetle development andsurvival. Populations across the beetle’swide range have adapted to local temperature regimes, resulting in variability in thetime required to complete a generationas well as timing of individual life-stagedevelopment. In low elevation standsand in warm years, mountain pine beetlerequire one year to complete a generation.At higher elevations, where summers aretypically cooler, life cycles may vary fromone to two years.Mountain pine beetle is associated withseveral fungal species including Ophiostoma montium and Grosmannia clavigera.Attacking beetles carry spores of the fungion their bodies and in specialized structures called mycangia. Fungi are inoculated into the tree by adult beetles, and developing larvae and new adults obtain vitalnutrients by feeding on fungal structuresthat spread throughout the phloem andsapwood. In addition to providing nutrients for developing brood, fungi can alsoalter patterns of water and resin flow in thetree, aiding in tree death. One to severalmonths after the tree is infested, the sapwood discolors to a bluish tint caused bythese “bluestaining” fungi (Figure 5).Female beetles initiate attacks. During theprocess of chewing into the inner barkand phloem, aggregating pheromones arereleased attracting hundreds of male andfemale beetles to the same tree within afew days. More attacking beetles producemore attractant pheromones, resulting in a mass attack that overcomes thetree’s defenses. Male beetles also producepheromones, including anti-aggregatingpheromones that help regulate the numberof attacks on an individual tree to preventovercrowding. This dynamic combinationof pheromones also results in attacks onadjacent live trees, producing groups ofdead trees across a landscape.Woodpeckers often feed on developingor overwintering larvae beneath the bark,and in doing so can make holes in thickbarked trees such as ponderosa pine, ormay completely remove the bark fromthinned bark trees such as lodgepole andMating occurs under the bark, after whichfemales construct straight, vertical egggalleries. Packed with boring dust, thesegalleries are mostly in the phloem, orinner bark, although they often slightly4

score the sapwood. Galleries may rangefrom 4-48 inches in length, depending onattack density, but average about 10 inches(Figure 6).and attack new trees or merely extend egggalleries in the same tree where they overwintered.Larvae are legless white grubs with brownheads that feed in the phloem, constructing galleries that extend at right anglesto the egg gallery. Development throughfour larval stages (instars) is temperaturedependent and therefore highly variablefrom year to year and site to site. Thefinal larval instar excavates an oval cellwhere pupation occurs and the new adultis formed. New, immature adults spend aperiod of time maturation feeding on fungal spores and associated tree tissue beforeemerging. When several feeding chamberscoalesce, adults may be found in groups,and one or more beetles often emergefrom the same exit hole. Within one ortwo days of emerging, adults seek out andattack new trees, to resume the yearly cycle(Figures 8, 9, 10).Tiny, pearl-white eggs are laid in nichesalong both sides of egg galleries. Timingof egg laying is dependent on temperatureand geographic location, but typically occurs from late June to late August. Eggshatch in 10-14 days, although they maytake longer in cooler weather (Figure 7).Eggs may also be laid in late spring byfemales that survived winter. Survivingfemales and males may either re-emergeFactors Affecting OutbreaksMountain pine beetle populations canexist at low levels in pine ecosystems fordecades infesting and killing stressedtrees, resulting in scattered and often inFigure 6. Mountain pine beetle eggsignificant levels of treeand larval galleries.mortality. Eruption to anoutbreak population andsubsequent widespreadtree mortality are dependent on a number of factors, including favorableclimatic and stand condiFigure 8. Mountain pine beetlelarvae in feeding galleries.Figure 7. Mountain pine beetleadult and eggs in egg gallery.Figure 9. Larvae transform topupae within cells constructedat end of feeding gallery.5Figure 10. “Callow” orimmature adults darken beforeemerging to complete life cycle.