SOME RISKS AND CAUSES OF MORTALITY IN MOUNTAIN PINE BEETLE POPULATIONS: A LONG-TERM ANALYSIS

Res. Ppul. Ecl. (1981) ~, 116--144. SOME RISKS AND CAUSES OF MORTALITY IN MOUNTAIN PINE BEETLE POPULATIONS: A LONG-TERM ANALYSIS Walter E. COLE Intermuntain Frest and Range Experiment Statin, USDA Frest Service, Ogden, Utah 84401, U. S. A. INTRODUCTION Thrugh the years, discussin has persisted abut the value f mrtality factrs as regulatrs and/r cntrlling factrs within ppulatins, particularly insect ppulatins. The pssibility f manipulating mrtality factrs t bilgically cntrl insect ppulatins has had an appeal that has given directin t thught f integrated pest management prpsals. This paper deals with the prbabilities f lss caused by a single mrtality factr acting alne r in cmbinatin with ther factrs. In any ppulatin study, there remains "unknwn" mrtality that is nt readily definable r measurable simply because measurement techniques are inadequate. Previus wrk by COLE (1962, 1973a, 1973b, 1974 and 1975) dealt with measurement and evaluatin f selected mrtality factrs r with effects f these factrs ver a shrt perid f time. The study reprted here deals with knwn factrs ver three infestatin levels fr 13 years. The apprach was t determine the prbability f death, using cmpeting risks analysis f life tables (CHIANG, 1968) t evaluate the effects f these factrs f mrtality--singly and in cmbinatin. In the end, it will be imprtant t nte the variability (r lack f it) in the amunt f mrtality caused by these factrs ver time. DATA SOURCE All knwn r suspected mrtality factrs were recrded by life stages f the muntain pine beetle fr each generatin year between 1964 thrugh 1977, andby tree diameter and stage f infestatin; i.e., preepidemic (1964-70), epidemic (1971-73) and pstepidemic (1974-77). This was dne in cncurrence with the mdel develped by COLE et al. (1976). The infested tree was the sampling unit and, t minimize between-tree variance, trees were stratified by diameter classes; i.e., 23cm d.b.h. and less, 28 t 36 cm d.b.h., and 38 cm d.b.h, and greater. The randm sampling technique described by CARLSON and COLE (1965) fcused n critical within-tree measurements and sampling effrts. The bjective f the within-tree measurement was t characterize mrtality by cause and life stages f the muntain pine beetle. Tw 15. 2 2-cm samples were taken at breast height per tree and the insect ppulatin was recrded as individuals living and dead (by cause f death)within

117 develpmental stages f the beetle. Previus life table wrk indicated that 6bservatins at five develpmental intervals within a generatin were sufficient t detect ppulatin mrtality by cause f death within the beetle's develpmental stages. Pertinent develpmental intervals within the life cycle fr bservatins f mrtality are as fllws: 0. Base ppulatin: The ttal number f eggs laid in starting ppulatin. 1. Late fall: Sample includes eggs and 1st and 2nd instar larvae that will enter winter. ~ 2. Early spring: Sample establishes the number f larvae that survived the winter. 3. Summer: Sample determines the late larval and pupal ppulatins. 4. Late summer: The final cunt f emerging adult ppulatin, btained by caging the sample area. Data were taken frm infestatins n three natinal frests: Wasatch, Tetn, and Targhee. Perids f infestatins by intensity were determined frm the muntain pine beetle mdel (COLE et al., 1976) and spanned years as fllws: Preepidemic (0-10% f trees killed annually)... 7 years (1964-1970) Epidemic (10-45% f trees killed annually)... 3 years (1971-1973) Pstepidemic (0-10% f trees killed annually)... 4 years (1974-1977) A special case ccurred n the Cache Natinal Frest in nrthern Utah. The muntain pine beetle has been active in this particular area frm 1971 t the present time, r lnger. The ppulatin has remained as a "high endemic" level ver this perid f time, as will be discussed later in this paper, and shws sme particularly interesting relatinships between the causes f mrtality and the hst ppulatins. DATA ANALYSES The cmpeting risks analysis (CmANG, 1968) was used in the majrity f analyses because death is nt a repetitive event and is usually attributable t a single cause. Varius risks, hwever, cmpete fr the life f an individual and must be cnsidered in cause-specific mrtality studies. This analysis is well reprted in the literature and was used by this authr in previus publicatins (COLE, 1974, 1975). Specifics f the analysis will nt be described here, except t state a few definitins: Risk f dying--a mrtality factr present in a ppulatin prir t death f an individual within that ppulatin. Cause f dying--a mrtality factr that actually resulted in the death f an individual in that ppulatin. Crude prbability--the prbability f death frm a specific cause in the presence f all ther risks acting in a ppulatin. It is als that mrtality witnessed in the uncntrlled insect ppulatin and that which we measured during sampling fr cnstructin f the life table fr the muntain pine beetle.

118 General prbability--prbability f death (r survival) when the cause f death is nt specified. The abridged chrt life table, in which a generatin f beetles is sampled at particular pints in time, was used thrughut ur study f mrtality factrs within the muntain pine beetle ppulatins. The abridged chrt life table was nt fllwed in its strictest sense because f destructive sampling. Death f the last individual was nt recrded. Instead, the emergence f the adult beetle was equated with the end f life fr that particular chrt. Cnsequently, flight mrtality was nt cnsidered. In additin, we assumed that all individuals within the sample were subject t the same frces f mrtality (risks) and that the survival f ne individual was independent f the survival f any ther grup. By making this assumptin, we avided unnecessary cmplicatins (CHIANG, 1968). RESULTS AND DISCUSSION The basic data fr the cnstructin f life tables and cmpeting risk analysis fr the muntain pine beetle are shwn in Table 1 and graphically in Figure 1. These data were stratified by tree diameter class and by perid f infestatin. Ppulatin numbers shwn in bth table and figure are n a sample basis; i.e., 15.2x15.2 cm f bark area. Prbabilities f survival and life expectatins Beetle ppulatins were greater numerically in the largest diameter tree class. The ppulatin change amng tree diameter classes in the preepidemic stage shwed a slight reversal; i.e., the starting ppulatin was greatest in the 30-cm trees, but survival was greatest in the 38-cm trees. Epidemic ppulatins fllwed expectatin, 140 10 23 cm,,~,,, " " '~ 80 ~, Preepidemic Epidemic Pstepidemic T l I "'~ I I l I 1 2 3 410 1 2 3 4/0 OBSERVATION PERIOD DURING BEETLE OE~:ER;,T:ON YEAR t I r I I 2 3 4 Fig. 1; Muntain pine beetle brd survival by bservatin, by tree diameter class, by stage f infestatin.

119 Z m "6 ~9 ~9 b, e~.~. 0] ~9 ~9 ~ c~.-g

120 c'q t~ ~ 0 r c~ c ct 00 ~ t~ 0 c ~ c~ c~ c5 ~ c~ c' c ~c~ c,i c~ c~ c,i ~ c,l b, =

121 ~0 O~ 9 ~ co LO ~ u~ ~0.=, v

122 with ppulatin density directly related t tree diameter. During the pstepidemic infestatin perid, an inverse trend cmpared t the endemic perid ccurred. 38-cm diameter class trees cntained nt nly the largest starting ppulatins, but shwed the greatest prprtinal mrtality. diameter classes was similar. The The trend between the 23- and 30-cm This culd be since the few 38-cm diameter trees remaining after the epidemic simply culd nt supprt and/r prduce much beetle ppulatin because f excessive drying resulting frm the increased attack density (COLE et al., 1976). The general survival trend within each stage f infestatin fr each diameter class was apprximately f like magnitude with the greatest mrtality ccurring between the late fall and spring perids; i.e., winter kill. The prbability f an individual surviving the entire grwth perid (egg t adult) was determined fr each diameter class within each stage f infestatin (Table 2 and Figure 2), i.e., when the cause f death is nt specified, Pu:Pr {an individual alive at age x~ will survive t age xj}, i~j; i, j=o, 1... The prbability increased Table 2. General prbabilities f an individual surviving the entire grwth perid frm egg t adult stages by tree diameter class and stage f infestatin. Tree diameter class Stage f infestatin 23 cm 30 cm 38 cm Preepidemic 0. 00196 0. 00551 0. 00850 Epidemic 0. 00900 0. 01403 0. 00850 Pstepidemic 0. 00360 0. 00776 0. 00223 O. 014 0.012 O. 010 O, 008 i 0.006 Pstepide mic O. 004.,~\~ Epidemic O. 002 / 23 30 38 TREE D~AMEEERCLASS (CM) Fig. 2. Prbability f any ne egg surviving t the adult stage by tree diameter class ver stage f infestatin.

123 frm the 23- t the 38-cm trees within the preepidemic perid; during the epidemic and pstepidemic perids, the: prbability peaked within the 30-cm diameter class and was apprximately equal in the 23- and 38-cm diameter classes. If we cnsider the habitat and fd supply nly, then these trends are nt unusual and relate well t the expected survival by phlem thickness within diameter class distributins within a stand structure (AMMAN, 1969; COLE and AMMAN, 1969). As the infestatin prgresses, these prbability-f-survival trends reflect the change in tree distributin and characteristics. One might interpret this t mean that if the prbability f survival increases ver diameter class during the preepidemic stage f infestatin then an epidemic is likely t develp. The peaks within the 30-cm diameter class during the epidemic and pstepidemic perids f infestatin prbably ccur because the larger diameter trees have been killed, leaving either fewer trees in each case t cntribute t the survival data r residual trees within this diameter class are slwer grwing and therefre prvide less fd (thinner phlem) fr beetle survival. These assumptins will be carried further as we develp the influence and rle f the mrtality factrs. Life expectatins Life table studies fcus centrally upn life expectatin and survival rates. By cmparing these, we can evaluate the intensity f risks measured during the stages f ppulatin grwth. The rate f survival (r cnversely, mrtality) can mre r less gvern life expectatin. Fr the muntain pine beetle, the ttal life interval frm egg t egg can be assumed t be apprximately 365 days. High mrtality rates can lessen life expectatin, whereas lw mrtality rates culd lengthen life expectatin. Table 3 shws an abridged life table fr determining prprtins f death and survival, life expectatin, and the variances fr bth. Life expectatins fr the muntain pine beetle ppulatins fluctuate smewhat within diameter classes amng stages f infestatins and amng diameter classes within each stage f infestatin, but generally increase frm the smallest t the largest diameter class and peak ver time during the epidemic stage (Figure 3). Exceptins are within bservatin times f brd develpment. Life expectatin within the 30-cm diameter tree class appears t be cnsistent. Less change wuld indicate that the 30-cm diameter class culd well be the carrying habitat fr the muntain pine beetle. A strng psitive trend ver diameter ccurred in the preepidemic stage. This trend disappears in the epidemic and pstepidemic stages with survival being higher in the 30-cm class. Survival peaks during the epidemic stage in all diameter classes. These data suggest that the high survival in the 38-cm trees during the preepidemic stage triggers the epidemic. The increased survival in the 30-cm trees during the epidemic and pstepidemic stages suggests that nce the epidemic starts this diameter class is the mst imprtant cntributr t beetle ppulatin increases. The greatest life expectatins ccur within the epidemic stage f infestatin and

124 0 N ~ 2 ~ ~ " 0) 9 0 ~.,~ L~ ~88 e-, ~a5 00 9 0"~ 0 N~ ~'~ 00 co

:125 v c~ c~ c~ c5,~ :~ ~ ~.~ ~ ~, bo~ 0 v c'q c c O4

126..-.. v = ~ [.~ 0.~ 0,-~ 0% e't &- v ~,.m ~.r.-~ ~J - co

127 1601~ P repi~,mic go ]REF O[AMI:TER CLASS 38 (CM) 0 1 2 3 4 2. ~- Epider'ic _g 1~ x 80 38 t~ 30 0 " Ps!epldernic '2 3L ~0 3e 0 1 2 3 4 OBS[RVA'rlOK TIME OVrR GENE~AT IOL Y~'AR Fig. 3. Life expectatins (days) at each bserved time ver generatin year by diameter class within stage f infestatin. tend t equalize amng diameter classes during the early brd develpmental stages, but shift in favr f the smaller diameter class in the late brd develpment stage. There is a direct reversal between life expectatins during the preepidemic and pstepidemic stages f infestatin. During the preepidemic stage, the greatest life expectatins ccur in brds within the larger diameters; during the pstepidemic stage, they ccur within the 30-cm diameter class. This may be a reflectin f the diameter-phlem distributins within the stand befre and after a muntain pine beetle infestatin r, as recent bservatins indicate, an effect f beetle quality generated in large diameter trees. We cnsider life expectatin (e~) fr discussin purpses because sme interpretatin can be made f its value based n a priri knwledge f the beetle. The increases f life expectatin, when they d ccur, are prbably assciated with decreased attack density f the beetle by diameter frm the preepidemic t the epidemic stage (COLE et al., 1976). During the pstepidemic perid in particular, a minimal number f large diameter trees remain, hwever, and these are usually nt cnducive t brd prductin. Assciated with this lack f large trees is an increase in attack density (COLE et al., 1976). Increased attack density increases the rate f tree drying and beetle mrtality due t this drying. General prbability f survival The general prbability f survival frm ne grwth interval t the next fllws

128 1.0 ~ ~T~I ~ Preepi~mic 11.6. TREE DIAhLrTER CLASS O. 4 38 (C ~,;) v v v ~v23 0 1 2 3 4 z ~ 0.8 Epidemic Z O. 6 < m 0.4 38 2i-/i-~17~7h--/-~ v v v v v23 0 l 2 3 4 1.0 " ic 0.8 (I. 6 0.4 0,2 0 1 2 3 4 OBSERVATION TIME OVER GENERATION year Fig. 4. Prbability f survival in the next interval by tree diameter class during stage f infestatin. the cnfiguratin f life expectatins (Figure 4). During the preepidemic perid f infestatin, the general prbability f survival increases ver diameter class within each brd develpmental perid. The chance f survival shifts slightly in favr f the 30-cm diameter class during the epidemic and cntinues t strengthen in this diameter class during the pstepidemic stage f infestatin ver each brd develpmental perid. Within diameter class, ver stage f infestatin, the prbability f survival generally peaks during the epidemic. This prbability is greater during the pstepidemic stage than during the preepidemic stage in the 23- and 30-cm diameter classes, but the reverse is true within the 38-cm diameter class (Figure 5). This again may be due t the pattern f tree killing during the epidemic, i.e., the beetle prgressively destrys its preferred fd supply (large diameters) ver the life f the infestatin. The data and analyses t this pint supprt the thery that the 30-cm diameter trees are supprting the beetle ppulatin in the main. The 38-cm trees may in fact supply the impetus fr starting epidemics, but, because f their fewer numbers and early eliminatin frm the stand, these trees quickly lse their cmmanding psitin. Cnsequently, the 30-cm diameter trees, which are still mre than sufficient fr beetle ppulatin grwth, carry the bulk f ppulatin grwth thrugh the epidemic and int the pstepidemic stage f infestatin. The beetle prduces l~ss brd in these

129 TREE DIAMFER CLASS 23 c'n 0.8 O. 6 STAGE OF IN~-ES~ATIO',~ O. ~, Pstepidem~c O. 2 ' mic 0 I 2 3 4 Preepidemic 1.0 30cn 7 0.8 N O. ~ Pcs!epiOe mic O. c," i 1 0 38cm 0.8 0.6 O. 4 Ps!epidemTc OBSFRVATION Tl',~" OVER GENERAT[O~ year Fig. 5. Prbability f survival in the next interval by stage f infestatin within tree diameter class. 23-cm trees, prbably thrugh the lack f adequate habitat fr the beetle and increased rate f drying. The chance-f-survival increase in these trees during the peak f the epidemic is prbably due t the greater number f these trees being infested, the reduced attack density, and pssibly t an artifact f ppulatin sampling. Crude prbability f death The general mrtality is the ttal mrtality fr a particular pint in time. The cmpnent prbabilities f death (crude prbabilities) caused by the specific mrtality factrs are additive t and cnstitute the general mrtality. As in mst cases, the greatest cause f mrtality is shwn as "unknwn." This "unknwn" categry accunted fr apprximately 50% f the mrtality that ccurred during any generatin f beetles, within any diameter class f trees, and during any ne stage f infestatin (Figure 6). We believe, hwever, it quite reasnable and prper t assume by reasn f sampling thery, that if this cause was knwn, it wuld be prprtinally distributed amng the ther causes f mrtality in respect t their ccurrence. Thus, in the interpretatin f the fllwing analyses, we will be cncerned with the factrs as they were recrded and evaluated. The crude prbabilities f death due t specific mrtality factrs are shwn in Table 4 and Figure 7. The general prbability f death is the rear prfile in each case (Figure 7). Smthed curves have been drawn thrugh mrtality estimates fr

130 ST,~GE OF INFESTATION I [Pre~ lide mic EpTdemic Pstepidernic ] 1.0-0.9 0.8 - i 0.7-0.6-0.5-0.4 -, ~ E Risks f Mrtality.Unknwn,Pitch If, Drying ii'.temperature U.3-0.2-0. I-- 0- '::, i i 23 30 38 23 30 38 23 30 38 I i A~dpeckers -Path~ens -Medetera Celides petitin Fig. 6. TREE DIAI',~ETER CLASS (C~I) Crude prbability f death by specific mrtality factrs. discrete pints in time t facilitate visual appraisal f mrtality trends ver time. Mrtality read frm these graphs, hwever, is nly pertinent at that particular pint in time. These data cntinue t substantiate previus studies (COLE, 1974, 1975) shwing that temperature (winter-kill) fllwed by drying f phlem in the early summer remain the tw greatest causes f mrtality f muntain pine beetle brds. These factrs generally decreased as diameter increased, and they varied with the stage f infestatin, with drying shwing a steady increase in the 23-cm diameter class. Specific mrtality factrs Individual mrtality factrs (risks) measured were: within cmpetitin, i.e., mr- tality frm crwding f larvae within a single brd r egg gallery; between cm- petitin, i.e., mrtality frm crwding f larvae frm tw r mre separate r different brds r egg galleries; the insect predatrs, Medelera amrichii (Diptera: Dlichp- didae); Thanasimus undatulus and Enclerus sphegeus (Cleptera: Cleridae); the insect parasite, Celides dendrctni (Hymenptera: Bracnidae); wdpeckers; lw winter temperatures; drying f the phlem; pitch; and pathgens. The prbability f death due t a mrtality factr is the prprtin that factr cntributes t the ttal lss frm all mrtality factrs. Interpretatins are in the cntext f prbabiety f death ccurring and nt in numerical ccurrence f the mrtality factr. Within cmpetitin decreased ver diameter during the preepidemic stage f infestatin, fluctuated during the epidemic stage, and decreased with diameter during the pstepidemic stage (Figure 8). Between cmpetitin fllwed smewhat the same

131 ~ O ~ ~ 0 ~ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ~ 0 0 ~ 0 0 C:~ 0 0 O 0 0 t'~ ce~ ~ ~ ~ <:~ 0 ~ r 0 0 C:~ 0 0 0 0 0,'~ (v 0 0 0 ~ ~ "~ r O 0 ~ ~ 0 0 ~ e.

~ 0 0 0 0 0 ~; 0 0 0 0 0 ~ 0 0 0 0 ~ 0 ~ ~ ~ 0 0 00 ~0 0 c;; c~ O d d d d t',- ~ 0 0 0 0 0 0 0 0 84 ~, 8 0 0 0 0 0 0 0 0 0 0 84 0 ~ ~.~ O ~ ~ ~ ~ ~ ~ ~ ~.~. ~., O4,,~ N,.,,. "~ ~ ~ 0 00

133 0 0 0 0 0 0 0 0 0 0 0 ~ O 0 0 0 0 0 0 0 0 ~ O 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 O 0 0 0 0 0 ~ 0J 00 ~ ~ ~ ~ ~ ~ 0 O0 CO,-~ 0.-- ~ = ~ v ~ ~.~ N.r, ~ 0 0

134 "IRE DIAMETER CLASS A PreepJdem~/ i L Epidemk:!~ I Ps epi6emic 012 34 p reepidemic Epidemic Pstepidemic O. 1 -v~" /Y 0.~ 01234 O.O4 0.~ Preepidemic Epidemic J Pstepiclemic / ~s 0 [ 234 ION OBSERVAT ION I IME OVER GENERAl YEAR y Fig. 7. Graphic display f crude prbability f death frm specific mrtality factrs by tree diameter class within stage i infestatin ver bservatin during generatin year.

135 pattern as within cmpetitin (Figure 9). Cmbining the effects f these tw frms f cmpetitin, r crwding, we find sme minr fluctuatins f these patterns during the preepidemic and epidemic stages f infestatin ver diameter, but a definite decrease f mrtality due t these factrs during the pstepidemic stage (Figure 10). Cmpetitin had the least influence within the 38-cm diameter class trees. Mrtality frm bth within and between cmpetitin was the same during the preepidemic and ;~ 23 crn 30 cm 38 c m 0.10 0.08 > 0.06 4 8.~.r.,~' Fig. 8. Crude prbability f death due t within brd cmpetitin by tree diameter class..8 >- O. 06 4 O, 04 < 0.02 m /-" ~" i-".~,~".~r.# Fig. 9. Crude prbability f death due t between brd cmpetitin by tree diameter class.,,~ 23 cm A,~.81-1 \l ~ ~W/,/,d~ /~!y 38cm Fig. 10. Cmbined crude prbability f death frm within and between brd cmpetitin by tree diameter class.

136 epidemic stages and peaked slightly during the pstepidemic stage. Even thugh ppulatins can be expected t be greater within the 38-cm diameter tree class, mrtality frm cmpetitin was prbably ffset by the better habitat and survival cnditins. Cmpetitin ffers the greatest influence within the 23-cm diameter tree class fr the ppsite reasns. The peak mrtality frm cmpetitin ccurred in the 30-cm trees during the epidemic stage and again prbably reflects ppulatin density, which ccurred under these cnditins. Medetera aldrichii shwed a density dependence ver time (Figure 11). Nt nly did Medetera shw preference fr the greater beetle ppulatins by diameter class, 23 cm 38 cm 0.03 4 0.0~ 3.a - 2.~<. 02 L/ y V Fig. 11. Crude prbability f death frm Medetera by tree diameter class. but preference increased by stage f infestatin. This culd be a prprtinal increase since ppulatins were decreasing. Because f this tendency, Medetera was the principal insect predatr/parasite that culd warrant further investigatin as a bilgical cntrl agent f the muntain pine beetle. Further, NAGEL and FITZGERALD (1975) have shwn that M. aldrichii has a rather vracius appetite. SCHMID (1971) attributed a majr share f beetle mrtality t M. aldrichii, but ne shuld remember these past studies refer t numbers f prey cnsumed by the predatr. This current study deals with prbability f lss. Thanasirnus undatulus and Enclerus sphegeus were recrded tgether as Clerids. The prbability f death by these Clerids was extremely minr and generally can be (at least in this case) disregarded as exerting any real influence "n reducing muntain pine beetle ppulatins (Figure 12). Any appreciable prbability f death by Celides dendrctni was restricted t the 23-cm diameter trees and shwed sme increase ver stages f infestatin (Figure 13). This restrictin t the smaller diameter trees is prbably due t the thinner bark--thicker bark restricts egg depsitin by Celides, which has a rather shrt vipsitr. Prbability f death due t wdpeckers generally ccurred in the 23- and 30-cm

137 23 cm 30 cm 38 cm O. lor 4 0.06 3 9 x~,c..c, Fig. 12. Crude prbability f death frm Clerids by tree diameter class. 23 cm 30 cm 38 cm ~- 00,l:.-r-k-)--~4 ~ E.61- /:'----~:'----r:3~..~" V'---W---~ 0.02 ~------7~0 1,~,~" Fig. 13. Crude prbability f death frm Celides dendrctni by tree diameter class. 23cm,/~ 30cm }8cm 0,0r _rx //... 0. 0 8 ] - ~ 4 / /'-/--)--/-~/":---~r'--7/~/.< ' V.6F //'--/7"--P7"3.~,~'I/-7"-/-7'7" L: v-r-/--r/ V s / // Zl Fig. 14. Crude prbability f death frm wdpeckers by tree diameter class. diameter trees and shwed an increase ver stage f infestatin in the 23-cm diameter trees and a decrease in the 30-cm diameter tree classes. Little wdpecker-caused mrtality ccurred in the 38-cm diameter tree class during any stage f the infestatin (Figure 14). This is partly a reflectin f sampling at breast height, i.e., snw depths prevented wdpecker activity during much f the winter. In additin, thick bark tends t discurage wdpeckers, and generally wdpeckers d nt nrmally seek fd inthe lwer ble when fd is readily available higher in a tree. Temperature alne presented the greatest influence f all mrtality factrs measured (Figure 15). The evident peaks f prbability f death during the epidemic stage f infestatin are prbably due t unusually lw temperatures during that stage ratlaer than t the beetle ppulatin level. Prbability f death due t temperature ver diameter class by stage f infestatin, hwever, decreased (as wuld be expected) by diameter class. The exceptin, surprisingly, was the increased prbability by diameter class within the pstepidemic stage. Mrtality frm drying culd have well preceded the mrtality frm temperature in thefiw f events. Als, mrtality frm cld, dry

138 23 cm 30 cm 38 cm ~ O. lo 0.08 0.06 4 A Y 7 0.4 / I /I / / / 0.02 _~ ic,~,~ " ~ ~,~.~..~ Fig. 15. Crude prbability f death frm lw winter temperatures by tree diameter class. 23 c r~ 30cm 38cm 0.'10 >~ 0. 08 0.06 0.04.z 9 ~.~.~,.~,~.~,~".~" Fig. 16; Crude prbability f death frm drying f phlem by tree diameter class. trees is usually greater than frm cld, wet trees. Prbability f death due t drying f the phlem increased ver stage f infestatin within each diameter class, but decreased ver diameter class within stage f infestatin (Figure 16). These bservatins reflect the general trend f phlem and sapwd (an indicatr f tree misture) being thinner in trees remaining after each stage f an infestatin. Within each stage f infestatin, drying in small diameter trees is greater than in large diameter trees. This is cnsistent with bth phlem and sapwd thickness being directly related t tree diameter (AMMAN, 1978). The large increase in drying in the pstepidemic stage is prbably influenced by increased attack and gallery densities that ccur in this stage f infestatin (COLE et al., 1976). Drying f the phlem and the resulting mrtality are als related t wdpecker feeding and

139 muntain pine beetle ppulatin level. At the mst, we experienced nly 1.4 percent chance f an egg surviving t an adult within the 30-cm diameter tree class, and then nly during the epidemic stage f infestatin--all ther stages were less than 1. Des this mean we are dealing with an apprximate difference f 0. 5 prbability f survival between epidemics and endemics? One must remember first, that these prbabilities d nt reflect ppulatin numbers per se. The prbability f any ne individual surviving frm the egg t the adult stage is interesting frm a statistical pint f view and useful as an indicatr f infetatin grwth. Frm an entmlgical pint f view, the percent survival frm egg t adult is prbably mre descriptive. In this case, percent survival varied frm an 11% increase frm preepidemic t epidemic stage in the 23-cm trees, and nly 4% ver the same perid f time in the 30-cm trees. Again, we are dealing with a rather small increase in survival between preepidemics and epidemics. In bth cases, prbability f survival and percent survival, the trends reflected are apprximately the same; nly the magnitude f index has changed. The muntain pine beetle is synchrnized s clsely with stand develpment and grwth that increased fd supply, as a cntributr t ppulatin explsin, prbably far utweighs the influence f ppulatin reductin by bilgical and physical factrs f mrtality. A special case We have maintained ne particular study plt n the Cache Natinal Frest in nrtheastern Utah that may best illustrate sme f these pints f interest. The muntain pine beetle has been active in this particular area fr 7 years r lnger (10-155 f trees killed annually). The ppulatin has remained at a cnstant "high endemic" level ver this perid. If certain factrs (risks) f mrtality were t be density dependent r independent, r if a steady muntain pine beetle ppulatin was t prvide an pprtunity fr these factrs t increase, then certainly this situatin shuld have prvided such an pprtunity. In this case, the prbability f any ne egg reaching the adult stage was 0. 00358 fr the ppulatins within the 23-cm diameter class; 0. 00639 fr the 30-cm diameter class; and 0.00560 fr the 38-cm diameter class. These prbabilities and the percent survival fr the Cache were cmparable t, but greater than, survival recrded fr the pstepidemic ppulatins in the ther plts (10.4% in the 23-cm, 10.5% fr the 30- cm, and 9. 9% fr the 38-cm diameter classes). The crude prbabilities fr each mrtality factr recrded are shwn in Table 5 and Figure 17. Once again, these curves are cmparable t the pstepidemic ppulatin levels fund in the ther study plts. Within cmpetitin mrtality is greatest within the 23-cm diameter tree class, while between cmpetitin mrtality is greatest within the 38-cm diameter tree class. Cmbined, these tw mrtality factrs shw the greatest effect n ppulatin reductin in the smallest and largest diameter classes (Figure 18).

140 23 c m 30 c m ttat O. 10..T~c.,~,. 00~ ~vmm >- 9 00~ T E ~ R A ~ ~moooaec~e ms 9 mat.m, Gel, m. UE~ma O. 04 COEL~E8 00Z CLER~OS 0 I 234 OBSERVAHOfl Fig. 17. Crude prbabilities f death frm specific factrs by tree diameter class fr the special case, Cache Natinal Frest. ~,'~illlin cmpelilin (a) Beh'~een ~.mpetitin (b) bine( 2 3 O. 10 0.% 23 30 38 0 "[R[[ DIAt~:IER CLASS (Cb~) Fig. 18. Crude prbability f death frm within (a), between (b) and cmbined within-between cmpetitin (c) fr a special case, Cache Natinal Frest. Frm the measured lsses caused by parasites, predatrs, pathgens, and pitch, abut the same picture fund in the ther data sets emerged. That is, Clerids had a minr influence n the amunt f mrtality (Figure 19a); Celides activity was greatest in the smallest diameter class and least in the largest diameter class (Figure 19b) ; Medetera shwed sme density dependence with the greatest predatin ccurring in the largest diameter class and least in the smallest (Figure 19c); prbability f death caused by wdpeckers was rather evenly distributed amng diameter classes, peaking in the spring within the 23-cm and in midsummer within the 30- and 38-cm diameter classes (Figure 19d); mrtalities causd by pathgens and pitch were highest

141 ~ ~ ~ O~ " ~ C~ ~ ~O ~ ~ ~ I I I I I ~ ~ c~ ~n ~ ~ ~ c~ ~ ~ ~ c~ ~- ~ ~ ~ ~ CO O OO ~ ~O O = ~.=-.- I I I I I D ~-~ rn D~D ~0

142 =: (a) ' O. 10 3 4 0.08 2 006 0.04 0.02 30 38 ~ (d) (e) (f} 0. l0 3 >- O. 08 2 ~fi" 0.06 23 30 38 0 TREE DIAMETER CLASS (CM) Fig. 19. Crude prbabilities f death frm Clerids (a), Celides (b), Medetera (c), wdpeckers (d), pathgens (e), and pitch (f) fr a special case, Cache Natinal Frest. (b) ~-.86.~~ 0.{)4 v ////' 23 30 38 0. ~,,. ~,2 ~.~.4,~,~,.~ TREE DIAMETER CLASS (CM) Fig. 20. Crude prbabilities f death frm lw winter temperatures (a), drying f phlem (b) and cmbined temperature-drying (c) fr a special case, Cache Natinal Frest. within the 23- and 30-cm diameter classes and during the fall (Figures 19e and if). Temperature, again, had the greatest influence in the smallest diameter class. The influence f drying fllwed a like pattern. Tgether, fr any generatin, these tw mrtality factrs caused the greatest reductin f muntain pine beetle ppulatins (Figures 20a, b, and c). The interpretatin f the prbabilities presented here is that nne f these risks acting in the presence f ther risks ffers much, if any, regulatry influence upn a

143 muntain pine beetle ppulatin. This agrees with REID (1963) that predatrs ccurred in t lw numbers t be cnsidered imprtant cntrl factrs. Cnsequently, because n single risk r cmbinatin f these risks ffer much regulatry influence, the cntentin that muntain pine beetle ppulatins are fd regulated is further strengthened (COLE and AMMAN, 1969). The evidence shws that the muntain pine beetle is fd regulated at ptimum temperature cnditins and temperature regulated at high elevatins where ptimum fd cnditins prevail. Reducing and/r minimizing tree lss t the muntain pine beetle is thus dependent upn manipulating the fd supply by management f the tree (stand) grwth. SUMMARY The interpretatin f the prbabilities presented in this paper is that nne f the cmpeting bilgical risks, acting in the presence f ther risks, ffers much, if any, regulatry influence upn a muntain pine beetle ppulatin. Cnsequently, if n single risk, r cmbinatin f these risks, ffers much help, then the cntentin that muntain pine beetle ppulatins are fd-regulated is nce again strengthened (COLE and AMMAN, 1969). The evidence remains (r cntinues) that the muntain pine beetle is fd-regulated at ptimum temperature cnditins and temperature-regulated at ptimum fd cnditins. Reducing and/r minimizing tree lss t the muntain pine beetle is thus dependent upn manipulating the fd supply r management f the tree (stand) grwth. REFERENCES AMMAN, G.D. (1969) Muntain pine beetle emergence in relatin t depth f ldgeple pine bark. USDA Fr. Serv. Res. Nte INT-96, 8 p. Intermt. Fr. and Range Exp. Stn., Ogden, Utah. AMMAN, G.D. (1978) The bilgy, eclgy and causes f utbreaks f the muntain pine beetle in ldgeple pine frests. In Thery and practice f muntain pine beetle management in ldgeple pine frests, p. 39-53. A.A. BERRYMAN, G.D. AMMAN and R.W. STARK, eds. Cll. Fr., Wildl. Range. Sci., Univ. Idah, Mscw. CARLSON, R.W. and W.E. COLE. (1965) A. technique fr sampling ppulatins f the muntain pine beetle. USDA Fr. Serv. Res. Pap. INT-20, Intermt. Fr. and Range Exp. Stn., Ogden, Utah. CHIANG, C.L. (1968) Intrductin t stchastic prcesses in bistatistics. Inc., N.Y. 313 pp. Jhn Wiley & Sns, COLE, W.E. (1962) The effects f intraspecific cmpetitin within muntain pine beetle brds under labratry cnditins. USDA Fr. Serv. Res. Nte INT-97, Intermt. Fr. and Range Exp. Stn., Ogden, Utah. COLE, W.E. (1973a) Crwding effects amng single-age larvae f the muntain pine beetle, Dendrctnus pndersae (Cleptera: Sclyidae). Envirn. Entml. 2: 285-293. COLE, W.E. (1973b) Interactin between muntain pine beetle and dynamics f ldgeple pine stands. USDA Fr. Serv. Res. Nte INT-170. Intermt. Fr. and Range Exp. Stn., Ogden, Utah.

~144 CL~, W.E. (1974) Cmpeting risks analysis in muntain pine beetle dynamics. Res; Pput. Ecl. 15: 183-192. COLS, W.E. (1975) Interpreting sme mrtality factr interactins within muntain pine beetle brds. Envirn. Entml. 4: 97-102. Ct.~, W.E. and G.D. AMMAN. (1969) Muntain pine beetle infestatins in relatin t tdgeple pine diameters. USDA Fr. Serv. Res. Nte INT-95, Intermt. Fr. and Range Exp. Stn., Ogden, Utah. Ci.~, W.E., G.D. Au~tAs and C.E.J~.Nss};. (1976) Mathematical mdels fr the muntain pine beetle-ldgeple pine interactin. Envirn. Entml. 5: 11-19. NA~sI., W.P. and T. D. FiTz~.am.D. (1975) Medetera aldrichii larval feeding behavir and prey cnsumptin (DIPT: DOLICHOPODODAE). Entmphga 20: 121-127. R~sn, R.W. (1963) Bilgy f muntain pine beetle, Dendrctnus mnticlae Hpkins n the East Ktenai Regin f British Clumbia. III Interactin between the beetle and its hst with emphasis n brd mrtality and survival. Can. Entml. 95: 225-238. Scnu~, J.M.(1971) Medetera aldrichii (Diptera: DOLICHOPODIDAE) in the Black Hills. II Bilgy and densities f the immature stages. Can. Entml. 103: 848-853. W.E. CL~ Purchased by the USDA Frest Service fr Official Use