Population Dynamics of the Spruce Budworm Choristoneura Fumiferana Author(s): T. Royama Source: Ecological Monographs, Vol. 54, No. 4 (Dec., 1984), pp. 429-462 Published by: Ecological Society of America Stable URL: http://www.jstor.org/stable/1942595 . Accessed: 26/03/2011 12:59 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at . http://www.jstor.org/action/showPublisher?publisherCode=esa. 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Box 4000, Fredericton, New BrunswickE3B 5P7, Canada Abstract.Using the latest observations, experiments, and theoreticalstudies, I have reanalyzed sprucebudworm data fromthe Green River Project, and now proposea newinterpretation of the populationdynamics of the species. Sprucebudworm populations in theProvince of New Brunswickhave been oscillatingmore or less periodicallyfor the last two centuries, the average period being z35 yr.Local populationsover theprovince tend to oscillatein unison,though their amplitudes and meanlevels are notalways the same. The local populationprocess in the sprucebudworm is composedof two majorparts, a basic oscillation,and secondaryfluctuations about this basic oscillation.The basic oscillationis largely determinedby the combined action of severalintrinsic (density-dependent) mortality factors during thethird to sixthlarval instars. These factorsinclude parasitoids and, probably, diseases (e.g., mi- crosporidianinfection), and, most important, an intriguingcomplex of unknown causes, which I term "thefifth agent" (a largenumber of larvae with no clearsymptoms died during the population decline in thelate 1950s). Othermortality factors, including predation, food shortage, weather, and lossesduring the spring and falldispersal of young larvae, are notcauses of the basic, universally occurring oscillation. Becauseof immigrationand emigrationof egg-carryingmoths, the ratio of all eggslaid to the numberof locally emerged moths (the E/M ratio, or theapparent oviposition rate) fluctuates widely fromyear to yearbut independently of thebasic oscillationof densityin thelocal populationsthat werestudied. The fluctuationin thisratio is themain source of the secondary fluctuation in density aboutthe basic oscillation,and is highlycorrelated with the meteorological conditions that govern theimmigration and emigration of moths. The E/Mratio is themajor density-independent component ofbudworm population dynamics. Contraryto commonbelief, there is no evidenceto indicatethat invasions of egg-carrying moths fromother areas upsetthe assumedendemic equilibrium state of a local populationand trigger outbreaks.Moth invasions can onlyaccelerate an increasein a localpopulation to an outbreaklevel, butthis happens only when the population is alreadyin an upswingphase of an oscillationcaused by highsurvival of thefeeding larvae. In otherwords, the "seed" ofan outbreaklies in thesurvival of feedinglarvae in thelocality, and mothinvasions can act onlyas "fertilizers." The weightof evidence is againstthe idea thatan outbreakoccurs in an "epicenter"and spreads to thesurrounding areas through moth dispersal. Rather, the egg-mass survey data in NewBrunswick since1952 favor an alternativeexplanation. If the trough of a populationoscillation in a certainarea stayscomparatively high, as in centralNew Brunswickin the 1960s, or ifthe area is moreheavily invadedby egg-carryingmoths when the populationsin thatarea are in an upswingphase, these populationsmight reach an outbreaklevel slightly ahead of the surrounding populations, all ofwhich are oscillatingin unison. If a local populationoscillates because of theaction of density-dependent factors intrinsic to the local budwormsystem, it mayappear to be difficultto explainwhy many local populationsover a widearea oscillate in unison.However, Moran's (1953) theory shows that density-independent factors (suchas weather)that are correlatedamong localities will bring independently oscillating local pop- ulationsinto synchrony, even if weather itself has no oscillatorytrend. I illustratethis with a simple time-seriesmodel. The samemodel also illustratesa principle behind the fact that outbreaks occurred fairlyregularly in New Brunswickand Quebecduring the past two centuries but rather sporadically in otherregions of eastern Canada. Finally,I reviewthe commonly accepted theory of outbreaks based on thedichotomy of endemic and epidemicequilibrium states and arguethat the theory does not applyto thespruce budworm system. Keywords: Choristoneurafumiferana (Clem.); Green River Project; insect life-table analysis; in- sectoutbreaks; moth dispersal; population dynamics; spruce budworm. INTRODUCTION Almost 20 yr have passed since publication of the budwormpopulation study of the Green River Project. monograph(Morris 1963a) based on the classic spruce During those 20 yr, spruce budworm populations in I Manuscriptreceived 23 December1982; revised and ac- theProvince of New Brunswickdeclined once and sub- cepted15 August1983; final version received 28 November sequentlyhave risento thecurrent outbreak level. Now, 1983. again,we are in themidst of controversy about whether 430 T. ROYAMA Ecological Monographs Vol. 54, No. 4 "to spray or not to spray" insecticidesto protectthe the Baskerville(1976) task-forcereport. In particular, forests.A few years ago, a task forcewas formedto I criticallyexamine the notion of a dichotomyof en- evaluate budwormcontrol alternatives for better forest demic and epidemic states and the alleged role of cli- resource management of the province (Baskerville mate and moth dispersal in the initiationand spread 1976). This task forcerelied heavily on a foresteco- of outbreaks. systemmodel thatwas based primarilyon the Morris (1 963a) monograph.However, thispioneering work is Life cycle 20 yr old and, in many respects,inadequate from a The spruce budworm (Choristoneurafumiferana currentpoint of view. Besides thelack ofadequate data, [Clem.],Lepidoptera: Tortricidae) is univoltinein east- the major problems of the early work were its inap- ern Canada. Moths emergefrom mid-July to earlyAu- propriatetreatment of time-seriesdata and its inade- gust in the Green River area of northwesternNew quate understandingof the concept of densitydepen- Brunswick.Females lay eggs over several days. Egg dence (Royama 1977, 198 la, b). Unfortunately,certain masses are laid on the foliageof conifers,mainly bal- of its interpretationshave continued to be accepted sam fir,Abies balsamea (L.), and severalspruce (Picea) virtuallyunchanged even in the most recent Royal species. Each egg mass contains an average of about Commission reporton the spruce budworm outbreak 20 eggs. Females raised in normal feedingconditions in Newfoundland(Hudak and Raske 1981). lay from 100 to 300 eggs, with an average of :200, The presentpaper reinterpretsthe originallife tables but heavydefoliation caused by a highdensity of larvae fromthe Green River Project,incorporating recent in- can reduce fecundityto one-half. The eggs hatch in formationfrom field observations, laboratory experi- 10 d. Soon afterhatching, the first-instarlarvae dis- ments,and theoreticalstudies. The task is somewhat perse withintree or stand, or even beyond by wind. like restoringa prehistoricanimal fromfragments of Survivinglarvae spin hibernacula withinwhich they fossilized bones. Morris's life tables provide a basic moltto thesecond instar. No feedingoccurs until spring. skeletal structurebut are insufficientfor full restora- Second-instarlarvae overwinterin hibernaculauntil tion; missingpieces have had to come frominference early May. Soon afteremergence, they disperse again or supposition.Whenever I need to deduce indirectly, and settleat feedingsites on host trees.They mine in I argue only qualitatively,steering between the riskof 1-2 yr old needles, or in seed and pollen cones when makinga falseinference and thatof hesitatingto adopt available. Duringthe thirdto sixthinstars, from about a potentiallycorrect hypothesis. earlyJune to earlyJuly, larvae feedon thecurrent-year This paper consists of four major sections. First, I shoots.If current-yearshoots become depleted,the lar- brieflydescribe the life cycle of the spruce budworm vae will feed on older foliage,but this oftenresults