Chap e 5
Fo es Insec Popula ion Dynamics
Je R. Ga nas, Ma hew P. Ay es, and Ma ia J. Lomba de o
5.1 In oduc ion
To he casual obse e , he a h opod auna o empe a e o es s may appea o be
domina ed by mosqui oes o o he bi ing insec s. Close inspec ion o he lea li e ,
he moss a he base o a ee, o lea su aces (o eading his book, in pa icula
his chap e ), quickly e eals ha insec di e si y in many o es ed landscapes can
be conside able. S ill, he deg ee o which insec s in e ac wi h ees, s ands and
landscapes o d i e o es communi y and ecosys em dynamics is a ely ob ious
wi hou in ensi e s udy. In ac , mos species o insec s a e a e mos o he ime.
Occasionally, insec popula ions inc ease o le els ha a e di icul o impossible
o igno e. Such e en s, o en e e ed o as “ou b eaks,” a e cha ac e ized by explosi e
inc eases in abundance (Be yman 1987) which a e o en episodic (Mye s 1988;
Williams e al. 2000) and whe e popula ion g ow h is la gely uncons ained by he
ecological o ces ha had held i in check a lowe densi ies. By i ue o he shee
numbe o indi iduals hey comp ise, ou b eaking popula ions can cause signi ican
damage o o es s, c ops, and o he ecosys ems and can dis up ecosys em se ices.
In he mos d ama ic examples, ou b eaking popula ions can each abundances in he
J. R. Ga nas (B)
Na u al Resou ces and he En i onmen , Uni e si y o New Hampshi e, Du ham, NH, USA
e-mail: Je [email p o ec ed]
Depa men o Zoology and En omology, Uni e si y o P e o ia, P e o ia, Sou h A ica
Fo es y and Ag icul u al Bio echnology Ins i u e (FABI), Uni e si y o P e o ia, P e o ia, Sou h
A ica
M. P. Ay es
Depa men o Biological Sciences, Da mou h College, Hano e , NH, USA
M. J. Lomba de o
Unidade de Xes ion Ambien al E Fo es al Sos ible, Uni e sidade de San iago de Compos ela,
Lugo, Spain
© The Au ho (s) 2023
J. D. Allison e al. (eds.), Fo es En omology and Pa hology,
h ps://doi.o g/10.1007/978-3-031-11553-0_5
115
116 J. R. Ga nas e al.
ens o billions, capable o ans o ming whole landscapes in ways ha can e en be
seen om space o ha wa an mul iple men ions in he Bible, as wi h he in amous
plagues o dese locus s which con inue o his day (Behme 2009).
Ou b eaks a e also common in o es sys ems. Recen ly, an unp eceden ed
ou b eak o he Moun ain pine bee le in he wes e n Uni ed S a es and Canada
p oduced ee mo ali y o e 374,000 km2 om 2000–2020; he ensuing i es, decay
and g ow h losses a e es ima ed o ha e eleased 270 mega ons (M ) o ca bon,
con ibu ing measu ably o global ca bon dioxide pools (Aukema e al. 2006;Ku z
e al. 2008; Reed e al. 2014). Some species expe ience cyclical dynamics wi h
peaks and oughs in abundance ha occu a s ikingly egula in e als anging
om a ew yea s o mul iple decades (Bal ensweile and Fischlin 1988; Tenow e al.
2013; Pu eswa an e al. 2016). O he s expe ience yea ly luc ua ions ha can appea
andom o chao ic and a e much mo e di icul o p edic . In his chap e we o e an
explo a ion o he ac o s ha in luence popula ion cycles and ha lead o ou b eaks
along wi h some o some o he p incipal app oaches o modeling such dynamics.
The ield o popula ion dynamics has deep oo s in en omology. S udies o luc ua-
ions in insec abundance—pa icula ly o o es insec s— ep esen some o he co e
empi ical wo k in he discipline and ha e in o med key heo y in he ield (Royama
1977, 1992; Speigh e al. 1999; Liebhold and Kama a 2000; Abbo and Dwye
2008;P ice
2011; Isae e al. 2017). This is due in pa o he ela i e ease by which
insec s can be moni o ed (ei he di ec ly ia apping o by measu ing de olia ion, o
example). Long ime se ies o popula ion abundance spanning a leas a ew decades
and/o de ailed li e ables ( allies o abundance ac oss li e s ages) a e equi ed o
e ec i ely examine hypo heses ela ing o pa e ns o abundance o e ime. Con em-
po a y abundance es ima es o su icien leng h exis o nume ous insec species,
pa icula ly o pes s o economic impo ance (Tu chin 2003). Dend och onological
( ee ing) s udies ha c oss- e e ence pa e ns o g ow h o xylem damage ac oss
li ing and dead ees (including na u ally p ese ed wood o s uc u al imbe ) allow
esea che s o econs uc abundance ime se ies o e cen u ies (Espe e al. 2007),
hough in e p e a ion o hese da a can be challenging (T o e e al. 2002). Finally,
paleoecological econs uc ion o insec abundance (e.g. using insec head capsules,
wing scales, ass, o damaged plan s p ese ed in bogs o sedimen s) can e en span
millennia ( Sonia e al. 2011; Mon o o Gi ona e al. 2018; Na a o e al. 2018).
5.1.1 Fo es Insec s on Plan a ion T ees
and on E olu iona ily Naï e Hos s
One inc easingly common si ua ion whe e he bi o ous o es insec s can become
se ious economic and/o ecological h ea s co esponds o he ela i ely s mall subse
o species ha espond o a supe -abundan and o en minimally de ended esou ce.
This occu s p ima ily (a) in plan a ion o es y whe e ees a e ypically g own in
high-densi y, low-di e si y monocul u es, and (b) as a consequence o biological
5 Fo es Insec Popula ion Dynamics 117
in asion in na u al o es s whe e na i e ee hos s a e exposed o insec s wi h which
hey ha e no e olu iona y his o y and agains which hey ha e li le capaci y o
de ense. In he i s case, any o he o en globally dis ibu ed insec s colonizing
pine o Eucalyp us plan a ions [e.g. he Eu asian woodwasp (Si ex noc ilio)o he
Red gum le p psyllid (Glycaspis b imblecombei)] could clea ly be labeled pes s
as hey educe yields and nega i ely impac o es plan a ion p o i abili y (Ga nas
e al. 2012; Hu ley e al. 2016). He e, hos ees a e nea ly always a ailable as
new compa men s o e en-aged coho s a e con inuously being plan ed. As such,
he plan a ion en i onmen comp ises a mosaic o di e en ages. This esul s in a
ela i ely s able and enewable esou ce om he pe spec i e o insec s (see Box 5.1
o a de ailed example). I is wo hwhile o no e ha such sus ained, ele a ed pes
densi ies can also occu when bo h ees and insec s a e na i e, such as is he case
wi h oo wee ils in No h Ame ican pine plan a ions (Rieske and Ra a 1990),
ch ysomelid bee les on Eucalyp us in Aus alia (S auss 2001), o pine shoo bee les
in Eu ope (Sch oede 1987) among o he s.
The second case a ises in la ge pa as an unin ended consequence o global
ade whe eby exo ic o ganisms es ablish in o es s o plan a ions wo ldwide. Whe e
a ec ed ees lack a co-e olu iona y his o y wi h newly a i ed insec s, esis ance o
he bi o y can be low o e en absen . This is la gely he si ua ion wi h Ame ican ash
(F axinus spp.) which lacks esis ance o he Eme ald ash bo e (Ag ilus planipennis)
in he Uni ed S a es and Eu ope (He ms and McCullough 2014)o pine(Pinus spp.)
and he Red u pen ine bee le (Dend oc onus alens) in China (Wing ield e al. 2016).
In such examples, insec popula ions can each ex emely high abundances ha o en
esul in widesp ead mo ali y o hos ees. Consequen ly, no el insec pes s o en
de as a e he local ee esou ce a e which hei own popula ions c ash due o he
lack o a ailable hos ma e ial. While i ’s emp ing o imagine ha pes popula ions
may go ex inc once hey ha e ea en all a ailable ees, in p ac ice, popula ions o en
pe sis on low-densi y “escape” ees ( hose ha we e missed by he ini ial wa e o
a ack) o on he s mall ee coho ha su i ed as seeds o seedlings bu become
suscep ible as hey age. In his case, he “ou b eak,” while d ama ic and de as a ing,
is likely o be sho -li ed as i mo es owa d some new equilib ium densi y on he
landscape.
5.1.2 Ou b eak Dynamics as an Eme gen P ope y
o Insec -Hos -Na u al Enemy In e ac ions
While some insec s eme ge as pes s p incipally as a consequence o speci ic ecolog-
ical condi ions (e.g. high hos densi ies/low di e si y o hos and/o a lack o co-
e ol ed esponses as discussed in he p e ious sec ion), an impo an subse o
damaging insec s includes a sui e o species ha a e na u ally p one o ola ile
popula ion dynamics. This ola ili y, cha ac e ized by wide hough o en ema kably
egula luc ua ions in abundance, a ises as a consequence o pa icula aspec s o
118 J. R. Ga nas e al.
hei biology, ecology, o communi y in e ac ions. These so-called “ou b eak species”
a e a ela i ely small, highly non- andom subse o insec s ha may be ei he na i e o
in oduced. Species cha ac e ized by ou b eak dynamics accoun o a highly disp o-
po iona e s ha e o managemen budge s and ha e been he ocus o in ense s udy
ela i e o non-ou b eaking species. Examining he combina ions o en i onmen al
condi ions, li e his o y ai s and communi y in e ac ions ha gi e ise o ou b eak
dynamics, o lack he eo , has p ac ical alue o managemen and con ibu es o basic
unde s anding o biological popula ions. Unde s anding he ea u es o popula ions
ha p omo e ou b eak beha io also helps us o unde s and why mos popula ions do
no display ou b eak dynamics and ins ead a e ela i ely a e and s able. Nume ous
books and jou nal a icles ha e been w i en on he opic, which we b oadly syn hesize
in his chap e . Much o his heo y is oo ed in classical popula ion dynamics.
5.1.3 In oduc ion o Popula ion Dynamics
Many ex books add ess he dynamics o popula ions in g ea dep h and om many
di e en pe spec i es. The ield is ac i e wi h sus ained, ongoing disco e y and
heo e ical de elopmen (Nicholson 1954; Royama 1992;Be yman
1999; Tu chin
2003; Go elli 2008; Vande mee and Goldbe g 2013; Isae e al. 2017). Much o he
concep ual basis o ou cu en unde s anding o how (sel - egula ed) popula ions
beha e is oo ed in he simple equa ion:
N = N0eR (5.1)
whe e is a disc e e numbe o gene a ions and N is he popula ion abundance
gene a ions om an a bi a y s a ing poin ( = 0). Following his logic, N0 is he
“s a ing” abundance a ime ze o. In he inal e m, eR , e is Eule ’s numbe (~2.178)
and R is de ined as he pe capi a popula ion g ow h a e, measu ed as he numbe
o indi iduals in he nex gene a ion o each indi idual in he cu en gene a ion.
The ela ionship be ween N and R is a he co e o why such an appa en ly simple
model can p oduce a wide ange o ecologically plausible dynamics wi h minimal
modi ica ion o i s pa ame e s. Bo h e ms ca y he subsc ip which means ha
hey a y in ime, and as i u ns ou , hey also a y as a unc ion o one ano he . Fo
N his ela ionship is anspa en : abundance is clea ly a unc ion o he g ow h a e
o popula ions (Eq. 5.1; le [blue] a ow in Fig. 5.1). In e es ingly (and c ucially o
he dynamics o popula ions), R is also a unc ion o N (Fig. 5.1). In o he wo ds, he
pe capi a g ow h a e (indi iduals pe indi idual pe uni ime) is dependen on he
numbe (o densi y) o indi iduals in ha popula ion. This eedback be ween densi y
and g ow h a e is a he e y co e o ou unde s anding o popula ion dynamics.
Special cases wi hin his eedback sys em p oduce ou b eak dynamics in a subse o
o es insec s.
Why does R a y wi h popula ion densi y? One majo eason is simply compe-
i ion o esou ces. When popula ions ha e ew indi iduals, esou ces (i.e. ood,
5 Fo es Insec Popula ion Dynamics 119
Fig. 5.1 Concep ual diag am showing eedback be ween popula ion abundance (N) and pe capi a
popula ion g ow h a e (R). The simula ed ime se ies on he bo om le depic s popula ion luc-
ua ion unde simple densi y dependence wi h he inclusion o a s ochas ic componen (ε) ha
app oxima es he exogenous (e.g. clima e o o he abio ic e ec s, impac o gene alis p eda o s)
con ibu ion o in e annual luc ua ions in abundance. The g aph in he bo om igh shows nega i e
densi y dependence while accommoda ing he po en ial o ime-delayed eedbacks (lags) ia he
equa ion R = F(N ,N
-1,…,N -x) + ε
o iposi ion si es, nu ien s, e c.) a e abundan . Thus, each indi idual is mo e likely
o con ibu e maximally o popula ion g ow h, ei he ia inc eased bi h a es, educed
dea h a es o bo h. A he o he ex eme, when N is high, esou ces become limi ing
and he a e age con ibu ion o each indi idual o he nex gene a ion is educed.
Popula ion egula ion ia compe i ion o esou ces is dubbed “ bo om-up” because
he esou ce pool (o en plan s, as in he case o he bi o ous insec s) is usually
depic ed as below he consume pool in isualiza ions o ophic ( ood) py amids,
webs o chains. The e can also be “ op-down” p essu e om na u al enemies (i.e.
p eda o s, pa asi oids o pa hogens) ha si “abo e” he consume pool and espond
o and some imes supp ess p ey densi y. Bo om-up e ec s can also occu ia he
induc ion o plan de enses ha limi esou ce quali y o a ailabili y o plan issues
o he bi o es. These de enses make plan s mo e challenging o less p o i able o ea .
Top-down con ol by na u al enemies as well as bo om-up con ol ia inducible
de enses can in oduce a ime lag (i.e. as p eda o popula ions espond o changes in
p ey densi y o as plan s espond o he bi o e a ack). Such ime lags u n ou o be
120 J. R. Ga nas e al.
e y impo an as hey can esul in p edic able, cyclical luc ua ions in abundance,
which will be discussed in mo e de ail below.
In many popula ions, he ela ionship be ween N and R is oughly linea and
nega i e (Fig. 5.1, bo om igh ). In such cases i is e e ed o as simple densi y
dependence. The e a e a ew impo an hings o ecognize abou he simple densi y
dependen ela ionship, some o which equi e ha we de ine a ew new e ms. Fi s ,
no e ha R can be ei he posi i e, nega i e o ze o ( Fig. 5.1). I is in ui i e ha a
high densi y, popula ion g ow h becomes nega i e. O he wise, popula ions would
end o g ow o e e and become in ini ely abundan . Popula ion g ow h a e mus
likewise be posi i e a low o in e media e densi y—species o which his is no he
case would ha e gone ex inc long ago. Whe e he densi y dependen line c osses
he R = 0 line (dashed line in Fig. 5.1, igh ) is a s able equilib ium poin ; in he
case o simple densi y dependence, his poin has a special name: he equilib ium
abundance, o K. The wo d “s able” when applied o an equilib ium poin is ano he
way o saying i is an a ac o . An a ac o in his con ex is an abundance owa d
which popula ions end, as he e m sugges s. Looking again a Fig. 5.1, his is easy
o isualize—when densi y is below K (N < K), R is posi i e and popula ions g ow;
when N > K, R is nega i e and popula ions sh ink. In he absence o any s ochas ic
a ia ion, popula ions exac ly a K (N = K) would nei he g ow no sh ink, hough
his a ely i e e occu s in na u e o e successi e gene a ions. In ac , anywhe e he
R unc ion c osses he R = 0 line is an equilib ium poin .
Wi h simple (nega i e) densi y dependence, he e is one addi ional pa ame e ha
eme ges om he R unc ion. Despi e he po en ial o be con using, his pa ame e
uses he same le e as he pe capi a popula ion g ow h a e, bu in he lowe case: .
“Li le ,” as i is some imes called, is he in insic g ow h a e o he popula ion.
Li le can be hough o as he maximum pe capi a g ow h a e when ha g ow h
a e is una ec ed by any o he limi a ions imposed by densi y. In o he wo ds, is
he alue o R o he special case when N = 0 (ne e mind ha popula ions wi h
ze o indi iduals a e echnically ex inc ). Thus, can be easily ead as he Y in e cep
o he R by N unc ion.
Figu e 5.2 shows some o he possible ela ionships be ween and K. Many o
hese concep s will ha e ele ance in subsequen sec ions and so a e wo h examining
he e. In all cases, he e a e h ee p ima y aspec s we a e conce ned wi h he: (1)
in insic g ow h a e ( ); (2) equilib ium abundance (K) o he popula ion; and (3)
he s eng h o he densi y dependen ela ionship, which can be unde s ood as he
slope o he line, and calcula ed as— /K.InFig.
5.2a, hal ing om 3.0 o 1.5 while
keeping he slope cons an has he e ec o shi ing K o he le , om 100 o 50.
In Fig. 5.2b, simila changes in while holding K cons an esul s in a signi ican ly
shallowe slope (weake densi y dependence). Finally, changing K om 100 o 50
while main aining a 3.0 leads o a doubling o he slope and he s eng h o densi y
dependence (Fig. 5.2c). O cou se, he e a e many examples whe e and K a e no
igh ly coupled, bu i is use ul o unde s and how each pa ame e in luences model
p edic ions independen ly.
5 Fo es Insec Popula ion Dynamics 121
Fig. 5.2 Th ee g aphical examples depic ing he ela ionship be ween he pe capi a popula ion
g ow h a e (R) and popula ion abundance (N) unde simple (nega i e) densi y dependence using
he Ricke model: N +1 = N e 1− N
K. In sub igu e a, shi ing om K1 o K2 while p ese ing
he slope, o “s eng h,” o he densi y dependen ela ionship has he consequence o educing he
in insic g ow h a e ( ). In b and c, changes in ei he o K while p ese ing he o he esul s in
changes in he densi y dependen slope, wi h consequences o popula ion beha io o ola ili y
5.2 D i e s o Popula ion Vola ili y
How do he models discussed abo e help us o unde s and o p edic how eal popu-
la ions beha e? In la ge pa , he popula ion dynamics o o es insec s (and o he
o ganisms) can be unde s ood wi h h ee ela i ely simple modi ica ions o he pa am-
e e s o Eq. 5.1 o o he na u e o shape o endogenous eedback ha de ines he
ela ionship be ween N and R. Toge he , he inclusion o (1) a ia ion in in insic
g ow h a es; (2) ime-lagged endogenous eedbacks (be ween N and R); and (3)
sc amble compe i ion (in aspeci ic compe i ion de ined by all-o -no hing su i al
122 J. R. Ga nas e al.
o ep oduc ion leading o decele a ing non-linea i y in he R ~ N unc ion) can
p oduce dynamics ha app oxima e hose seen in o es insec s.
5.2.1 Va ia ion in he In insic G ow h Ra e o Popula ions
Up o his poin , we ha e deal only wi h simple (i.e. linea ) nega i e densi y depen-
dence, which is a use ul s a ing place bu is no always a good ma ch wi h na u al
popula ions (Tu chin 2003). By changing he s eng h o densi y dependence ( he
slope o he densi y line, as in Fig. 5.2) we can p oduce a ange o dynamics
ha app oxima es he ange o dynamics seen in na u e (May, 1976). Speci ically,
inc easing he in insic g ow h a e (which as we saw, inc eases he s eepness o
he nega i e densi y dependen unc ion) mo es he dynamic eedback sys em in
he di ec ion o mo e ola ile, complex dynamics. This shi is impo an om a
managemen pe spec i e, as inc eases in ola ili y/complexi y ine i ably esul in
lowe p edic abili y o popula ions (Be yman 1987).
He e we will use he ma hema ical o maliza ions o densi y dependen popula ion
g ow h known as he “Ricke model,” o iginally de eloped o p edic ing ishe ies
s ock (Ricke 1954):
N +1 = N e 1− N
K(5.2)
whe e N +1 is he abundance in he nex imes ep, N is he cu en abundance, K
is he equilib ium abundance (o ca ying capaci y) and is he in insic g ow h a e
o he popula ion. Any model (such as his one) ha conside s changes in popula ion
abundance a egula ime in e als (i.e. , + 1) is e e ed o as a disc e e ime model.
The in e al is a bi a y bu usually akes a alue wi h some biological meaning o he
popula ion in ques ion, o en one yea o insec s ha ep oduce annually. Semi ol ine
( hose ha ake 2 yea s o de elop) o mul i ol ine species ( hose wi h mul iple
gene a ions pe yea ) can be acked annually o by using a longe o sho e ime
s ep as app op ia e. The only equi emen is ha he acking in e al i sel does no
change o e ime. Mos disc e e ime models ha e con inuous ime equi alen s ha
employ calculus o model popula ion abundance e ec i ely “con inuously,” which is
o say o e in ini esimally small imes eps. Disc e e ime models a e ypically oughly
(o p ecisely) equi alen o hei con inuous ime coun e pa s, and o simplici y,
his chap e p esen s only disc e e ime models.
Figu e 5.3 shows i e dis inc ou comes ha a ise simply as a consequence o
a ying , anging om simple con e gence ( o he equilib ium abundance, o K)
h ough damped oscilla ions, simple and complex cycles, o chaos. In his con ex ,
simple cycles e e o he si ua ion whe e popula ions cycle be ween wo abundances,
one on each side o K, while in complex cycles he e a e ou o mo e abundance
alues ( o example, wo high and wo low) ha epea o as long as he models a e
un. The mos ola ile luc ua ions a e cha ac e ized as chao ic dynamics. All he
5 Fo es Insec Popula ion Dynamics 123
models discussed a e en i ely de e minis ic wi h no s ochas ic, o andom, elemen s.
He e chaos does no e e o andomness. Ra he , i e e s o he ac ha luc ua ions
in abundance a e highly dependen on ini ial condi ions whe e e en sligh di e ences
(i.e. o a ew indi iduals) p edic as ly di e en abundances e en a ew ime s eps
in he u u e. Thus, o chao ic sys ems accu a e o ecas ing is nea ly impossible
(Has ings 1993).
Al hough in insic g ow h a es a e o clea impo ance o popula ion dynamics
and species wi h highe in insic alues ha e a g ea e p opensi y owa d apid and
d ama ic changes in abundance, he e is li le suppo o he idea ha popula ion
cycles a e mainly a p oduc o high . To gene a e popula ion cycles o he mechanisms
a e needed—in pa icula , ophic dynamics.
Fig. 5.3 Depic ion o i e dis inc model beha io s (le ) anging om low o high ola ili y (o
high o low p edic abili y) using he Ricke model: N +1 = N e 1− N
K. Co esponding densi y
dependen ela ionships a e s hown in he igh mos sub igu e. No e ha he only di e ence among
he models is he alue o li le (which d i es he s eng h o densi y dependence [nega i e slope]
a cons an K, as in Fig. 5.2). Delayed eedbacks and sc amble compe i ion a e likewise majo
con ibu o s o popula ion ola ili y—see ex
130 J. R. Ga nas e al.
o a ack o whe he EAB will go locally ex inc once mos o he ash ees a e
killed. To a la ge ex en , he a e o ash on he con inen depends on he long-
e m, endemic equilib ia ha es ablish in he a e ma h o in asi e sp ead and
may also be in luenced by he sui e o na i e and in oduced na u al enemies
ha ha e es ablished. Such is he case wi h many in asi e insec s o which
high abundance pos -a i al is mo e e lec i e o ansien dynamics, namely
a “ eeding enzy” on highly s uscep ible ees o geno ypes on he way o a
lowe , s able, long- e m equilib ium.
In he LBM sys em, hos plan quali y appea s o change as a unc ion
o p e ious ca e pilla densi y, making i a delayed eedback. La ch ees
a e deciduous coni e s. T ees de olia ed in a gi en yea p oduce lea es in
subsequen yea s ha a e sho e , less diges ible, and con ain less p o ein.
La ch oliage becomes less nu i ious o LBM popula ions o 1–4 yea s
pos -de olia ion. This has consequences o la al su i al and adul ecun-
di y, which de e mine R in he yea s a e de olia ion. This eedback is c ucial
o he mo h’s ecology as i in oduces 2nd-o de (lagged) dynamics ha can
la gely explain popula ion oscilla ions. In his case, he leng h o lag asso-
cia ed wi h each eedback mechanism was also impo an o he dynamical
beha io o LBM; induced e ec s on ood quali y pe sis o up o ou yea s,
while pa asi ism a es p incipally lag LBM densi ies by wo yea s.
In e es ingly, despi e being a classic example o egula ou b eak cycles,
LBM popula ion beha io ab up ly and inexplicably changed a ound he 1980’s
such ha hese ou b eak cycles ha e disappea ed in ecen yea s. Modeling
e o s using popula ion es ima es om he pas 1,200 yea s (Espe e al. 2007)
clea ly shows how ou b eak epicen e s egula ly shi up and downslope in
esponse o changes in empe a u e (Johnson e al. 2010). Recen wa ming
has shi ed op imal condi ions o LBM popula ion g ow h o he e y edge
o he ange o hos ees, dampening abundance luc ua ions and dis up ing
ecological in e ac ions (i.e. wi h na u al enemies and compe i o s). In ac , his
is among he s onges known examples o a clima e change-d i en collapse in
popula ion beha io (Espe e al. 2007; Johnson e al. 2010).
5.3.3 T ee-Killing Ba k Bee les
Nume ous species o ee-killing ba k bee les also display ou b eak dynamics, bu he
mechanisms appea o be di e en han o cyclical lepidop e a (Kaus ud e al. 2011;
Ko iche a e al. 2012; Weed e al. 2015). The sou he n pine bee le (Dend oc onus
on alis; he ein SPB) is a classic example o an insec ha exhibi s wide luc ua ions
in abundance (Fig. 5.6a). SPB is pa icula ly use ul o explo e since many aspec s
o he biology and ecology o his insec ha e been s udied in g ea de ail, in la ge
5 Fo es Insec Popula ion Dynamics 131
pa because i is a majo pes o highly p oduc i e pine o es s in he sou heas e n
Uni ed S a es (Coulson and Klepzig 2011). In ac , he e a e nume ous species o
ba k bee les (Sub amily Scoly inae, wi hin he wee il amily, Cu culionidae) ha a e
impo an in di e en egions h oughou he wo ld, hough he ou b eak species a e
a small mino i y o he o al scoly ine auna (see Chap e s 10 and 11). We no e ha
ou pe cep ion o “impo ance,” whe he ecological o economic, is s ongly linked
wi h he p opensi y o a species o ou b eak. Insec s wi h popula ions ha inc ease
o ou b eak s a us a e pa icula ly ele an o managemen since hei impac s a e
o en e y di icul o p edic in bo h space and ime and can be locally o egionally
de as a ing o a esou ce. Figu e 5.6a shows he abundance o SPB in es a ions om
1958 o 2015. Though his beha io is no unique among he ba k bee les, SPB is
amous o i s abili y o apidly agg ega e on pine ees in huge numbe s, which
allows hem o exhaus esin de enses and kill heal hy, igo ously g owing ees.
Fig. 5.6 The Sou he n pine bee le is one o he mos damaging o es pes s in he wo ld. This is
due in la ge pa o i s po en ial o ou b eak whe e huge numbe s o bee les mass-a ack o he -
wise heal hy ees, o e coming esin de enses and killing hem, ypically wi hin a ew weeks.
Sub igu es depic in e annual luc ua ions in he abundance o SPB “spo s” (agg ega ions o bee le-
killed ees) in Texas om 1958–2016 (a); an SPB adul (ac ual leng h = 2–4 mm; b); “pi ch
ubes,” o esin de enses p oduced by ees in esponse o a ack (c); ae ial pho o o an ac i e SPB
spo (d); widesp ead SPB damage ha can esul when ou b eaks a e le unmanaged (e). Pho o
c edi s (cou esy o o es y-images.com): (5.6b) UGA0013093: USDA Fo es Se ice, USDA
Fo es Se ice, Bugwood.o g; (5.6c) UGA1929027: Tim Tigne , Vi ginia Depa men o Fo es y,
Bugwood.o g; (5.6d) UGA1510001: USDA Fo es Se ice - Region 8 - Sou he n, USDA Fo es
Se ice, Bugwood.o g; (5.6e) UGA0007064: Richa d Sp iggs, USDA Fo es Se ice, Bugwood.o g
132 J. R. Ga nas e al.
Local ou b eaks o SPB can be obse ed om he ai due o he cha ac e is ic
o ma ion o bee le “spo s,” which a e local agg ega ions o ens o hund eds o dead
o dying pine ees ha appea ed agains a sea o g een ees/needles (Billings and
Wa d 1984). Why is i ha in some o es s in some yea s he e a e housands o
SPB spo s, while in mos o es s in mos yea s he e a e ze o? I appea s ha he
answe lies in some in e es ing popula ion dynamical beha io whe eby SPB popu-
la ions can be egula ed a ound wo di e en equilib ia and swi ch be ween hem a
unp edic able in e als (Ma inson e al. 2012). Mo e speci ically, popula ions can
be egula ed a low, “endemic” le els whe e ins ead o a acking and killing heal hy
ees, hey u ilize p ima ily ligh ning-s uck o o he s essed ees ha a e a low
densi y on he landscape. E en ually, ia chance exogenous e ec s hey exceed a
nume ical escape h eshold (an uns able equilib ium) beyond which hei de e min-
is ic endency is o inc ease o an uppe “epidemic” equilib ium. Figu e 5.7a depic s
his al e na i e s able s a es model as i is unde s ood o SPB (Ma inson e al.
2012; Weed e al. 2017). The g aphical model ep esen s he wo s able equilib ia
as solid black do s and he single uns able equilib ium as an open ci cle (Fig. 5.7a).
Below he escape h eshold, popula ions end o emain nea he lowe , endemic
equilib ium, while abo e i , popula ions end o “escape” he lowe a ac o and
ise o epidemic equilib ium. The ac ion o hese wo a ac o s esul s in a bi-modal
dis ibu ion in abundance whe eby low and high densi ies a e mo e common han
in e media e densi ies, which a e ansi ional and a e (Fig. 5.7b).
This dynamical beha io is sa is ying as i app oxima es obse ed abundance
dis ibu ions. Bu wha o ces c ea e hese wo equilib ia and wha accoun s o
he swi ches be ween hem? The i s ques ion is equi alen o asking wha d i es
nega i e densi y dependence a lowe and hen again a highe abundance alues. In
he case o SPB, i appea s ha he lowe equilib ium is gene a ed by p eda ion by
he cle id bee le, Thanasimus dubius, and compe i ion om o he ba k bee le species
(Ma inson e al. 2012). The egion o posi i e eedback (co esponding o a posi i e
slope in R s. N) gene a es an uns able equilib ium. The equilib ium is e e ed o
as uns able since a he han ac ing as an a ac o in i sel , popula ions below his
densi y end o be d awn owa d he lowe a ac o and abo e i o he highe a ac o .
This abundance alue can also be hough o as an “escape h eshold.” Abo e his
alue he e is a ange o abundances o which SPB ep oduc i e success con inues
o imp o e as he e a e mo e and mo e indi iduals a ailable o join in mass a acks o
hei hos ees. Swi ches be ween al e na i e s able s a es equi e ha he e also be
impo an exogenous (densi y-independen ) e ec s on abundance. In he case o SPB,
his could come, o example, om changes in he abundance o a blues ain ungus
(Ophios oma minus), which is a powe ul an agonis o SPB and whose abundance
wi hin ees seems la gely independen o SPB abundance (Ho s e e e al. 2006;
Weed e al. 2017).
5 Fo es Insec Popula ion Dynamics 133
Fig. 5.7 Hypo hesized dual equilib ium o “al e na e a ac o s” model p oposed o he Sou he n
pine bee le in Ma inson e al. (2012). Sub igu e (a) shows he by N unc ion whe e wo s able
equilib ia (solid poin s) ep esen a ac o s and p edic wo dis inc abundances a ound which
popula ions a e p edic ed o luc ua e. An uns able equilib ium (open ci cle) exis s be ween hem
and ac s as a epello . A equency his og am (b) e eals wo dis inc peaks in expec ed abundances
which co espond concep ually o obse ed bee le popula ion beha io which end o luc ua e
be ween ei he low (endemic) o high (epidemic) abundances
5.3.4 Insec Popula ion Dynamics in Managed Sys ems
In an inc easingly globalized wo ld whe e (a) high-densi y and high-yield p oduc-
ion s ys ems using a hand ul o ee species a e elied upon o mee g owing local,
egional and global demand o ibe and uel; (b) non-na i e pes insec s a e accu-
mula ing in na u al and plan a ion o es s; and (c) clima e is changing, leading o
shi ing geog aphic anges and al e ed dynamics, i is highly likely ha managing
134 J. R. Ga nas e al.
damaging insec s (and pa hogens) will be o inc easing impo ance in yea s o come.
While ou comes o b and c abo e a e gene ally di icul o p edic , s hi s owa d
monocul u e plan a ions yield gene al p edic ions o sho - and long- e m impac s
on insec popula ions. Pe haps mos salien is he ac ha con e sion o ecosys ems
in o monospeci ic p oduc ion o es s ends o inc ease he K o po en ial pes s o he
ee species ha is being p opaga ed (Box 5.1). I he K o an insec species exceeds
economic damage h esholds (one de ini ion o a pes species), hen he e may be need
o ac i e supp ession. Since he na u al endency o popula ions is o g ow owa d
K when popula ions a e below i , i should be expec ed ha con ol e o s will need
o be sus ained inde ini ely. A he same ime, homogeniza ion o plan species and
landscapes in such highly managed o es s also ends o dec ease K o pollina o s,
endange ed species, gene alis na u al enemies and o he elemen s o biodi e si y.
This could lead o an ele a ed ex inc ion isk, especially whe e popula ions exhibi
a endency owa d ex inc ion when abundance alls below a minimum h eshold.
The exis ence o his ex inc ion h eshold, o mo e speci ically he beha io o small
popula ions o end owa d ze o, is called an “Allee” e ec .
Allee e ec s e e o he endency o some popula ions o exhibi a posi i e co e-
la ion be ween abundance (N) and pe capi a g ow h a es a low popula ion densi ies
(Allee 1932). This egion o posi i e densi y dependence (whe e he slope is posi-
i e in he R ~ N unc ion; Fig. 5.8) can a ise ia a sui e o ecological mechanisms
including coope a i e beha io (e.g. he d igilance, co-ope a i e hun ing, o mass
a ack on hos ees), ma e inding, o escape om he nega i e e ec s o inb eeding,
all o which a e pa icula ly ele an when popula ions a e small (Liebhold and Tobin
2008). In each case, highe popula ion densi ies lead o inc eased pe capi a con i-
bu ions o he nex gene a ion. In he case o insec s, aposema ically colo ed indi-
iduals (b igh ly o conspicuously ma ked) expe ience lowe p eda ion a es when
he e a e enough indi iduals o p eda o s o e ec i ely lea n he wa ning signal
(Swo d 1999). Ma e inding can likewise be impo an and may in pa explain he
o e - ep esen a ion o pa henogene ic, emale-only species o aces among in a-
si e popula ions (Kana ek e al. 2015) which e y o en expe ience small popula ion
sizes a he ime o in oduc ion, o sho ly he ea e . In ac , he success ul “Slow
he Sp ead” p og am a ge ing he spongy mo h speci ically akes ad an age o Allee
e ec s, exploi ing he di icul y o indi iduals o loca e ma es in small, sa elli e popu-
la ions along he ad ancing on o he egional in es a ion. In ensi e phe omone
ap moni o ing in hese a eas can de ec incipien popula ions; ae ial o g ound-
based sp aying can hen be used o educe popula ion size o nea o below he Allee
h eshold (open ci cle; Fig. 5.8), below which he na u al endency o each local
popula ion is o go ex inc (Liebhold and Tobin 2008, 2010).
In addi ion o changes in he equilib ium abundance, popula ion beha io is
p edic ed o espond o changes in habi a o communi y. Fo example, dec eases in
he abundance o gene alis na u al enemies can some imes p omo e pes p oblems,
no simply ia he loss o hei supp essi e e ec s, bu by al e ing he eedback sys em
o p oduce popula ion cycles. Dec eases in immedia e nega i e eedbacks ( om
gene alis enemies) could inc ease he ela i e impo ance o delayed nega i e eed-
back ( om specialis enemies), which may cause inc eased popula ion ola ili y and
5 Fo es Insec Popula ion Dynamics 135
Fig. 5.8 Densi y dependen
popula ion g ow h unc ion
showing a egion o posi i e
densi y dependence a low
densi y, o an Allee e ec .
The lowe , uns able
equilib ium (open ci cle)
ep esen s he Allee, o
ex inc ion h eshold.
Popula ions below his
h eshold end owa d ze o
abundance. Popula ions
exceeding he Allee
h eshold a e egula ed in
his case by simple
(nega i e) densi y
dependence a he ca ying
capaci y (K; solid ci cle)
could induce cyclical o ou b eak dynamics (Ruohomäki e al. 2000; Klemola e al.
2009). In e es ingly, he in en ional addi ion o specialis na u al enemies o biolog-
ical con ol could, in p inciple, ha e simila e ec s, inc easing popula ion ola ili y.
Clea empi ical examples o expe imen al demons a ions o his phenomenon a e
lacking, howe e (Mye s 2018).
Finally, he e is an unusually s ong a gumen o conside ing ac i e supp es-
sion when pes popula ions ha e al e na i e s able s a es (low abundance and high
abundance sepa a ed by an uns able equilib ium) such as explained abo e o SPB.
In his case, moni o ing o abundance coupled wi h occasional supp ession when
popula ions i s app oach he escape h eshold can hold po en ial pes s a endemic
le els (whe e hey a e egula ed by na u al o ces) o sus ained pe iods o ime
(Billings 2011). In con as , ac i e supp ession o popula ions wi h na u ally cyclical
dynamics can heo e ically ha e he undesi able e ec o p olonging he ou b eak
phase by in e up ing na u al p ocesses (i.e. op-down p essu e om na u al enemies)
ha would ha e led o declines wi hou human in e en ion.
5.4 Conclusion
Fo es insec s ep esen some o he mos well-s udied o ganisms in he ield o
popula ion ecology, due a leas in pa o hei economic and ecological impo -
ance and amenabili y o moni o ing and/o his o ical econs uc ion o abundance.
The a ailabili y o ime s e ies spanning decades o e en millennia, oge he wi h
comp ehensi e mechanis ic s udies pa icula ly in ou b eaking lepidop e an species,
o m a s ong basis o o ecas ing om which key p inciples ha e been de i ed
and es ed. In his chap e we ha e e iewed some o he basic models o simple
136 J. R. Ga nas e al.
densi y dependen egula ion, expanding on hese ideas o include g ea e ecological
complexi y by inco po a ing lagged and nonlinea eedbacks. We demons a e how o
concep ualize and in eg a e s ochas ic a ia ion in o hese models and discuss a sui e
o plausible model beha io s ha app oxima e eal-wo ld luc ua ions in abundance.
Th ough case s udies and examples, we explo e he dominan ecological d i e s o
popula ion dynamics in o es insec s including in e ac ions wi h hos plan s and espe-
cially specialis na u al enemies ha la gely d i e cyclical dynamics in many o es
lepidop e an species. We conside mul iple equilib ia models o “al e na i e s a e”
models ha e ec i ely app oxima e Sou he n pine bee le dynamics, and explo e he
ole and unc ional o m o posi i e densi y dependence when popula ions a e small
(Allee e ec s). Finally, we conside how popula ion egula ion can be concep ualized
in highly managed sys ems such as high-yield, high-densi y monocul u e plan a ion
se ings as well as in “naï e” ecosys ems, whe e insec s and ees in e ac unde
no el condi ions wi h li le co-e olu iona y his o y, mos o en as a consequence o
biological in asion. While his o e iew e lec s many o he basic ene s o a ield
ha has ma u ed conside ably, accu a e o ecas ing o insec s ac oss ime and space
s ill ep esen s a majo challenge o o es manage s and popula ion ecologis s alike,
especially gi en complex, a iable and changing en i onmen s.
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