ANRV363-EN54-11 ARI 27 August 2008 20:44
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Bionomics of Bagworms
(Lepidoptera: Psychidae)
?
Marc Rhainds,
1
Donald R. Davis,
2
and Peter W. Price
3
1
Department of Entomology, Purdue University, West Lafayette, Indiana, 47901;
email: mrhainds@purdue.edu
2
Department of Entomology, Smithsonian Institution, Washington D.C., 20013-7012;
email: davisd@si.edu
3
Department of Biological Sciences, Northern Arizona University, Flagstaff, Arizona,
86011-5640; email: peter.price@nau.edu
Annu. Rev. Entomol. 2009. 54:209?26
The Annual Review of Entomology is online at
ento.annualreviews.org
This article?s doi:
10.1146/annurev.ento.54.110807.090448
Copyright c? 2009 by Annual Reviews.
All rights reserved
0066-4170/09/0107-0209$20.00
?
The U.S. Government has the right to retain a
nonexclusive, royalty-free license in and to any
copyright covering this paper.
Key Words
bottom-up effects, flightlessness, mating failure, parthenogeny,
phylogenetic constraint hypothesis, protogyny
Abstract
The bagworm family (Lepidoptera: Psychidae) includes approximately
1000 species, all of which complete larval development within a self-
enclosing bag. The family is remarkable in that female aptery occurs in
over half of the known species and within 9 of the 10 currently recog-
nized subfamilies. In the more derived subfamilies, several life-history
traits are associated with eruptive population dynamics, e.g., neoteny
of females, high fecundity, dispersal on silken threads, and high level
of polyphagy. Other salient features shared by many species include
a short embryonic period, developmental synchrony, sexual segrega-
tion of pupation sites, short longevity of adults, male-biased sex ratio,
sexual dimorphism, protogyny, parthenogenesis, and oviposition in the
pupal case. The unusual mating behavior of bagworms, characterized
by an earlier emergence of females than males and a high proportion
of females that do not mate as adults, challenges conventional wisdom
regarding the evolution of mating systems.
209
ANRV363-EN54-11 ARI 27 August 2008 20:44
Psychidae: one of five
tineoid families of
Lepidoptera, partially
characterized by
case-bearing larvae
(bagworms)
Tineoidea: the most
basal superfamily
within the Lepidoptera
clade Ditrysia
Ditrysia: the largest,
most recently derived
clade of Lepidoptera
characterized
primarily by the
presence of two female
genital openings
INTRODUCTION
The bagworm family (Lepidoptera: Psychidae)
includes approximately 1000 described species
and 300 genera distributed worldwide (48, 94),
most of which share an unusual life history (30,
39, 63). All existing reviews of bagworms pre-
date the 1970s and are limited in scope, focus-
ing on taxonomic aspects or on species occur-
ring in restricted geographic regions (14, 20,
21, 29, 41, 56, 59, 109). Understanding the biol-
ogy and ecology of bagworms is important from
an applied perspective, because several species
are economic pests of cultivated crops, espe-
cially in tropical regions (8, 59). From a fun-
damental perspective, bagworms may serve as
model systems for studying the principles of
population dynamics (99) and for understand-
ing life-history strategy related to intraspecific
variation of reproductive success (85). The Psy-
chidae represent the only lepidopteran family
with a large proportion of species with flightless
females (94), thereby providing a ground plan
for an evolutionary biology of eruptive popula-
tion dynamics (79). In this article, we review the
literature on bagworms, focusing on aspects re-
lated to their taxonomy, life history, phenology
and population dynamics, and provide a sum-
mary of life-history traits for the 18 best-studied
species of the Psychidae (Table 1).
SYSTEMATIC REVIEW
The Psychidae is one of five families of moths
composing the basal superfamily (Tineoidea)
within the most successful and recently evolved
clade (Ditrysia) of Lepidoptera (28). Adults are
small- to medium-sized moths, with forewings
ranging from 4 to 28 mm in length. Males
are always fully winged; the females are either
fully winged, brachypterous, apterous, or ver-
miform (with all body appendages vestigial or
lost). The larvae construct portable cases, as do
genera in at least 10 other families of Lepi-
doptera. The morphology of the larval stage
is conservative and exhibits diagnostic features
that indicate the monophyly of the family, in-
cluding (a) four pairs of epipharyngeal setae;
(b) pronotum expanded laterally and fused to
the lateral pinnaculum to include the spiracle
and all three prespiracular setae; and (c) abdom-
inal crochets on segments 3?6 arranged in a
uniordinal, lateral penellipse (23). The pupa is
less diagnostic, with major morphological dis-
tinctions according to phylogenetic hierarchy,
as well as between males and females in derived
subfamilies. Particularly noteworthy are pupal
specializations involving the dorsal abdominal
spines. The plesiomorphic condition present in
the more basal subfamilies Naryciinae and Tale-
porinae consists of an anterior, transverse patch
of 3?20 scattered rows of short, stout spines
variably present on abdominal terga 3?8 (males)
or 3?7 (females). Pupae in derived subfamilies
typically possess a single anterior row of short,
stout, caudally directed spines variably present
on terga 3?8 and a single posterior row of slen-
der, recurved spines variably present on terga
2?8 (22, 24, 73, 96).
Determining the monophyly of the family
on the basis of adult morphology is difficult be-
cause of a broad range of morphological vari-
ation. However, molecular evidence is begin-
ning to emerge that confirms the monophyly
of the Psychidae as currently recognized (115).
One important synapomorphy for the Psychi-
dae, the presence of fused metathoracic fur-
cal bridges, is shared with the sister group Ar-
rhenophanidae (26, 91). The female psychid
has evolved a greater array of morphological
specializations, especially involving appendage
reductions, than any other family in the Lepi-
doptera. This is particularly evident when the
broad range of these specializations is applied
to the subfamily classification of the Psychidae
(Table 1). Assessing the family relationships of
psychid species with fully winged females was
a consistent problem for early lepidopterists,
who often assigned such taxa to Tineidae or
Yponomeutoidea (24). As a result, previous at-
tempts to classify the family have varied from
the recognition of as many as 10 families (109)
to as few as two subfamilies (59), or a superfi-
cial division into two paraphyletic families: Mi-
cropsychidae ( = Micropsychiniidae) to include
tineid-like forms and the ?true? Psychidae, or
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Table 1 Life-history traits of 18 species of bagworms
Species Distribution
Length of
female
(mm)
a
Development
time (day)
b
Family of
host plants
(number) Fecundity
a
Reference(s)
c
Psychinae
Psyche viciella Palearctic 13 730 7 225 (59)
Psyche casta Palearctic 7 365?730 10 115 (11, 59, 109)
Metisinae
Metisa plana Paleotropical 12 69?120 7 130 (8, 57, 85?87,
104)
Pteroma pendula Paleotropical 8 80 19 81 (57, 60, 104)
Pteroma plagiophleps Paleotropical 6 40?88 22 174 (52, 53, 71)
Oiketicinae
Apterona helix Palearctic 6 365 24 43 (11, 21, 59,
113)
Canephora unicolor Palearctic 23 365?730 8 450 (60)
Pachytelia villosella Palearctic 18 365?730 10 375 (44, 109)
Sterrhopterix fusca Palearctic 13 365?730 9 180 (59)
Thyridopteryx ephemeraeformis Nearctic 24 365 50 792 (4, 7, 21, 56,
58, 99, 112,
114)
Eumeta variegata Paleotropical 20 365 32 3000 (35, 57, 76,
108, 116)
Eumeta crameri Paleotropical 14 84?365 31 628 (1, 3, 62, 67,
106)
Eumeta moddermanni Paleotropical 20 365 9 3270 (42, 59)
Chaliopsis junodi Paleotropical 20 110?365 16 1228 (29, 41)
Hyalarcta huebneri Paleotropical 18 303 16 1200 (45, 49)
Mahasena corbetti Paleotropical 31 124 25 2009 (57, 104)
Cryptothelea surinamensis Neotropical 13 168 8 875 (11, 21, 22)
Oiketicus kirbyi Neotropical 34 212?288 40 5752 (20, 21, 31, 86,
102)
a
Values were averaged across different studies.
b
Range of values observed in different studies.
c
In addition to the references cited above, the HOSTS web database was used to assess the family of host plants of the different species of bagworms
(92). Families of host plants for different species of bagworms are listed in Supplemental Table 1 (follow the Supplemental Material link from the
Annual Reviews home page at http://www.annualreviews.org).
Bombyx-like forms (33, 34). Although the first
modern character analysis of psychid genera in-
volved only the Palearctic fauna (96), it repre-
sented an improvement over previous classifi-
cations and provided a basis for hypothesizing
suprageneric relationships. Utilizing 29 almost
exclusively morphological characters, these au-
thors group 77 Palearctic genera phylogeneti-
cally, without any formal cladistic analysis, into
17 tribes and eight subfamilies. Other subfam-
ilies currently recognized include the Pseu-
darbelinae (new status; 28, 91) and the Scori-
odytinae (Table 2) (40). The 10 subfamilies
currently recognized within the Psychidae may
be characterized in part by the broad range of
morphological and behavioral plasticity present
in the adult female (Table 2). Significant
behavioral modifications appear within the
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ANRV363-EN54-11 ARI 27 August 2008 20:44
Table 2 Summary of the subfamilies of Psychidae and their major morphological/behavioral characteristics as typified by
the adult female and male pupa
Adult female Male pupa
Subfamily Wings Legs
Labial palpi:
segments Ocelli
Female
emergence
behavior
a
Location for
copulation
(to bag)
Rows of abdominal
spines
Naryciinae + or ? + 3, R, ? + or ? Type 1 Distant or on bag Anterior: multiple
Posterior: 0
Taleporiinae + or ? + 3, R, ? + or ? Type 1 Distant or on bag Anterior: multiple
Posterior: 0
Pseudarbelinae + + 3 + ? ? Anterior: ?
Posterior: ?
Scoriodytinae ? R 1 (males have 3) ? Type 1 On bag Anterior: multiple
Posterior: 0
Placodominae + or ? + 3, R ? ? Distant or on bag Anterior: ?
Posterior: ?
Typhoniinae +,R,? + 3, R, ? ? Type 2 Distant Anterior: 1
Posterior: 1
Psychinae ? + Ror? ? Type 3 On bag Anterior: 1
Posterior: 1
Epichnopteriginae ? +,R,? ? ? Type 4 Within bag Anterior: 1
Posterior: 1
Metisinae ? Ror? ? ? Type 4 Within bag Anterior: 1
Posterior: 1
Oiketicinae ? Ror? ? ? Type 5 Within bag and/
or in pupal case
Anterior: 1
Posterior: 1
a
Type 1: female emerges from bag with pupal exuviae extruded; Type 2: female emerges without exuviae extruded and usually leaves bag; Type 3: female
emerges without exuviae but remains on bag; Type 4: female emerges only partially from bag; Type 5: female remains inside bag.
Table modified from Reference 96.
Abbreviations: +, present; ?, absent; R, reduced.
family as morphological specializations have
evolved.
Our current knowledge of psychid system-
atics is based largely on the better-studied Ho-
larctic fauna. Because a greater proportion of
pantropical taxa than taxa in temperate regions
exhibit generalized, plesiomorphic morphol-
ogy (e.g., less reduced mouthparts, fully winged
females), the Psychidae most likely arose first in
tropical regions. Ongoing investigations on the
American Psychidae (25) reveal that approxi-
mately 55% of Neotropical Psychidae possesses
generalized females with no wing reduction,
80% of which represent new taxa. The psychid
fauna over much of the paleotropics appears
similarly unstudied. In contrast, females in over
90% of the Palearctic species exhibit wing re-
duction. Genera with females exhibiting little
or no wing reduction occur within the five basal
subfamilies (Naryciinae, Taleporiinae, Pseudar-
belinae, and possibly Placodominae and Ty-
phoniinae). Because genera with brachypterous
or apterous females occur in four of these sub-
families, as well as in Scoriodytinae (Table 2)
(28, 95), wing reduction may have evolved in-
dependently several times within the family, as
is confirmed by the molecular phylogeny of 30
psychid species (115). Female brachyptery may
have evolved within each subfamily as a result of
biological traits (e.g., heavy abdomen and high
fecundity) that favored or facilitated flightless-
ness of females (101).
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LIFE HISTORY
Egg Stage
Eggs are cylindrical, smooth, and relatively
large compared with the size of females and
are smaller in primitive subfamilies (11, 59,
109). Embryonic development is usually com-
pleted within one month (2, 8, 11, 13, 29, 50,
67, 104, 113), with the exception of temperate
species that overwinter in the egg stage (70, 74).
Eggs laid by mated females are generally fertile
(8, 50, 87, 102).
Larval Stage
Larvae emerge synchronously, usually within
their maternal bag, where they sometimes feed
on the remains of their dead mother (3, 11, 89),
ova shells (13, 44), or sibling eggs (87). Follow-
ing a residence time of up to 5 days, neonates
exit the maternal bag by a silk thread from
its posterior opening (Figure 1e) (16, 29, 58,
102, 106), although some individuals die be-
fore leaving the bag (8, 11). Stimuli that induce
departure from the maternal bag include suit-
able weather, sunlight, and disturbance caused
by wind (11, 16, 20, 29, 41). Larval progeny
exit the maternal bag either simultaneously (56,
67) or in small groups over a delayed period
(16). Early instars either remain on the plant
where they emerged or disperse by ballooning
(Figure 1e) (16, 29, 41, 42, 52, 56, 68, 83, 86,
89, 102).
Factors associated with an enhanced rate
of dispersal by neonates include high den-
sity of conspecifics (81, 83), severe defoliation
(16), poor quality of the plant on which lar-
vae emerge (68), and strong wind currents (32,
47, 102). Neonates engage in ballooning be-
fore or after constructing their primary bag,
which may affect their survival or the range at-
tained during wind dispersal (16, 32, 41, 42).
Although ballooning larvae sometimes disperse
over great distances, most settle within a few
meters from their point of origin (16, 32, 66,
83, 86). Neonates enhance their dispersal range
by climbing to the top of plants before balloon-
Ballooning: mode of
dispersal by which
larvae drop on a silk
thread to become
windborne
ing (20, 29, 42, 102). The rate of ballooning
declines with the age of larvae and is low after
first instars start to feed (16, 32, 68), although
larvae have the capacity to silk through develop-
ment (83). The reproductive output of females
relative to the carrying capacity of host plant
influences the incidence of dispersal among
neonates: In species with small females and
low fecundity, the larval progeny commonly
remain on their natal host, where they initi-
ate feeding and development. In species with
large and highly fecund females, in contrast,
neonatal larvae often disperse by ballooning ir-
respective of the quality of the food plant, be-
cause the offspring of a few females have the
capacity to overexploit their host (32, 41, 42,
86).
First instars construct a self-enclosing bag
before they start to feed (11, 24, 30, 44, 59, 109),
and the availability of suitable material for bag
construction sometimes has priority over the
palatability of plants as food as the determinant
of host choice (8, 29). The primary bag to which
foundation larvae incorporate various materials
(such as plant tissue and organic and inorganic
debris), consists of a 1- to 2-mm-long conical
structure with two openings and is made of silk
spun from labial glands (11, 13, 30, 37, 50, 58,
113).
The head and thoracic legs protrude from
the anterior opening of the bag, allowing the
larva to feed and move over the plant; the small
posterior opening serves to expel feces and cast
skins (Figure 1a) (30, 41, 56). Larvae maintain
the interior of their bag clean and dry through-
out development. Prolonged wet conditions are
detrimental to the maintenance of a suitable mi-
croclimate within the bag and allow for the de-
velopment of entomopathogenic fungi (41, 59).
Larvae are capable of repairing damaged bags,
but the ability to construct a bag de novo is re-
stricted mostly to first instars (12, 30, 58). To ac-
commodate their increasing size, larvae contin-
ually enlarge their bag in length and diameter
by adding pieces of plant and other material in-
terspersed with silk (12, 13, 21, 37, 78). Bags of
first instars are relatively uniform in the Psychi-
dae, but bags of mature larvae vary considerably
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ANRV363-EN54-11 ARI 27 August 2008 20:44
ba
c d e
Figure 1
Photographs illustrating the life history of Metisa plana Walker. (a) Bag of early instars. (b) Pupal bag of male (right) and female (left).
(c) Male mating with a female by intruding his abdomen into the lower section of the female?s bag. (d ) Male mating with a receptive
female; the female bag has been opened and the pupal case removed to illustrate how the extensible abdomen of the male reaches the
female?s genitalia. (e) Neonates emerging from their maternal bag suspended from a silken thread. Photographs are reproduced with
permission of Applied Entomology and Zoology (see Reference 87).
between species in both shape and dimension
(20, 21, 28, 42, 56, 109).
The possession of a self-enclosing bag gen-
erates microclimatic conditions (e.g., elevated
temperatures) that may accelerate development
(6, 100) and favor the survival of overwintering
eggs (90). Larvae close the anterior part of their
bag before molting or when disturbed, and as
such the bags physically protect them against
natural enemies (3, 37, 58, 62, 67, 102). The
cryptic appearance of bags (3, 41, 42, 75, 109)
also serve as camouflage.
The duration of the larval stage is extended
in temperate bagworms because mature larvae
are the most common overwintering stage (59,
109). In tropical and subtropical species, the
duration of larval development varies between
32 days in Pteroma pendula Joannis to more
than 200 days in Oiketicus kirbyi Guilding (60,
102). Females have a longer larval development
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ANRV363-EN54-11 ARI 27 August 2008 20:44
and greater feeding requirements than males
do, which in some species involves a supernu-
merary feeding instar (2, 8, 24, 41, 71, 104,
106).
Larval Performance
Bagworms in the more basal subfamilies are
omnivorous scavengers with a diet similar to
that of some Tineidae, feeding on lower plant
forms such as lichens and mosses, as well as
on small insects and organic debris (21, 27,
59, 109). Although omnivory is observed in
Oiketicinae (21, 50, 110), most species are
polyphagous defoliators with a broad range of
hosts (Table 1). A high midgut pH may allow
larvae to effectively extract leaf nutrients while
detoxifying secondary metabolites from several
families of plants (15). Larvae can tolerate long
periods of starvation lasting a few weeks to sev-
eral months (21, 44, 58, 59), especially late dur-
ing development (41).
Even though most species have a broad host
range, larvae often die when they are trans-
ferred to a new plant species during develop-
ment, suggesting that host preference is in-
duced through larval experience (21, 56, 111).
Both the level of polyphagy (20, 29, 47, 59,
112) and the rate of feeding (8, 58) increase
with the age of larvae. In a highly polyphagous
species such as Thyridopteryx ephemeraeformis,
larvae are thought to adapt to different plant
species, leading to the evolution of sympatric
races based on distinctive host plant exploita-
tion (43, 56). However, the performance of lar-
vae is only weakly influenced by the host of
origin in the parental generation, indicating a
lack of specialized adaptations to different plant
species (75, 112). Polyphagy may be maintained
at the species level by inherent variation among
neonates in their tendency to disperse, for ex-
ample, some larvae disperse irrespective of the
quality of the host plant on which they emerge
(68). The dispersal behavior of larvae may also
affect the exploitation of different plants re-
gardless of their nutritional quality, for exam-
ple, tall or exposed trees are more likely to in-
tercept ballooning larvae (32, 75).
Sexual segregation of
pupation sites:
distinctive dispersal
behavior of male and
female larvae, which
results in different
positions of male and
female pupae on a
plant
Pupation
Upon completion of feeding, larvae tightly at-
tach the anterior portion of their bag onto a
substrate (Figure 1b) and reverse their position
within the bag, with the head oriented down-
ward (21, 59); larvae that do not reverse their
position usually fail to emerge as functional
adults (55, 56, 58). The larva of one species,
Brachygyna incae Davis, does not invert its po-
sition prior to pupation, with the adult conse-
quently emerging from a subapical opening in
the bag (25). Male larvae in some species un-
dergo a nonfeeding instar before pupation (45,
50, 59, 109). Larvae either pupate on the same
host plant where they fed or disperse before
pupation (14, 42, 109). Larvae that seek pu-
pation sites are often gregarious (41, 42, 43,
56, 59). Sexual segregation of pupation sites
has been recorded in several species: Compared
with males, female larvae are more likely to pu-
pate away from the host where they fed or in
higher locations (19, 20, 55, 59, 102, 109). This
behavior likely results from sex-specific selec-
tive pressures. Female larvae enhance their fit-
ness as sessile adults by pupating in locations
most suitable for mate attraction or for the per-
formance of their progeny; male larvae are not
subjected to these constraints because winged
adults are capable of dispersal (17, 38, 80, 82,
83, 86).
Female pupae are larger and differ morpho-
logically from male pupae. In the more derived
subfamilies, the female often does not exhibit
well-defined appendage sheaths. The elongate,
obtect pupa of the male is typical of moths, ex-
hibiting enclosed structures readily identifiable
as wings, legs, eyes, or antennae; the abdomen
characteristically possesses an anterior row(s) of
tergal spines that aid the male in moving down-
ward in its bag before emerging (22, 59, 73).
The pupal stage is shorter for females than for
males (2, 8, 20, 29, 41, 45, 102, 106).
Adult Stage
Shortly before emergence, the male pupa
pushes itself partway outside of the caudal end
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ANRV363-EN54-11 ARI 27 August 2008 20:44
of the bag; a terminal cremaster, sometimes as-
sisted by a posterior row of recurved spines on
abdominal tergites 2?8, prevents the male from
falling to the ground (73). The male emerges af-
ter rupturing the anterior segments of its pupal
case and takes flight on the same day (69). The
male is a typical winged moth, often with well-
developed bipectinate antennae, relatively long
legs, and reduced mouthparts. Males are capa-
ble of sustained flight and are active a few hours
each day, either during daytime or at night de-
pending on the species (21, 29, 41, 50, 56, 59,
109).
Females in most primitive species are fully
winged and undergo the same process as males
during emergence, yet they are less active as
adults (21, 94). Females in about half the species
are apterous, possibly due to an inhibition of cell
proliferation in wing disks caused by hemopoi-
etic organs among late instars (76). Apterous
females with fully developed legs leave their
pupal case upon emergence and spend their
adult life clinging on the exterior surface of the
bag (21, 41, 59, 61, 109) (Table 2). In many
species, neotenic females lack functional ap-
pendages; their emergence is indicated by a de-
hiscence of the anterior segments of the pupal
case. The head and thoracic segments are fused
and poorly developed, and most of the body
consists of a weakly sclerotized abdomen tightly
packed with ova (2, 21, 39, 59, 89, 109). The
length of females varies between 6 and 34 mm
depending on the species (Table 1). Adults with
vestigial appendages remain in their pupal case
and protective bag until shortly before death.
Neither male nor female bagworms feed as
adults. The longevity is longer for females (up
to two weeks) than for males (usually one or
two days) (3, 8, 29, 41, 50, 52, 56, 58, 59, 61,
62, 67, 71, 102, 105, 106, 108, 109). Sex ratios
of laboratory-reared bagworms are often male
biased (3, 8, 45, 52, 67; but see 69). In field pop-
ulations, males are either more abundant than
females (4, 17, 47, 55, 108), less abundant than
females (13, 18, 38, 85), or equally abundant
(41). A high population density of bagworms
is often correlated with a male-biased sex ratio
(17, 31, 51, 55), possibly because female larvae
have greater feeding requirements than males
and are more susceptible to intraspecific com-
petition for food (85). The sex ratio of adult
bagworms is also influenced by the higher level
of mortality among male pupae than female
pupae (19, 85, 99).
Mating Behavior
Females attract conspecific males over large dis-
tances by releasing sex pheromones that consist
of chiral esters (35, 63, 84, 103). In the more
basal subfamilies, females clinging to their bag
exhibit a calling behavior characterized by pe-
riodic pulsations of the abdomen (61). In some
Oiketicinae, the pheromone is synthesized in
glands located on thoracic segments and on the
first abdominal segment (10, 35, 64, 103). The
pheromone is sometimes released from decid-
uous thoracic setae shed by females outside of
the pupal case into the lower portion of the
bag (2, 10, 42, 63, 64, 84). In the subfamily
Metisinae, females periodically protrude their
thorax outside of the lower section of the
bag, possibly to further the dissemination of
pheromone (Table 2). Females eventually drop
onto the ground to die if they fail to attract a
mate (8, 13, 29, 30, 41?43, 55, 58, 62, 102),
and the sexual attractiveness of females declines
with age (105).
In the more basal genera with winged fe-
males, or with females that cling outside of
their bag upon emergence, the mating proce-
dure is similar to that of typical moths (21, 50,
59). Species with vermiform females that re-
main within their bag exhibit a highly modified
mating behavior. Upon being attracted by the
pheromone plume of a virgin female and land-
ing on her bag, the male pneumatically inserts
his extensible abdomen through the posterior
opening of the female?s bag all along her body
inside her pupal case to reach the caudal geni-
talia (Figure 1c,d ) (39, 43, 55, 63, 105). Males
of Oiketicinae typically possess paired sclero-
tized apophyses, arising from the male eighth
sternum, that assist in the considerable exten-
sion of the abdomen (up to three times its orig-
inal length) during copulation (21, 59). Males
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ANRV363-EN54-11 ARI 27 August 2008 20:44
sometimes die with their abdomen inserted into
the female?s bag after unsuccessful mating at-
tempts (41, 58, 105). The abdomen of males
is retractable, and they are capable of multiple
copulations (21, 55, 109), although their mat-
ing capacity is limited by their brief life span
(14, 41, 50, 52, 61, 105, 109). The fertility of
females declines when they copulate with pre-
viously mated males (61).
Females mate only once and cease being at-
tractive shortly thereafter (29, 41, 55, 59, 109).
A high proportion (up to 30%) of unmated fe-
males (hereafter called mating failure) has been
reported in several species (7, 42, 52, 62, 80,
82, 85, 99, 105), which may be attributed to the
complex mating procedure, limited mating ca-
pacity of males, short longevity of adults, or late
emergence of males; alternatively, flightlessness
of female bagworms per se may constrain their
mating success (85). Male-biased sex ratios may
have evolved as a strategy to compensate for the
low mobility of females (8, 45, 108). The high
incidence of mating failures likely selected for
behavioral adaptations of females to enhance
mating success, e.g., pupation in locations most
suitable for mate attraction (82). Because large
female bagworms effectively attract mates inde-
pendently of their location, the fitness impact
of pupation site in terms of enhanced mating
success is most pronounced for small females
(80).
Although rare in Lepidoptera, parthenogen-
esis has evolved independently in many gen-
era of the Psychidae (46, 61, 65, 72). Studies
on the genetics of Dahlica triquetrella H ?ubner
reveal the existence of sexual and partheno-
genetic (diploid and tetraploid) races whose dis-
tributions closely match recent geological his-
tory and biotic changes (97, 98). Facultative
parthenogenesis has not been demonstrated for
any sexual species.
Oviposition Behavior
Winged females use their long ovipositor to lay
eggs in crevices, usually some distance from the
pupation site (59, 94, 109). In the predatory
bagworm Perisceptis carnivora Davis, females
Mating failure: an
adult female fails to
attract a male for
mating during her
lifetime and
consequently dies as
virgin
Parthenogenesis:
production of
offspring by a female
with no genetic
contribution from a
male
Neoteny:
preservation in
reproductively mature
adults of traits usually
associated with
immature stages
wrap their eggs individually inside cocoon-like
cases made from abdominal setae, a behavior
that appears unique in insects and may have
evolved to protect the eggs from carnivorous
siblings (27). Apterous females with functional
legs stay on their bag upon mating and insert
their telescopic abdomen into the lower open-
ing of the bag to oviposit in their pupal case (21,
59, 61, 109). In species with vermiform females
that remain inside their pupal case, peristaltic
contractions of the abdomen allow females to
discharge their eggs intermixed with abdomi-
nal setae inside the upper section of their pu-
pal case, progressively shrinking in the process
(14, 21, 59, 109); oviposition is initiated imme-
diately after copulation and completed within
two days (21, 39, 55, 56, 89, 102, 104). Mated
females lay a high proportion of their eggs (67,
87, 102), and more than two thirds of larval-
derived resources are allocated to egg produc-
tion (87, 88). Large female T. ephemeraeformis
are more effective than small females at con-
verting adult biomass into eggs (88). The high
rate of conversion of adult biomass into repro-
ductive tissue in bagworms may be related to
the neoteny of short-lived females that invests
little in somative tissue (88). Females usually de-
part their bag upon completing oviposition to
perish on the ground (2, 13, 29, 52, 55, 58, 102,
104), but occasionally they remain in their bag
(8, 14, 30, 39, 109). The presence of a dead fe-
male inside the bag may provide neonates with a
food source (3, 11, 89), or with material for con-
structing the primary bag (14, 30, 42), while po-
tentially obstructing the lower segment of the
bag through which neonates emerge (11).
Fecundity
The reproductive success of females in field
populations of insects is inherently difficult
to quantify owing to the small size and high
mobility of adults. Apterous female bagworms
complete all reproductive activity within their
bag, and as such provide a model system to
estimate realized fitness in natural conditions
(85). For example, the reproductive output of
females that fail to mate as adult is null. The
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ANRV363-EN54-11 ARI 27 August 2008 20:44
Protogyny: early
emergence of females
relative to males
lifetime fecundity of mated females can be as-
sessed by counting the number of eggs laid in
pupal cases, which is a good estimate of fitness
because eggs laid by mated females are gener-
ally fertile.
The distinctive reproductive strategies of
bagworms (neoteny to apterous to winged, sex-
ual and asexual reproduction) affect the real-
ized fecundity of females. The number of eggs
laid by females in different species varies by
two orders of magnitude (Table 1), with over-
all trends of enhanced fecundity with increasing
body size, both within (19, 20, 31, 85, 87, 88,
102) and among species (Table 1). The fecun-
dity tends to be higher for tropical species than
for temperate species (Table 1), and for species
with vermiform females than those with winged
females (41). The latter trend possibly reflects
the metabolic cost associated with the produc-
tion of wing structures (93). Similar fecundity of
coexisting parthenogenetic and sexual species
suggests no direct cost related to sexual repro-
duction (61), although a high incidence of un-
mated females in sexually reproducing species
may indirectly reduce the reproductive output
of females.
SEASONAL HISTORY
The development of temperate bagworms is
usually completed in one or two years (Table 1).
Populations often exhibit developmental syn-
chrony, e.g., individuals overwinter as late in-
stars and emerge as adult in late spring (21, 59,
109). Individuals also overwinter within their
maternal bag, either as first instars (Apterona
helicoidella Vallot) (21, 113) or diapausing eggs
(T. ephemeraeformis) (70, 74). Diapausing eggs of
T. ephemeraeformis that undergo a short period
of chilling exhibit asynchronous development
and a prolonged interval of adult emergence
(69, 74).
The development time of tropical bagworms
is variable (Table 1). In allopatric species that
exploit the same host plant, the duration of
development increases with the size of adults
(Table 2) (8, 29, 41, 42, 86). The abundance or
performance of individuals may fluctuate sea-
sonally or along a latitudinal gradient in re-
sponse to variation in abiotic conditions or
availability of host plants (3, 47, 52, 53, 62, 67,
69). The continuous development of bagworms
in tropical regions allows for the occurrence
of asynchronous development and overlapping
generations (all developmental stages simulta-
neously present) (41, 59), although some pop-
ulations exhibit synchronous development (8,
82, 85, 108). The level of developmental syn-
chrony affects population dynamics through its
effect on natural enemies (9) or on the mating
success of females (8, 74, 82, 104).
Temporal pattern of adult emergence is of-
ten characterized by protogyny (31, 62, 69, 71,
82, 85, 89, 102, 106), although males may also
emerge in synchrony with (8, 41) or before (45,
104) females. Protogyny is not an incidental
consequence of sexual dimorphism in the adult
stage, because the pupal stage, which is shorter
for females than for males, is offset by the longer
developmental period of female larvae (2, 8,
41, 106). Protogyny did not evolve to enhance
the mating success of emergent adults, at least
not in populations with nonoverlapping gen-
erations in which early-emerging females and
late-emerging males encounter few receptive
partners (82, 85). The presumably high level
of genetic relatedness within local populations
of bagworms (21, 22, 61, 85, 109) suggests that
protogyny may have evolved as a strategy to
reduce inbreeding depression (82, 85), assum-
ing that the timing of adult emergence varies
among different sites and that effective disper-
sal by males between sites connects local popu-
lations of bagworms (88).
POPULATION DYNAMICS
An Evolutionary Hypothesis
Outbreaks, defined as explosive increases in
abundance over a short period, have been
documented in temperate and tropical bag-
worms (9, 29, 43, 44, 45, 51, 71, 104, 109). The
Psychidae share several life-history traits with
other lepidopteran species with flightless or
poor-flying females and eruptive population dy-
namics, such as high fecundity, no adult feeding,
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broad host range, exploitation of persistent
host plants, eggs laid in clutches, clustering of
progeny, and dispersal by ballooning on silken
threads (5, 54, 77). These traits are more com-
mon within certain families of Lepidoptera than
in other taxa (77), suggesting that phylogenetic
constraints are associated with flightlessness of
females and eruptive population dynamics.
The Psychidae share with other ditrysian
Lepidoptera the presence of anal papillae, a
pair of caudal sensory lobes that assists in plac-
ing the eggs onto a substrate, but not insert-
ing the eggs into plant tissue (36). Informa-
tion regarding plant quality for larval feeding
is therefore superficial for ovipositing females,
and substrate specificity relating to larval food
quality may decline. A lack of ovipositional
preference requires larvae to forage as first in-
stars, which selects for an ability to feed on
a range of plant species. Reduced oviposition
preference is compensated by high fecundity,
achieved through reduced activity, flightless-
ness, aptery, loss of appendages, and eventu-
ally neoteny. With declining mobility of fe-
males, selection will promote dispersal among
larvae, with ballooning on silken threads a com-
mon evolved response. The ecological conse-
quences, or emergent properties, of indiscrimi-
nate oviposition as a phylogenetic constraint,
and the adaptive syndrome of traits outlined
above, will be a high probability of eruptive
population dynamics in the taxon (79). Females
with low or no mobility, and larvae with lim-
ited dispersal, will result in a population rapidly
reaching the environmental carrying capacity
under favorable ecological conditions (13, 16,
17, 31, 32, 41, 43, 56, 71, 89).
In the ditrysian Lepidoptera, the Psychidae
were among the first external folivores (36),
with the bags providing essential protection
against the elements for the poorly adapted ex-
ternal feeders. This adaptation of the bag be-
came a preadaptation for oviposition within the
bag (94). The evolution of aptery and neoteny is
widespread in the Psychidae and evolved early
in the lineage. This contrasts with other lepi-
dopteran families in which aptery has evolved
more at the genus and species levels (54) and no
Phylogenetic
constraints: critical
plesiomorphic
characters that limit
adaptive trajectories in
a lineage, and
therefore ecology
neoteny occurs. The presumably tropical origin
of aptery and neoteny in the Psychidae also con-
trasts with other groups with wingless females
that occur predominantly in temperate regions
(5). In the tropics a major selective advantage of
aptery and neoteny may have been the short-
ening of the life cycle as well as increased fe-
cundity, thereby greatly increasing population
growth rates and the potential for pest status.
The Relative Influence of Bottom-Up
and Top-Down Forces
Bagworms are excellent subjects for popula-
tion studies, because postmortem dissections
of bags reveal several demographic parame-
ters related to population dynamics (99). Even
though the relative effect of bottom-up and
top-down forces on population regulation has
not been investigated for any species of bag-
worms, the literature reveals consistent trends
relative to the effect of plant attributes and nat-
ural enemies on the population dynamics of
bagworms.
Bottom-up effects strongly influence pop-
ulation dynamics. The performance of larvae
varies on different host plants (19, 53, 62, 75,
111, 112), which influences the fitness of adults
(88). Stress factors that negatively affect the
growth of plants either enhance or reduce the
nutritional quality of plants as a food resource
for larvae (47, 71, 107). Plants with a low level
of tannins or a high content of nitrogen may be
nutritionally superior (8). Defoliation caused
by bagworms can reach high levels, sometimes
causing the death of plants (13, 16, 31, 32, 41,
43, 56, 71, 89). A high density of conspecifics or
defoliation of host plants limits the availability
of food and reduces the performance of larvae
(8), often resulting in larval death (56, 89),
reduced body size (31, 42, 55, 56, 81, 85), and
low reproductive output of females (7, 17, 41,
42, 85). Individual variation in fitness caused by
a limited availability of food has consequences
at the population level: For example, the body
size and reproductive output of females are
negatively correlated with population density
(7, 17, 85, 87), which may influence the size of
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ANRV363-EN54-11 ARI 27 August 2008 20:44
bagworm populations in offspring generation.
Resource partitioning is often mediated by
density-dependent dispersal of larvae that
emigrate from crowded plants by walking or
silking (16, 41, 43, 81, 86, 89), especially late
during development when feeding is at its peak
(8, 56, 83, 104).
Bagworms often suffer a high level of mor-
tality owing to natural enemies (4, 9, 20, 102),
but few studies have evaluated the relation be-
tween population density and the incidence
of parasitism, predation, or disease. Top-down
population regulation is generally believed to
arise from an increasing level of mortality
caused by natural enemies with an increased
population density of herbivores, either on a
temporal or spatial scale. The level of pupal
mortality caused by natural enemies often de-
creases with population density (9, 18, 51, 85),
which suggests a limited impact of top-down
forces in terms of regulation of bagworm pop-
ulations. The considerably higher level of mor-
tality for male pupae than for female pupae,
combined with the low level of mortality among
large, mostly fecund females (19, 85), also im-
plies that natural enemies do not significantly
reduce the overall reproductive output of fe-
males in parental generations. However, the
lack of field data documenting the relation be-
tween population density and the rate of larval
mortality caused by different natural enemies
precludes definitive conclusions with regards to
the role of top-down forces on the dynamics of
bagworm populations.
CONCLUSION
The unusual biology of bagworms, character-
ized by the possession of a larval bag and ex-
treme forms of appendage reduction in females,
has long attracted the attention of entomolo-
gists, leading to a vast accumulation of liter-
ature. The existence of broad patterns in the
Psychidae, most notably in terms of erup-
tive population dynamics and mating failures
among females, provides excellent yet largely
neglected material to further life-history the-
ory. For example, only two reviews on the
evolutionary significance of flightlessness have
included the Psychidae as prime examples,
and in both cases only one temperate species,
T. ephemeraeformis, was considered (5, 79). We
hope this review helps to promote bagworms as
a model system to study population dynamics
and intraspecific variation of reproductive suc-
cess, in particular to explore consequences at
the population level of traits that influence the
fitness of individuals.
SUMMARY POINTS
1. The bagworm family includes approximately 1000 species, all of which complete larval
development within a self-enclosing bag. Aptery of females has evolved several times
independently in the family and is present in over half the species.
2. In the more basal subfamilies, larvae are omnivorous scavengers and mobile females leave
their bag upon emergence. Some species are parthenogenetic.
3. Bagworms in the most derived subfamilies are higher-plant feeders with a broad host
range. Larvae disperse suspended from a silk thread to be windborne. Male and female
larvae exhibit distinctive dispersal and pupation behavior. Sexual dimorphism at the adult
stage is extreme. Neotenic females remain in their bag within their pupal case and release
a sex pheromone to attract males. Copulation involves the insertion of the telescopic ab-
domen of males inside the female bag. Females oviposit within their pupal case upon mat-
ing. A high incidence of mating failures among females has been reported in many species.
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4. Populations often exhibit developmental synchrony and discrete generations. Females
often emerge before males, which may have evolved as a strategy to reducing inbreeding.
5. Life-history traits of bagworms are associated with eruptive population dynamics. Con-
sistent trends observed for several species include strong bottom-up effects and resource
partitioning mediated through larval dispersal.
FUTURE ISSUES
1. The biology and systematics of bagworm species with winged females remain poorly
understood. Further studies on these taxa, especially in tropical regions, are needed.
2. Species with neotenic females that reproduce within their bags represent model sys-
tems for studying the principles of population dynamics or for quantifying intraspecific
variation of reproductive success. Future studies need to explore consequences at the
population level of traits that influence the fitness of individuals.
3. Species with apterous females are ubiquitous worldwide, providing valuable yet neglected
comparative data on temperate and tropical species.
4. Extreme wing reduction in females is generalized in the Psychidae, which appears to lead
to eruptive population dynamics. Long-term studies are needed to unravel bottom-up
and top-down forces that contribute to the regulation of bagworm populations.
5. The high incidence of mating failures among females challenges conventional wisdom
relative to the evolution of mating systems, most notably with respect to the prevalence
and significance of protogyny, behavioral adaptations of female larvae to enhance their
mating success, and the cost of sexual reproduction.
DISCLOSURE STATEMENT
The authors are not aware of any biases that might be perceived as affecting the objectivity of this
review.
ACKNOWLEDGMENTS
MR acknowledges G. Gries and C. Sadof for their support. P. Haettenschwiler kindly shared his
insight on the Psychidae. We thank D. Quiring, R. Johns, T. Tammmaru, C. Wiklund, and S.H.
Yen for comments on earlier versions of the review. Photographs were generously provided by
C.T. Ho.
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