Colorado State University
Fort Collins, CO
17 April, 1996
Chrysomelidae are phytophagous beetles.а They specialize on a small number of plant
species, including the Salicaceae family.а Characteristically, life cycles are
simple, with all stages occurring on the same plant and the ground below it.а Larvae
and adults secrete a variety of different chemicals, perhaps in response to
different sources of predation.а The most common chemical groups of secretions among
adults are izoxazolin-5-one glucosides and saturated hydrocarbons, Larvae secrete
mostly monoterpenes or phenolic glycosides.а The former are synthesized by the
organism.а These secretions are thought to be part of the primitive defense
mechanism.а The latter are derived from host plant compounds.а A shift in food
preference may account for species that secrete phenolic glycosides.а In particular,
salicin (from Salicaceae) is used to produce salicylaldehyde.а Salicylaldehyde is an
effective repellent against ants, but has not been thoroughly tested on other
predators that feed on chrysomelideae.а Furthermore, exocrine secretions may be used
to repel competitors for the same resource.а The role of larval secretions can be
clarified with more research.а Research shows that host plant chemicals are also
used as feeding cues.а Chrysomelid beetles species have become highly specialized on
their plant hosts.
Plant feeding cues are quite specific.а For example, while P. vitellina prefers to
feed on willows/diets rich in phenyl glycosides -especially salicin and salicortin-
and poor in proanthocianidins, Plagiodera versicolora prefers S. caprea, which has
small amounts of many phenolic compounds.а Chemical cues may also serve a role in
oviposition.а Salicaceae escape herbivory when environmental factors -climate and
isolation- are unfavorable to chrysomelid reproduction.а In beetle-rich
environments, selection for willows low in phenolic glycosides may be occurring.
Key words: chrysomelidae, leaf-eating beetles, phenolyc glycosides, Salicaceae,
salicin, salicylaldehyde, willow
The family of Salicaceae includes such well known species as willows (Salix spp.)
and poplars (Populus spp.) In Europe, Salicaceae are being studied as crop plants,
particularly for the production of wood pulp.а Since plant the densities required of
commercial production may increase pest infestations, it behooves producers to
understand the biology of herbivores which rely on Salicaceae.а Chrysomelid beetles
(Chrysomelidae) are one such group.а Specialization of chrysomelids on Salicaceae
can be extreme, with some species carrying out their entire life cycle and feeding
on the same plant.а In fact, it appears that these leaf beetles select willows with
particular chemical characteristics, and then use chemicals for their own defense.
This paper will review research on the chemical ecology of chrysomelids that utilize
Salicaceae.а I will focus mostly on larval secretions.а The first part is an
overview of chrysomelid life history.а Then, the biochemistry of chrysomelid
secretions is reviewed, followed by several aspects of Salix-beetle chemical
ecology.а Finally, ecological implications of this relationship are discussed.
The Chrysomelidae are a family of leaf-eating beetles.а They are found in damp
habitats, such as wetlands and riparian areas.а Larvae are mono- or polyphagous on
riparian forbs, shrubs and trees (Table 1).а The life cycle of the chrysomelidae is
simple.а For example, the adults of Phratora vulgarissima (L) overwinter under the
bark of trees.а In spring they lay large numbers of eggs on the underside of Salix
viminallis leaves, There are 3 larval instars, which remain on .5. viminallis.а
Pupation occurs after six weeks, and adults emerge two weeks later (Kelly and Curry,
1991 a).а Depending on the length of warm weather, chrysomelidae may have one or
several cycles per year (Kelly and Curry 1991 a, Rowell-Rahler 1984a).
In many species, the larvae posess eversible glands.а Generally, the larvae have 9
pairs of glands; the first two are located on the meso- and metathorax, the others
are on the first seven abdominal segments.а When the larva is disturbed, a droplet
of chemical(s) is everted.а These chemicals are purported to have defensive
properties.а The droplet is retracted after a brief exposure during which volatile
compounds are released.а In Plagiodera versicolora, these glands are shed with the
last larval skin, apparently conferring some protection to the pupa (Hinton 195 1,
Wallace and Blum 1968).а Adults use a variety of mechanisms, including behavioral,
chemical and physical defenses.а Behavioral mechanisms include evasive action such
as flying, jumping or falling to the ground, or offensive techniques like reflexive
bleeding.а Physical defenses include aposematic colorations and sharp, spiked
tegumenta (DeRoe and Pasteels 1982, Pasteels et al. 1982) . Defensive secretions
ooze from elytral and pronotal glands of adult Ortina beetles (Pasteels, 1994).The
classification of chrysomelinae into tribes and genuses is based on morphological
features of both larvae and adults.а It is given in Table 1.
Exocrine secretions vary among larval and adult instars.а These differences are
summarized in Pasteels et al., 1982, Pasteels et al., 1984 and Pasteels et al.,
1994.а They are given in tables 2 and 3. The secretions are chemically diverse.а
Reported secretions by adults include cardenolides, alkaloids, amino acid
derivatives, lipids and glucosides (Pasteels et al., 1984 and Pasteels et al.,
1995).а Compounds are secreted de novo or sequestered from host plant secondary
chemicals.а Larval secretions fall into two broad categories: phenolics, and
monoterpenes.а It is thought that monoterpenes and derivatives are synthesized by
the larvae, while the precursors to juglone and salicylaldehyde are produced by host
plants (Pasteels et al. 1984).
Pasteels et al. (1983) showed a positive correlation between the amount of salicin
-a phenyl glucoside- contained in Salix leaves and salicylaidehyde secreted by
Chrysomela larvae. Furthermore, Phratora spp. uses host plant salicin in the
biosynthesis of salicylaidehyde (Rowell-Rahier and Pasteels, 1982).а As seen in
Table 1, the host plants of phenol producing larvae are Salicaceae.а Phenolics are
the only class of secondary compounds that have been isolated in Salicaceae.а
Furthermore, salicin is the chemical marker of Salicaceae (Palo, 1984). Salicin is
present in the bark of 11 European Salix species, while other phenolic compounds
vary in type, location and concentration.а Julkunen-Tiitto (1986) confirmed that of
15 Salicaceae species studied, all contained variable amounts of condensed tannins
and phenolic glucosides. Salicin was isolated in fourteen of these species.а
Julkunen-Tiitto (1986) did not isolate salicin in S. triandra, but Palo (1984) had
previously found trace amounts in this species.а The chemical structure of salicin
and salicylaldehyde is given in Figure 1. Consequently, specificity among willow
feeding insects and their hosts can be quite high.а This is apparent among
chrysomelids that conduct their entire life cycle on one host plant species.
1. FOOD SELECTION
There is ample evidence that host-plant selection is based on the quantity and types
of phenolic compounds.а Field evidence suggests that Chrysomela anaecolis feeds
preferentially on salicylate rich willows. (Smiley, 1985).а In comparative feeding
trials, P. vitellina preferred to feed on willows/diets rich in phenyl glycosides
-especially salicin and salicortin- and poor in proanthocyanidins (Pasteels et al.
1983, Tahvanainen et al. 1985, Kolehmainen et al. 1995). Plagiodera versicolora feed
preferentially on S. caprea, which has small amounts of many phenolic compounds
(Tahvanainen et al., 1985).а Galerucella lineola and Lochmanea caprae were most
attracted to diets rich in another phenolic glucoside, tremulacin (Kolehmainen et
Both larval survival and growth of P. vulgatissima are negatively affected by high
concentrations of phenolic glycosides (Kelly and Curry, 199 lb), but not by the high
tannin and moderate salicin levels found in their host plant, S. viminalis.а Denno
et al. (1990) found that P. vitellina oviposits preferentially on salicylate rich
willows.а In feeding choice tests, larvae and adults of C anaecolis preferred the
salicylate-fich S. orestra, but oviposition occurred on both S. orestra and
salicylate-poor S. lutea (Rank 1992).а Orians et al. (personal conununication),
found that beetles differed markedly in their feeding preferences among different
willow species and their hybrids.а These authors suggest that preference for a
willow species is related to its salicortin content.а However, preference for one
willow type was not indicative of performance.
Note that some investigators have suggested that the physical characteristics of
leaves, such as pilosity, water content and toughness may affect the feeding
behavior of leaf beetles and their larvae (Horton 1989, Pasteels et al. 1993).а Rank
(1992) found no correlation between leaf characteristics and feeding, but observed
that phenolic concentrations strongly influenced host plant selection.а Similarly,
Kelly and Curry (1991) remarked that neither macronutrient content nor physical
attributes of willow species were responsible for the food preferences of P.
Although certain chrysomelid species select their host plants based on salicin
content, it is apparent that the phenolics produced by Salicaceae have a much
broader role.а They are used by chrysomelids as feeding cues, both positive and
negative.а The activity of these cues depends on the type and concentration of each
compound.а Long term, these chemicals affect not only feeding, but growth and
reproduction.а The synergistic (or antagonistic) effects of these compounds remains
to be explored.
2.CHRYSOMELID DEFENSE AGAFNST PREDATORS
Reported natural predators of chrysomelids include the lady beetle Neoharmonia
venusta, the fly Parasyrphus melanderi, mites, wasps (Symmorphus cristalus,
Ancistrocerus spp.) and spiders (Whitehead and Duffield 1982, Smiley 1985, Rank and
Smiley 1994, Rank 1994).а Jolivet (1950) catalogued over 40 insect species, as well
as vertebrates that feed on European Chrysomelidae.а Two parasitoids are common: a
fly and a wasp (Rank, 1992).а Cannibalism -C. anaecolis larvae eating their own
eggs- has also been observed (Rank, 1992).
Larvae discharge secretions in response to disturbance.а This behavioral mechanism
leads us to believe that the biological function of these glands is defensive.а
Salicylaldehyde has been shown to repel ants (Wallace and Blum, 1968, Matsuda and
Sugawara 1980), a ladybird beetle (Denno et al., 1990) and spiders (Palokangas and
Neuvonen, 1992) in laboratory experiments. Field evidence is mixed.а Whether the
paucity of evidence is due to the efficacy of larval secretions or lack of research
is unclear.а Rank (1994) notes that both the wasp S. cristatus and P. melanderi
display complex avoidance behavior towards larval secretions.а The former wipes
larvae on wood before placing them in a nest, the latter usually attacks and eats
the larva from it's underside.а Smiley (1985), demonstrated that larvae feeding on
salicin rich willows were more likely to produce salicin, pupate and survive.а In a
study relating altitudinal gradient to mortality in C anaecolis, Smiley and Rank
(1986) hypothesized that larvae were limited at higher elevations by cold, and by
predation on lower ground.а They suggested that predation increased because of low
levels of phenolic glycoside precursors in Salicaceae.а However, reported
concentrations in salicin were similar at both sites, and predator activity was not
clearly demonstrated.а Furthermore, Rank and Smiley (1994) showed that P. melanderi,
a natural predator of C. anaecolis eggs and larvae, feeds on larvae that have been
exposed to salicylate rich willows.а In fact, P. melanderi is most abundant on
salicylate rich willow species. Furthermore, Rank (I 994) showed that survival of C.
anaecolis larvae under field conditions was correlated with leaf water content, not
salicylate richness of host willows.а Other studies showed that survival rates for
aggregated C. anaecolis larvae were higher (Smiley 1991), but this was not related
to increased concentrations of defensive chemicals (Breden and Wade, 1987).а
Finally, Whitehead and Duffield, (1982) reported that while N. venusta is a major
predator of P. versicolora larvae and pupae, it is not affected by P. versicolora
larval secretions (which contain both monoterpenes and salicylaldehyde).
Pasteels et al. (1988) suggested that larval secretions might be used to protect
their resources.а Salicylaldehyde arrested the forward movement of gypsy moth
(Lymantria dispar) and tentmaker (Ichthyura inclusa) larvae.а Additionally, it is a
feeding deterrent to mourning cloak (Nymphalis antiopa) larvae.а Incidentally,
Orians (personal communication, 1996) reported that the secretions of chrysomelid
larvae are quite noticeable, and unpleasant, to humans. Clearly, the role of
chrysomelid secretions needs further investigation.
1. Why do larvae and adults produce different defenses even though they utilize the
Pasteels et al. (1984) proposed that defenses are tailored to the more frequent
predator. They suggest that larvae are more likely to be attacked by predators
located on the willows themselves.а Adults are more visible, therefore more
vulnerable to "airborne" predators such as birds.а Consequently, larvae produce
volatile repellents such as salicylaldehyde, while adults have marked aposematic
coloration associated with bitter tasting toxins.а Since larvae do not have a hard
tegument, one bite may be fatal.а Thus, it is important that predators be held off
before they approach the larva, by recognizing volatile toxins.а Volatile irritants
are not effective against birds unless applied in large quantities (Boeve and
Pasteels, 1983). Several arguments can be made against this hypothesis.а First,
larvae are dark and not readily visible.а In some species, larvae aggregate.а By
aggregating, they become more visible, but not more vulnerable (Smiley, 1991).а
Second. wasps and sawflies are airborne predators of larvae. The sawfly Tenthredo
olivacea can be conditioned to eat either salicylaldehyde or monoterpenes secreting
larvae, thus learning to ignore volatile warnings (Pasteels and Gregoire, 1984).а
Third, repellence of larval secretions has been tested on species that may not be
natural predators, such as ants.а Much may be learned by "cross testing" predators
and prey, by comparing defensive success rates of phenologically close beetles with
each others predators.
2. Why don't chrysomelids completely decimate host willow populations?
Smiley et al. (1985) suggested that plants which produce chemicals useful to insect
herbivores may be decreasing their own fitness, by increasing herbivory.а These
authors demonstrated that C. anaecolis did indeed prefer to feed on salicin rich
willows.а Along a stream gradient, herbivory was linearly related to salicin content
of willows, but only at lower elevations.а Furthen-nore, where willows stands were
patchy, patch size and isolation may have been the predominant factors influencing
herbivory.а Where the linear correlation held, Smiley et al. (1985) hypothesized
that previous bouts of herbivory had genetically selected willow clones that
produced low levels of salicin.а A year later, Smiley and Rank (1986) noted that C.
anaecolis mortality was similar along the stream gradient, but that the causes of
mortality were different. At lower elevations, predators were the main cause of
mortality, whereas temperature limited C. anaecolis at higher elevations.а It
appears that willows, regardless of secondary chemistry, escape herbivory when
environmental factors are unfavorable to beetle growth.а When environmental factors
are not limiting, selection pressures may favor selection of low salicin producing
3. Why are chrysomelid secretions different among species?
As seen above, the chemistry of chrysomelid secretions is quite varied.а Pasteels et
al. (1984) suggest that evolutionary pressures have influenced the nature of
chemical secretions (and other defensive strategies) of the chrysomelidae.а These
authors proposed that monoterpenes are the primitive defensive secretion of
chrysomelidae, since they are found in a wide variety of species.а Species of
beetles that secrete juglone and salicylaldehyde result from
12ааа shifting food preferences, with concomitant specialization on the host plant.а
From a study of published literature, Rowell-Rahier (1984b) concludes that
specialist herbivores are more likely to be found on willows that produce phenyl
glycosides.а Generalist herbivores are more frequent on non phenyl glycoside
producing Salicaceae.а Likewise, the faunas of phenyl glycoside producing Salicaceae
are more similar to each other than the faunas of Salicaceae without phenyl
Refined mechanisms for recognizing and exploiting host plants have evolved among
phytophagous chrysomelid beetles.а The efficacy of these mechanisms is attested to
by the fact that they occur in distinct geographic locations and among various
species of plants and beetles. For example, both European and American species of
chrysomelids utilize salicin in the biosynthesis of the defensive compound
salicylaldehyde.а The defensive secretion of Gastrolilna depressa larvae also
originates from plant secondary compounds, even though the host plant is not Salix
but Juglans (walnut).
Chrysomelid beetles have become extremely adapted, indeed specialized, on their host
plants.а In several instances, the entire life cycle occurs on the same plant.а
Moreover, research has shown that beetles are adept at recognizing specific plant
chemicals as feeding cues, and perhaps even egg laying cues.а Unless environmental
factors limit insect development, extreme host recognition and specificity may have
detrimental effects on host plants.а Under intense pressure from herbivores, we may
expect genetic variation to occur in favor of plants with low levels of target
The chemical nature of chrysomelid secretions are quite different within species and
among life stages.а While the reasons for these differences have not been
elucidated, several interesting points have surfaced.а First, the chemical nature of
secretions may be used for taxonomic purposes (Pasteels et al., 1984).а Second, the
knowledge of host plant and chrysomelid chemistry may further our understanding of
(co)evolution.а Finally, the role of these chemicals is not fully known.а These
compounds may guard the individual, or perhaps its resource.а Perhaps they are used
for intra species communication?а Clearly, more studies involving both actual and
potential predators are needed.а If these compounds are repellent to other
herbivores, practical applications can surely be found in agriculture.
BOEVE, J.L. and J.M. PASTEELS (1985).а Modes of defense in the nematine sawfly
larvae. Efficiency against ants and birds.а J. Chem.а Ecol. 4: 47-53.
BREDEN, F. and M. WADE (1987).а An experimental study of the effects of group size
on larval growth and survivorship in the imported willow leaf bettle Plagiodera
versicolora (Coleoptera:Chrysomelidae).а Environ.а Ent. 16:131-140.
DENNO R.F, S. LARSSON and K. OLMSTEAD (1990).а Role of enemy free space and plant
quality in host-plant selection by willow beetles.а Ecology 71:124-137.
DEROE, C. and J.M. PASTEELS (1992).а Distribution of adult glands in Chrysomelids
(Coleoptera: Chrysomelidae) and its significance in the evolution of defense
mechanisms within the family.а J. Chem.а Ecol. 8: 67-82.
HINTON, H.E. (1951).а On a little known protective device of some chrysomelid
pupae (Coleoptera).а Proc.а Roy.а Ent.а Soc.а London (A) 26: 67-73.
HORTON, D.R. (1989).а Performance of a willow-feeding beetle, Chrysomela knaki
Brown, as affected by host species and dietary moisture.а Can.а Entomol. 121: 777-
JOLIVET, P. (1950).а Les parasites, predateurs et phoretiques des Chrysomeloidea
(Coleoptera) de la faune Franco-Belge.а Bull.а Inst.а Sci.а Nat.а Beig. 26: 1-39.
JULKUNEN-TIITTO, R. (1986).а A chemotaxonomic survey of phenolics in leaves of
northern Salicaceae species.а Phytochemistry 25: 663-667.
KELLY, M. T. and J. P. CLJRRY (1991a).а The biology and population density of the
willow beetle (Phratora vulgatissima [L.]) on Salix viminallis in reclaimed cutaway
peat.K. Appl.а Ent. 111: 44-56.
KELLY, M.T., and J.P. CURRY (1991b).а The influence of phenolic compounds on the
suitability of 3 Salix species as hosts for the willow beetle Phratora vuIgatissima.а
Entomol. exp. appl. 6:25-32.
KOLEHMAINEN J., R. JULKUNEN-TIITTO, H. ROININEN and J. TAHVANAINEN (1995) Phenolic
glucosides as feeding cues for willow-feeding leaf beetles.а Entomologia
Experimentalis et Applicata. 74:235-243.
MATSUDA, K. and F, SUGAWARA (1980).а Defensive secretion of chrysomelid larvae
Chrysomela vingintipunctata costella (Marseul), C. populi L. and Gastrolina
depressa Baly (Coleoptera: Chrysomelidae).а Appl.а Ent.а Zool. 15: 316-320.
PALO, R.T. (1984).а Distribution of birch (Betula spp.), willow (Salix spp.) and
poplar (Populus spp.) Secondary metabolites and their potential role as chemical
defense against herbivores.а J. Chem.а Ecol. 10:499-520.
PALOKANGAS, P. and S. NEUVONEN (1992) Differences between species and instars of
leaf beetles on the probability to be preyed on.а Ann. Zool. Fenn. 29273-278.
PASTEELS, J.M. and J.C. GREGOIRE (1984) Selective predation on chemically defended
chrysomelid larvae.а A conditioning process.а J. Chem.а Ecol. 10: 1693 -1700.
PASTEELS J.M., J.-C. BRAEKEMAN and D. DALOZE (1982).а Chemical defence in
chrysomelid larvae and adults.а Tetrahedron 38:1891-1897.
PASTEELS J.M., M. ROWELL-RAHIER and M.J. RAUPP (1988) Plant-defived defense in
chrysomelid beetles.а In P. Barbosa and D, Letourneau, Eds.а Novel aspects of
Insect-Plant Interactions. pp. 235-272.
PASTEELS J.M., M. ROWELL-RAHIER, J.-C. BRAEKEMAN and D. DALOZE (1984). Chemical
defenses in leaf beetles and their larvae: The ecological, evolutionary and
taxonomic significance.а Bioch.а Syst.а Ecol. 12:395-406.
PASTEELS J.M., M. ROWELL-RAHIER, J.-C. BRAEKEMAN and D. DALOZE (1994). Chemical
defense in chrysomelid leaf beetles updated.а Iii P.H. Jolivet, M.L. Cox and E.
Petipierre, Eds.а Novel aspects of the biology of Chrysomelidae. pp. 289-301.
PASTEELS J.M., M. ROWELL-RAHIER, J. -C.а BRAEKEMAN and A. DUPONT (I 983). Salicin
from host plants as a precursor of Salycyaldehyde in defensive secretion of
Chrysomelina larvae.а Physiol.а Entomol. 8: 307-314.
PASTEELS J.M., S. DOBLER, M. ROWELL-RAHIER, A. E and T, HARTMANN (1995),
Distribution of autogenous and host-derived chemical defenses in Oreina leaf
beeteles (Coleoptera: chrysomelidae).а J. Chem.а Ecol. 21: 1163-1179.
RANK, N.E. (1991).а Effects of plant chemical variation on a specialist herbivore:
Willow leaf beetles in the Eastern Sierra Nevada.а Iii : C.A. Hall, V. Doyle-Jones
and B. Widawski, Eds. Natural History of California and High Altitude Research.а
Proc.а White Mountain Research Station Symposium, Vol 3. pp 148-160.
RANK, N.E. (1992).а Host plant preference based on salicylate chemistry in a willow
leaf beetle (Chrysomela aeneicolis) Oecologia 90:95-101.
RANK, N.E. (1994).а Host-plant effects on larval survival of a salicin-using leaf
beetle Chrysomela aeneicollis Schaeffer (Coleoptera: Chrysomelidae).а Oecologia
RANK, N.E. and J.T. SMILEY (1994).а Host-plant effects on Parasyrphus melanderi
(Diptera: Syrphidae) feeding on a willow leaf beetle Chrysomela aeneicollis
(Coleoptera: Chrysomelidae). Ecol.а Entomol. 19: 31-38.
ROWELL-RAHIER, M. (1984a).а The food plant preferences of Phratora vitellinae
(Coleoptera: Chrysomelidae).а A. Field observations. 369-374.
ROWELL-RAHIER, M. (1984b).а The presence or absence of phenylglycosides in Salix
(Salicaceae) leaves and the level of dietary specialization of some of their
herbivorous insects. Oecologia 62: 26-30.
ROWELL-RAHIER, M. and J.M. PASTEELS (1982).а The significance of salicin for a Salix
feeder, P. vitellinae.а In: Proc. 5th Int.а Symp.а Insect-Plant Relationships, J.H.
Visser and A.K. Minks, Eds.а P 73-79.а Pudoc, Wagenigen.
SMILEY, J. T. (1991).а Aggregation benefits in a willow leaf beetle along an
elevational gradient. In: C.A. Hall, V. Doyle-Jones and B. Widawski, Eds.а Natural
History of California and High Altitude Research.а Proc.а White Mountain Research
Station Symposium, Vol 3. pp. 148-160.
SMILEY, J.T. and N.E. RANK (1986).а Predator protection versus rapid growth in a
montane leaf beetle.а Oecologia 70:106-112.
SMILEY J.T., J.M. HORN and N.E. RANK (I 985).а Ecological effects of salicin at
three trophic levels: New problems from old adaptations.а Science 229-649-651.
TAHVANAINEN J., R. YULKUNEN-TIITTO and J. KETTUNEN (1985).а Phenolic glycosides
govern the food selection pattern of willow feeding leaf beetles.а Oecologia 67:52-
WALLACE, J.B. and M.S. BLUM. (1969).а Refined defensive mechanisms in Chrysomela
scripta.а Ann.а Entomol.а Soc.а Am. 62:503-506.
WHITEHEAD, D.R. and R.M. DUFFIELD (1982).а An unusual specialized predator prey
association (Coleoptera: Coccinellidae): Failure of a chemical defense and possible
practical application.а The Colleopterists Bulletin 36:96-97.ааааааааааааааа