A Review of Leaf-Eating Chrysomelid Beetle (Chrysomelideae: Coleoptera) Interactions with Salicaceae Host Species

Ann Dernburg
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.
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 
al. 1995).а 
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.
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 
same host?
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 
secondary compounds.
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.
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