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

ABSTRACT

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

INTRODUCTION

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.

BIOLOGY

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.

BIOCHEMISTRY

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.

CHEMICAL ECOLOGY

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 

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. 

vulgatissima.

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.

ECOLOGICAL IMPLICATIONS 

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 

clones.

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 

glycosides.

CONCLUSION

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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