Latitudinal variation in resistance and tolerance to herbivory of a salt marsh shrub

Interactions between plants and herbivores often vary on a geographic scale. Although theory about plant defenses and tolerance is predicated on temporal or spatial variation in herbivore damage, no single study has compared the pattern of herbivory, plant defenses and tolerance to herbivory of a single species across a latitudinal gradient. In 2002–2005 we surveyed replicate salt marshes along the Atlantic coast of the United States from Florida to Maine. At each field site we scored leaves of Iva frutescens for herbivore damage. In laboratory experiments we measured constitutive resistance and induced resistance in I. frutescens from high and low latitude sites along the Atlantic Coast. In another common garden experiment we studied tolerance to herbivory of I. frutescens from various sites. Theory predicts that constitutive resistance should matter more when damage is high, and induced resistance when herbivory is high but variable. In the field, average levels of herbivore damage, and spatial and temporal variation in herbivore damage were all greater at low versus high latitudes, indicating that constitutive as well as induced resistance should be stronger at low latitudes. Consistent with this prediction, constitutive resistance to herbivory was stronger at low latitudes. Induced resistance to herbivores was also stronger at low latitudes: it was deployed faster and lasted longer. Theory also predicts that tolerance to herbivory should be greater where average herbivory damage is greater; however, tolerance to herbivory in Iva did not depend on geographic origin. Our results emphasize the value of considering multiple ways in which plants respond to herbivores when examining geographic variation in plant–herbivore interactions.     

Plant chemical defense against herbivores and pathogens: generalized defense or trade-offs?

Plants are often attacked by multiple enemies, including pathogens and herbivores. While many plant secondary metabolites show specific effects toward either pathogens or herbivores, some can affect the performance of both these groups of natural enemies and are considered to be generalized defense compounds. We tested whether aucubin and catalpol, two iridoid glycosides present in ribwort plantain (Plantago lanceolata), confer in vivo resistance to both the generalist insect herbivore Spodoptera exigua and the biotrophic fungal pathogen Diaporthe adunca using plants from P. lanceolata lines that had been selected for high- and low-leaf iridoid glycoside concentrations for four generations. The lines differed approximately three-fold in the levels of these compounds. Plants from the high-selection line showed enhanced resistance to both S. exigua and D. adunca, as evidenced by a smaller lesion size and a lower fungal growth rate and spore production, and a lower larval growth rate and herbivory under both choice and no-choice conditions. Gravimetric analysis revealed that the iridoid glycosides acted as feeding deterrents to S. exigua, thereby reducing its food intake rate, rather than having post-ingestive toxic effects as predicted from in vitro effects of hydrolysis products. We suggest that the bitter taste of iridoid glycosides deters feeding by S. exigua, whereas the hydrolysis products formed after tissue damage following fungal infection mediate pathogen resistance. We conclude that iridoid glycosides in P. lanceolata can serve as broad-spectrum defenses and that selection for pathogen resistance could potentially result in increased resistance to generalist insect herbivores and vice versa, resulting in diffuse rather than pairwise coevolution.     

Latitudinal variation in resistance and tolerance to herbivory in the perennial herb Lythrum salicaria is related to intensity of herbivory and plant phenology

Both the length of the growing season and the intensity of herbivory often vary along climatic gradients, which may result in divergent selection on plant phenology, and on resistance and tolerance to herbivory. In Sweden, the length of the growing season and the number of insect herbivore species feeding on the perennial herb Lythrum salicaria decrease from south to north. Previous common‐garden experiments have shown that northern L. salicaria populations develop aboveground shoots earlier in the summer and finish growth before southern populations do. We tested the hypotheses that resistance and tolerance to damage vary with latitude in L. salicaria and are positively related to the intensity of herbivory in natural populations. We quantified resistance and tolerance of populations sampled along a latitudinal gradient by scoring damage from natural herbivores and fitness in a common‐garden experiment in the field and by documenting oviposition and feeding preference by specialist leaf beetles in a glasshouse experiment. Plant resistance decreased with latitude of origin, whereas plant tolerance increased. Oviposition and feeding preference in the glasshouse and leaf damage in the common‐garden experiment were negatively related to damage in the source populations. The latitudinal variation in resistance was thus consistent with reduced selection from herbivores towards the northern range margin of L. salicaria. Variation in tolerance may be related to differences in the timing of damage in relation to the seasonal pattern of plant growth, as northern genotypes have developed further than southern have when herbivores emerge in early summer.     

Induced plant responses to multiple damagers: differential effects on an herbivore and its parasitoid

Herbivore-induced plants responses can affect the preference and performance of herbivores and their natural enemies. These responses may vary depending on the identity and number of herbivore species feeding on the plant so that when herbivores from different guilds feed on plants, the interactions between plants, herbivores, and natural enemies may be disrupted. Tomato plants were damaged either by the caterpillar Spodoptera exigua, or the aphid Macrosiphum euphorbiae, or damaged by both herbivores, or undamaged controls. We measured the preference and performance of S. exigua and its parasitoid Cotesia marginiventris, and activity of proteinase inhibitors (PI) as an indicator of induced resistance. Compared to undamaged plants, caterpillar damage reduced the number of eggs laid by S. exigua adults, reduced growth, consumption, and survival of larval S. exigua and C. marginiventris, and increased activity of PIs 43%; but did not increase attraction of C. marginiventris. While pupal mass of S. exigua was not affected, the pupal mass of C. marginiventris decreased on caterpillar-damaged plants compared to controls. In contrast, plants damaged by aphids were preferred for oviposition by S. exigua, and had increased larval consumption and survival, compared to controls. Aphid feeding did not affect the preference or performance of C. marginiventris, or PI activity, compared to controls. While oviposition was deterred on caterpillar-damaged plants, plants damaged by both herbivores received the same amount of oviposition as controls. The attraction of C. marginiventris to plants damaged by caterpillars and aphids was increased compared to controls. However, plants damaged by both herbivores had similar PI activity, larval growth and survival of S. exigua and C. marginiventris, as plants singly damaged by caterpillars. Overall, the preference component for both the herbivore and parasitoid was more strongly affected by damage due to multiple herbivores than the performance component.     

Poleward increase in feeding efficiency of leafminer Stigmella lapponica (Lepidoptera: Nepticulidae) in a latitudinal gradient crossing a boreal forest zone

Damage to plant communities imposed by insect herbivores generally decreases from low to high latitudes. This decrease is routinely attributed to declines in herbivore abundance and/or diversity, whereas latitudinal changes in per capita food consumption remain virtually unknown. Here, we tested the hypothesis that the lifetime food consumption by a herbivore individual decreases from low to high latitudes due to a temperature-driven decrease in metabolic expenses. From 2016 to 2019, we explored latitudinal changes in multiple characteristics of linear (gallery) mines made by larvae of the pygmy moth, Stigmella lapponica, in leaves of downy birch, Betula pubescens. The mined leaves were larger than intact leaves at the southern end of our latitudinal gradient (at 60°N) but smaller than intact leaves at its northern end (at 69°N), suggesting that female oviposition preference changes with latitude. No latitudinal changes were observed in larval size, mine length or area, and in per capita food consumption, but the larval feeding efficiency (quantified as the ratio between larval size and mine size) increased with latitude. Consequently, S. lapponica larvae consumed less foliar biomass at higher latitudes than at lower latitudes to reach the same size. Based on space-for-time substitution, we suggest that climate warming will increase metabolic expenses of insect herbivores with uncertain consequences for plant–herbivore interactions.     

Genetic and soil-nutrient effects on the abundance of herbivores on willow

The effects of soil-nutrient environment, plant genotype, and the interaction between the two on the resistance of the willow, Salix sericea, to insect species in a diverse herbivore community was measured. We found that soil-nutrient environment influenced plant growth and the abundance of most herbivores of S. sericea. However, environmental effects on herbivore abundance were often modified by plant genetics; the abundance of four of seven herbivores exhibited significant genotypeby-environment interaction effects. Pure genotype effects were mostly small and non-significant. The effects of fertilization differed among herbivores. Several herbivores were more abundant on fertilized plants, one was less abundant, and the abundance of others did not change. We found that feeding guild was a poor predictor of herbivore response. Finally we found significant phenotypic and genetic correlations among growth rate, internode length, and the abundances of several herbivores.     

Herbivore damage-induced production and specific anti-digestive function of serine and cysteine protease inhibitors in tall goldenrod, Solidago altissima L. (Asteraceae)

Plant protease inhibitors (PIs) are among the most well-studied and widely distributed resistance traits that plants use against their herbivore attackers. There are different types of plant PIs which putatively function against the different types of proteases expressed in insect guts. Serine protease inhibitors (SPIs) and cysteine protease inhibitors (CPIs) are hypothesized to differentially function against the predominant gut proteases in lepidopteran and coleopteran herbivores, respectively. Here, we test the hypothesis that tall goldenrod, Solidago altissima, can specifically respond to damage by different herbivores and differentially induce SPIs and CPIs in response to damage by lepidopteran and coleopteran herbivores. Moreover, we ask if the concerted induction of different types of PIs accounts for variation in induced resistance to herbivory. We altered and optimized a rapid and effective existing methodology to quantitatively analyze both SPI and CPI activity simultaneously from a single tissue sample and to use the same plant extracts directly for characterization of inhibitory effects on insect gut protease activity. We found that both SPIs and CPIs are induced in S. altissima in response to damage, regardless of the damaging herbivore species. However, only SPIs were effective against Spodoptera exigua gut proteases. Our data suggest that plant PI responses are not necessarily specific to the identity of the attacking organism but that different components of generally induced defense traits can specifically affect different herbivore species. While providing an efficient and broadly applicable methodology to analyze multiple PIs extracted from the same tissue, this study furthers our understanding of specificity in induced plant resistance.     

Different effects of two exotic herbivores on the pollinator-mediated effect of an exotic plant on a native plant

Exotic plants can affect native plants indirectly through various biotic interactions. However, combinations of the multiple indirect effects of exotic plants on native plants have been rarely evaluated. Herbivory can either positively or negatively influence plant–pollinator interactions. Here, we addressed whether the pollinator-mediated plant interaction between exotic and native plants is altered through the introduction of exotic herbivores by conducting a 2-year common garden experiment. We compared the effects of pollinator-mediated indirect effects of an exotic plant, Solidago altissima, on the co-flowering native plant Aster microcephalus in geographically different populations reflecting differences in insect herbivore communities. We found a positive effect of co-flowering S. altissima on pollinator visitation of A. microcephalus, which varied between gardens and years. The co-flowering S. altissima did not significantly affect the seed set of A. microcephalus in the first year but had a negative effect in the second year. The facilitative effect of S. altissima on A. microcephalus pollination was suggested to be negatively affected by an exotic aphid, while it was not significantly affected by an exotic lace bug. Our study suggests that the phenology and feeding guilds of the herbivores may be critical for predicting the effect of exotic plants on native plants through herbivore–pollinator interactions. Integrated effects between plant interactions via multiple species interactions under different abiotic and biotic environments are necessary to understand the impact of exotic plants under complex interactions in nature.     

The jasmonate pathway alters herbivore feeding behaviour: consequences for plant defences

The jasmonate pathway is a highly conserved defensive cascade in plants that regulates the induction of resistance against herbivores; however, its role in herbivore feeding behaviour remains unknown. We used a mutant tomato plant (def‐1) deficient in the production of jasmonate‐related defensive proteins to test the hypothesis that genotypes with a reduced ability to induce resistance have a higher and more concentrated pattern of herbivore damage. Wild‐type and def‐1 plants received either damage by Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) caterpillars or no damage. After treatment, we tested for systemic responses by allowing a free roaming S. exigua caterpillar to feed on the undamaged portions of plants. Weight‐gain and leaf consumption of S. exigua were highest on def‐1 plants, regardless of prior herbivore damage. Def‐1 plants also had fewer numbers of leaves and leaflets eaten, and fewer feeding holes, which was indicative of a more concentrated distribution of damage on mutant compared to wild‐type plants. Following these results, we mimicked the amount and distribution of feeding damage that wild‐type or jasmonate‐deficient plants would receive on wild‐type plants to test whether changes in feeding behaviour may feedback to influence the expression of induced resistance. We mimicked the distribution of damage in wild‐type and jasmonate‐deficient plants by allowing caterpillars to feed on either one (leaf 1 or 2) or two leaves (leaf 1 and 2). Increased herbivore damage resulted in higher proteinase inhibitor (PI) activity, a jasmonate‐regulated defensive protein, and lower S. exigua performance on wild‐type but not jasmonate‐deficient plants. Compared to undamaged plants, a concentrated pattern of herbivore damage increased systemic resistance; these induced responses were greater on leaflets with stronger vascular connections to the damaged leaf. A more dispersed pattern of caterpillar damage altered the expression of induced responses, but the outcome depended on the specific pattern of damage. When leaf 1 was damaged and then leaf 2, the undamaged (third) leaf (which is more strongly connected to leaf 1 than 2) expressed reduced the PI activity compared to plants receiving concentrated damage to leaf 1; whereas in plants where leaf 2 was first damaged and then leaf 1, there were no differences in PI activity in leaf 3 compared to plants receiving concentrated damage to leaf 2. Thus, induction of the jasmonate pathway may not only determine the amount and distribution of feeding damage by herbivores, but this may feedback to affect the subsequent expression of plant defence.     

Community structure of insect herbivores on introduced and native Solidago plants in Japan

We compared community composition, density, and species richness of herbivorous insects on the introduced plant Solidago altissima L. (Asteraceae) and the related native species Solidago virgaurea L. in Japan. We found large differences in community composition on the two Solidago species. Five hemipteran sap feeders were found only on S. altissima. Two of them, the aphid Uroleucon nigrotuberculatum Olive (Hemiptera: Aphididae) and the scale insect Parasaissetia nigra Nietner (Hemiptera: Coccidae), were exotic species, accounting for 62% of the total individuals on S. altissima. These exotic sap feeders mostly determined the difference of community composition on the two plant species. In contrast, the herbivore community on S. virgaurea consisted predominately of five native insects: two lepidopteran leaf chewers and three dipteran leaf miners. Overall species richness did not differ between the plants because the increased species richness of sap feeders was offset by the decreased richness of leaf chewers and leaf miners on S. altissima. The overall density of herbivorous insects was higher on S. altissima than on S. virgaurea, because of the high density of the two exotic sap feeding species on S. altissima. We discuss the importance of analyzing community composition in terms of feeding guilds of insect herbivores for understanding how communities of insect herbivores are organized on introduced plants in novel habitats.     

Nitrogen and water affect direct and indirect plant systemic induced defense in cotton

Plants have direct and indirect constitutively produced and inducible defenses against herbivores and pathogens, which can substantially aid in their ability to defend themselves. However, very little is known about the influence of agronomic factors on such defenses. Here, we tested the effects of nitrogen levels and water availability on the ability of cotton plants to deter feeding by Spodoptera exigua through induction of anti-feedants, and to attract Microplitis croceipes through systemic induction of volatile emission. Cotton plants were grown with various nitrogen levels and were either exposed to water stress or normal water before being exposed to S. exigua for 48 h for induction of defenses. Dual choices of various nitrogen and water treatments were provided to M. croceipes in flight tunnel bioassays. Dual choices of leaf tissue from the various nitrogen and water treatments were provided to S. exigua larvae. Both water stress and nitrogen levels under and over the recommended levels increased leaf tissue consumption and decreased attraction of M. croceipes to the plants. Analyses of induced volatiles released from herbivore damaged plants indicate that their concentrations differ among the nitrogen levels tested with plants receiving no nitrogen or twice the recommended dose having amounts much lower than plants receiving the recommended dose. Because both direct and indirect plant defense mechanisms are negatively affected by improper nitrogen and insufficient water, we argue that these factors should be considered for a better natural control of pests in cotton and most probably in other crops.     

Density dependence in insect performance within individual plants: induced resistance to Spodoptera exigua in tomato

Net intraspecific density dependence experienced by insect herbivores at the scale of single plants can be a function both of induced resistance in the plant and other interactions among individual herbivores. Theory suggests that non‐linearity in the form of this density dependence can influence the effects of plants on herbivore population dynamics. This study examined both net density dependence at the scale of single plants, and changes in plant quality with herbivore density for Spodoptera exigua caterpillars on tomato plants. One experiment measured the growth of caterpillars moving freely about the plant at different densities, the distribution of damage by these caterpillars, and the quality of the plant as food for caterpillars (growth of caterpillars on undamaged leaf tissue excised from the plant). A second experiment measured plant quality for plants with different amounts of damage by caterpillars confined to particular leaves in mesh bags. Growth of S. exigua caterpillars was found to be negatively density dependent, and this was in part due to decreases in plant quality both as herbivore density increased and as the amount of damage increased. The response of plant quality to herbivores was found to have non‐linear features; there was both a threshold below which no significant decreases in quality (as measured by herbivore growth) occurred, and the decrease in herbivore performance saturated at the highest damage levels. In addition, it was found that caterpillar damage was significantly more aggregated than expected when multiple caterpillars occupy a single plant. This study confirms that host plants have the potential to be a source of density dependence that affects herbivore performance.     

Can genetically based clines in plant defence explain greater herbivory at higher latitudes?

Greater plant defence is predicted to evolve at lower latitudes in response to increased herbivore pressure. However, recent studies question the generality of this pattern. In this study, we tested for genetically based latitudinal clines in resistance to herbivores and underlying defence traits of Oenothera biennis. We grew plants from 137 populations from across the entire native range of O. biennis. Populations from lower latitudes showed greater resistance to multiple specialist and generalist herbivores. These patterns were associated with an increase in total phenolics at lower latitudes. A significant proportion of the phenolics were driven by the concentrations of two major ellagitannins, which exhibited opposing latitudinal clines. Our analyses suggest that these findings are unlikely to be explained by local adaptation of herbivore populations or genetic variation in phenology. Rather greater herbivory at high latitudes can be explained by latitudinal clines in the evolution of plant defences.     

Competition among herbivores of Solanum carolinense as a constraint on the evolution of host-plant resistance

Interspecific competition (or facilitation) between herbivores sharing a host plant species can result in negative (or positive) correlations in damage levels, independent of a plant’s genetic covariance in resistance to the herbivores. Just like genetic correlations in resistance to herbivory, these “environmental correlations” in damage may affect the evolution of resistance in the host plant. In a field study of 960 ramets and 40 genets of the herbaceous plant Solanum carolinense, I looked for evidence of such environmental correlations in damage caused by 11 species of herbivores, including 10 insects and 1 mammal. There were 28 significant correlations in damage levels between species (21 negative and 7 positive) after plant genetic influences on resistance were statistically removed. Negative environmental correlations were more likely between species that fed upon the same type of plant organs than between those that fed on different types of organs, and the magnitudes of the correlations were inversely proportional to the abundance of the organ types. Taken together, these results offer strong evidence that competition is largely responsible for the pattern of environmental correlations in damage. Environmental correlations were just as common as genetic correlations in resistance, but the environmental correlations tended to be lower in magnitude, were more likely to be negative, and were more evenly spread out among the herbivore community than the genetic correlations. Damage levels by all 11 species were negatively correlated with damage by at least one other species. Thus the selective advantage a plant would receive from increased resistance any of these herbivores would be partially negated by increased damage by competing herbivores. As a result, competition has the potential to be an important constraint on the evolution of resistance in S. carolinense.     

Corymbia leaf oils,latitude, hybrids and herbivory: A test using common‐garden field trials

Foliar oils, particularly monoterpenes, can influence the susceptibility of plants to herbivory. In plants, including eucalypts, monoterpenes are often associated with plant defence. A recent analysis revealed an increase in foliar oil content with increasing latitudinal endemism, and we tested this pattern using three eucalypt taxa comprising a latitudinal replacement cline. We also examined the relative concentrations of two monoterpenes (α‐pinene and 1,8‐cineole), for which meta‐analyses also showed latitudinal variation, using hybrids of these three taxa with Corymbia torelliana. These, and pure C. torelliana, were then assessed in common‐garden field plots for the abundance and distribution of herbivory by four distinct herbivore taxa. Differing feeding strategies among these herbivores allowed us to test hypotheses regarding heritability of susceptibility and relationships to α‐pinene and 1,8‐cineole. We found no support for an increase in foliar oil content with increasing latitude, nor did our analysis support predictions for consistent variation in α‐pinene and 1,8‐cineole contents with latitude. However, herbivore species showed differential responses to different taxa and monoterpene contents. For example, eriophyid mites, the most monophagous of our censused herbivores, avoided the pure species, but fed on hybrid taxa, supporting hypotheses on hybrid susceptibility. The most polyphagous herbivore (leaf blister sawfly Phylacteophaga froggatti) showed no evidence of response to plant secondary metabolites, while the distribution and abundance patterns of Paropsis atomaria showed some relationship to monoterpene yields.     

Disarming the paradox of sublethal plant defense against insects: Trirhabda virgata larval development time and leaf tissue loss on Solidago altissima

Although the fitness benefits of traits that kill herbivores are obvious, the contention that sublethal antiperformance traits have evolved as plant defenses has proved more controversial. Traits that slow herbivore development seem particularly paradoxical, given the common assumption that a protracted feeding period will lead to greater total consumption. Whereas this assumption is superficially reasonable, there is very little evolutionarily relevant evidence to suggest that plants on which larval development is slower actually lose more tissue. For the assumption underlying the sublethal‐defense paradox to be valid, plant traits that affect larval development time and tissue loss must be positively correlated genetically within natural plant populations. In this study, we examined the relationship between larval development time of the beetle Trirhabda virgata LeConte (Coleoptera: Chrysomelidae) and plant tissue loss by its host plant Solidago altissima L. (Asteraceae). Plant genets on which the larval development time was longer ended up losing less leaf area than plant genets that allowed quicker larval development. This negative genetic correlation contradicts the common assumption that greater sublethal resistance leads to increased tissue loss. Combined with other hypothesized benefits of sublethal resistance, this result suggests that antiperformance traits may constitute a more potent form of resistance than is generally acknowledged.     

Latitudinal patterns of herbivore pressure in a temperate herb support the biotic interactions hypothesis

The longstanding biotic interactions hypothesis predicts that herbivore pressure declines with latitude, but the evidence is mixed. To address gaps in previous studies, we measured herbivory and defence in the same system, quantified defence with bioassays, and considered effects of leaf age. We quantified herbivory and defence of young and mature leaves along a continental gradient in eastern North America in the native herb Phytolacca americana L. Herbivory in the field declined with latitude and was strongly correlated with lepidopteran abundance. Laboratory bioassays revealed that leaf palatability was positively correlated with latitude of origin. Young leaves were more damaged than mature leaves at lower latitudes in the field, but less palatable in bioassays. Both defence and palatability displayed non‐linear latitudinal patterns, suggesting potential mechanisms based on biological or climatic thresholds. In sum, observational and experimental studies find patterns consistent with high herbivore pressure and stronger plant defences at lower latitudes.     

Genetic mapping shows intraspecific variation and transgressive segregation for caterpillar‐induced aphid resistance in maize

Plants in nature have inducible defences that sometimes lead to targeted resistance against particular herbivores, but susceptibility to others. The metabolic diversity and genetic resources available for maize (Zea mays) make this a suitable system for a mechanistic study of within‐species variation in such plant‐mediated interactions between herbivores. Beet armyworms (Spodoptera exigua) and corn leaf aphids (Rhopalosiphum maidis) are two naturally occurring maize herbivores with different feeding habits. Whereas chewing herbivore‐induced methylation of 2,4‐dihydroxy‐7‐methoxy‐1,4‐benzoxazin‐3‐one glucoside (DIMBOA‐Glc) to form 2‐hydroxy‐4,7‐dimethoxy‐1,4‐benzoxazin‐3‐one glucoside (HDMBOA‐Glc) promotes caterpillar resistance, lower DIMBOA‐Glc levels favour aphid reproduction. Thus, caterpillar‐induced DIMBOA‐Glc methyltransferase activity in maize is predicted to promote aphid growth. To test this hypothesis, the impact of S. exigua feeding on R. maidis progeny production was assessed using seventeen genetically diverse maize inbred lines. Whereas aphid progeny production was increased by prior caterpillar feeding on lines B73, Ki11, Ki3 and Tx303, it decreased on lines Ky21, CML103, Mo18W and W22. Genetic mapping of this trait in a population of B73 × Ky21 recombinant inbred lines identified significant quantitative trait loci on maize chromosomes 1, 7 and 10. There is a transgressive segregation for aphid resistance, with the Ky21 alleles on chromosomes 1 and 7 and the B73 allele on chromosome 10 increasing aphid progeny production. The chromosome 1 QTL coincides with a cluster of three maize genes encoding benzoxazinoid O‐methyltransferases that convert DIMBOA‐Glc to HDMBOA‐Glc. Gene expression studies and benzoxazinoid measurements indicate that S. exigua ‐induced responses in this pathway differentially affect R. maidis resistance in B73 and Ky21.     

Drought changes plant chemistry and causes contrasting responses in lepidopteran herbivores

Drought events are predicted to increase due to climate change, yet consequences for plant–insect interactions are only partially understood. Drought‐mediated interactions between herbivores and their host plants are affected by a combination of factors, including characteristics of the affected plant, its associated herbivore and of the prevailing drought. Studying the effect of these factors in combination may provide important insight into plant and herbivore responses to drought. We studied drought effects on plant resistance to two leaf‐chewing herbivores by considering differing growth conditions, plant chemistry and insect responses in concert. We exposed Alliaria petiolata plants from several wild populations to different intensities of intermittent drought stress and quantified drought‐mediated changes in plant chemistry. Simultaneously, we assessed behavior (feeding preference) and performance of two lepidopteran herbivores: Pieris brassicae, a specialist, and Spodoptera littoralis, a generalist. Drought led to lowest concentrations of secondary defense compounds in severely stressed plants, without affecting total nitrogen content. Additionally, drought evoked opposite patterns in feeding preferences (plant palatability) between the herbivore species. Pieris brassicae consumed most of well‐watered plants, while S. littoralis preferred severely drought‐stressed plants. Hence, feeding preferences of S. littoralis reflected changes in plant secondary chemistry. Contrary to their feeding preference, P. brassicae performed better on drought‐stressed than on well‐watered plants, with faster development and higher attained pupal mass (plant suitability). Spodoptera littoralis showed retarded development in all treatments. In conclusion, drought caused plant secondary defense compounds to decrease consistently across all studied plant populations, which evoked contrasting feeding preferences of two herbivore species of the same feeding guild. These results suggest herbivore specificity as a possible explanation for herbivore responses to drought and emphasize the importance of herbivore characteristics such as feeding specialization in understanding and predicting consequences of future drought events.