Nancy E. Stamp

OUT OF THE QUAGMIRE OF PLANT DEFENSE HYPOTHESES

Several hypotheses, mainly Optimal Defense (OD), Carbon:Nutrient Balance (CNB), Growth Rate (GR), and Growth‐Differentiation Balance (GDB), have individually served as frameworks for investigating the patterns of plant defense against herbivores, in particular the pattern of constitutive defense. The predictions and tests of these hypotheses have been problematic for a variety of reasons and have led to considerable confusion about the state of the “theory of plant defense.” The primary contribution of the OD hypothesis is that it has served as the main framework for investigation of genotypic expression of plant defense, with the emphasis on allocation cost of defense. The primary contribution of the CNB hypothesis is that it has served as the main framework for investigation of how resources affect phenotypic expression of plant defense, often with studies concerned about allocation cost of defense. The primary contribution of the GR hypothesis is that it explains how intrinsic growth rate of plants shaped evolutionarily by resource availability affects defensive patterns. The primary contribution of the expanded GDB hypothesis is that it recognizes the constant physiological tradeoff between growth and differentiation at the cellular and tissue levels relative to the selective pressures of resource availability, including explicitly taking into account plant tolerance of damage by enemies. A clearer understanding of these hypotheses and what we have learned from investigations that use them can facilitate development of well‐designed experiments that address the gaps in our knowledge of plant defense.

EFFECTS OF PLANT AGE, GENOTYPE, AND HERBIVORY ON PLANTAGO PERFORMANCE AND CHEMISTRY'

Plant performance and chemistry may vary due to a variety of factors, such as plant genotype, environmental conditions, presence of herbivores, timing of herbivory, and species of herbivore. The relative importance of these factors, and how the plants respond to them, may affect the dynamics of the plant population, as well as the insect herbivores feeding on those plants. To understand the relative importance of some of these factors on plant performance and chemistry, we used Plantago lanceolata L. (Plantagina- ceae). In an experimental garden at Binghamton, New York, we examined the effects of plant age, plant genotype, and herbivory by generalist or specialist caterpillars on P. lan- ceolata. There were two parts to this experiment. In the first, we examined variation in nutritional quality (nitrogen) and defensive chemistry (the iridoid glycosides aucubin and catalpol) as a consequence of plant age, plant genotype, and leaf age class. We compared these parameters for a set of four pairs of plants of each of five genotypes that were not exposed to herbivores, and were harvested in early July (control-start plants) with another set harvested 6 wk later in mid-August (control-end plants). The older plants harvested in August (control-end plants) had concentrations of aucubin, catalpol, total iridoid glycosides, and nitrogen approximately one-half that of plants harvested 6 wk earlier. Leaf age affected all of these variables, and plant genotype influenced the iridoid glycoside variables but not the nitrogen concentration. Overall, leaf age explained twice as much of the variation in iridoid glycosides as did plant age, which accounted for twice as much variation as did plant genotype. The second part of the experiment compared the effects of herbivory, genotype, and leaf age on plant performance (leaf biomass, scape (flower stalk) biomass, and total biomass) and chemistry (nitrogen, protein, and iridoid glycoside concentration). We compared these measures for a set of four pairs of replicate plants of each of five genotypes, exposed to one of three herbivory treatments: no herbivory, herbivory by specialist caterpillars, and herbivory by generalist caterpillars. The results of this experiment showed that herbivory had little effect on plant performance, and there was no difference due to herbivory by the specialist Junonia coenia Hibner (Nymphalidae), compared to the generalist Spilosoma congrua Wlk. (Arctiidae). However, plant chemistry was significantly affected by herbivory. Herbivory by both caterpillar species induced iridoid glycosides and resulted in an increased concentration of catalpol and an increase in the proportion of catalpol relative to total iridoid glycosides in those plants exposed to herbivores compared to controls. In addition, plants exposed to specialist caterpillars had higher concentrations of catalpol and a higher proportion of total iridoids that was catalpol than those exposed to generalist caterpillars. Overall, in this experiment, leaf age explained three times as much of the variation in iridoid glycosides as did plant genotype and herbivory. The results of the two parts of this experiment show that iridoid glycoside concentration in P. lanceolata leaves decreases over time, and that exposure to herbivores induces increased concentrations of iridoid glycosides.

VARIATION IN FOOD QUALITY AND TEMPERATURE CONSTRAIN FORAGING OF GREGARIOUS CATERPILLARS

We examined the separate and combined effects of changing food quality, thermal environment, and larval behavior on the foraging pattern of a gregarious caterpillar, Hemileuca lucina (Saturniidae), which specializes on the shrub Spiraea latifolia (Rosaceae). First-instar larvae, which encounter only new leaves under field conditions, showed no preference for new leaves over mature ones. In contrast, third-instar larvae, which normally encounter both leaf types, exhibited a preference for new leaves, in both 12-h and instar- length choice tests. In laboratory tests, larvae in groups were more likely to survive and exhibited less variation in biomass gained than solitary larvae. An experiment was con- ducted to determine the interactions of temperature (with daytime temperatures of 200, 250, or 300C), group size (solitary or in groups of 10), and diet (new or mature leaves) on third-instar larvae. The effect of diet and group size on relative growth rate depended on the daytime temperature. At a daytime temperature of 200, representing overcast and cool spring conditions, growth rates were similar for all group size-diet combinations. Thus, at such a cool temperature, there was no advantage in feeding on new leaves or in larval groups. In contrast, solitary larvae fed mature leaves gained less mass and developed more slowly than the other group size-diet combinations at the mean maximal day temperature (250) for field conditions. At 300, representing the average temperature reached through basking, larvae eating new leaves grew significantly faster than those on mature leaves, regardless of larval group size. These results indicate that in the absence of predators, in which case larvae eat new leaves, bask, and remain aggregated, the larvae can grow twice as fast and attain more biomass per instar than solitary larvae eating mature leaves in the shade, which is often the case when they avoid predators.

Chemical variation within and between individuals ofPlantago lanceolata (Plantaginaceae).

Variation in concentrations of leaf nitrogen and iridoid glycosides was examined in replicate plants of five genotypes ofPlantago lanceolata (Plantaginaceae) grown in an experimental garden. Nitrogen concentration and iridoid glycoside concentration were affected by leaf age. New leaves had nitrogen concentrations 1.7 to 2.7 times higher than mature leaves. Catalpol concentration was highest in new and intermediate-aged leaves. The concentration of aucubin, the biosynthetic precursor to catalpol, was higher in intermediate-aged leaves than in mature leaves, in three of five genotypes. Consequently, the proportion of aucubin relative to total iridoid glycosides increased as leaves aged. Concentration of iridoid glycosides was not correlated with plant size. Plant genotype significantly affected concentration of nitrogen and iridoid glycosides, as well as plant size. Thus, major indicators of hostplant quality for insect herbivores varied considerably both within and among plant genotypes and individuals.

DIFFERENTIAL RESPONSES OF GROWTH AND TWO SOLUBLE PHENOLICS OF TOMATO TO RESOURCE AVAILABILITY

We conducted two experiments to determine how resources influenced the intraspecific and within-plant allocation by tomato plants (Lycopersicon esculentum) to the soluble phenolics rutin and chlorogenic acid. We also measured the effect of resource availability on growth by measuring mass and other physical and cellular attributes of the plant. In the first experiment, we subjected plants to four levels of potassium nitrate fertilizer. In the second experiment, we subjected plants to high and low levels of potassium nitrate fertilizer and light. Both experiments yielded results consistent with the growth-differen- tiation balance hypothesis. Plants grown with low resources showed low levels of soluble phenolics and low plant mass. Plants grown with intermediate levels of resources showed high phenolic concentrations but inhibited growth. Plants grown with high resources had high growth but no increases in phenolic concentrations. The results were also consistent with the general prediction of the optimal defense hypothesis that there should be a negative relationship between growth and defense. We discuss possible adaptive explanations for the specific patterns observed. There were also consistent within-plant differences in phe- nolic concentrations. These differences in phenolic concentrations were large enough to have potential consequences for insect herbivores feeding on tomato plants.

Direct and indirect effects of predatory wasps (Polistes sp.: Vespidae) on gregarious caterpillars (Hemileuca lucina: Saturniidae).

SummaryWe examined how predation by vespid wasps,Polistes dominulus andP. fuscatus, affected the behavior, growth rate and survivorship of aggregated caterpillars ofHemileuca lucina (Saturniidae). Although these larvae can exhibit a variety of defense and escape behaviors, in general larvae reacted to wasp attacks by clinging to the hostplant. Neighboring larvae in the aggregation responded by leaving the feeding site and moving to the interior or base of the plant. To determine wheter wasp attack affected the behavior and growth of the caterpillars that escaped, a field experiment was conducted with treatments of: 1) larvae exposed to wasps, 2) larvae protected from wasps, and 3) larvae protected from wasps but with the attack of wasps simulated (=harassment). Over just one instar, protected larvae gained significantly more weight than the harassed larvae, which in turn weighed significantly more than the larvae that escaped the wasps. The behavior of attacked and harassed larvae differed from that of the protected larvae; the disturbed larvae often fed in smaller groups and in shaded portions of the plant where only mature leaves were available. A laboratory experiment showed that at 35° C (daytime temperature) larvae had significantly higher relative growth rates and significantly shorter instar duration than larvae reared at 25° C. Our results suggest that wasps, in addition to killing caterpillars, indirectly affect larval fitness by slowing larval growth, at least in part by forcing larvae into cooler microhabitats where leaves are of lower quality.

Early stage of host range expansion by a specialist herbivore, Euphydryas phaeton (Nymphalidae)

We examined whether larval and adult behavior, physiology, and chemical defense were altered as a result of host range expansion by the Baltimore checkerspot (Euphydryas phaeton, Nymphalidae) from the native host plant, turtlehead (Cheloneglabra, Scrophulariaceae), to the introduced weed, plantain (Plantago lanceolata, Plantaginaceae). We found that newly hatched larvae from eggs collected from a population using plantain were heavier than those from a population using turtlehead. Nonetheless, both the pre- diapause and postdiapause larvae derived from the turtlehead population and fed turtlehead in a laboratory experiment gained more mass than those from the plantain population fed plantain. Collections of diapausing larvae from field sites corroborated that pattern. Re- gardless of the population source (i.e., those using either turtlehead or plantain), postdia- pause larvae reared on turtlehead exhibited higher relative growth rate (RGR), efficiency of conversion of ingested food (ECI), and efficiency of conversion of digested food (ECD) than those fed plantain, even though approximate digestibility (AD) and leaf nitrogen concentrations were lower for turtlehead. Regardless of their population of origin, choice tests showed that newly hatched larvae preferred turtlehead. Likewise, adult females reared from larvae collected from both populations oviposited exclusively on turtlehead. Both C. glabra and P. lanceolata contain iridoid glycosides. The iridoid glycoside profile of but- terflies reared on these two plants differed, reflecting the differences of the host plants. The shift of some populations of E. phaeton onto plantain is most likely a function of several ecological factors rather than genetic differentiation between populations using turtlehead and plantain.

Enemy-free space via host plant chemistry and dispersion: assessing the influence of tri-trophic interactions

It has been argued that generalist natural enemies of insect herbivores provide a major selection pressure for restricted host plant range. This idea is a subset of the enemy-free space (EFS) hypothesis, whereby insect herbivores escape their enemies by being scarce in space and time and/or chemically defended via containing plant allelochemicals. To date, there are only two complete tests of EFS via host plant chemistry and two via host plant dispersion, and only two of these tests support the EFS hypothesis. However, three corollaries to existing views on EFS are sufficiently supported by data to warrant direct testing of the view that EFS is obtained via host plant chemistrys effects on enemies of insect herbivores. So the issue remains. Resolution will require a more collaborative, methodological approach to examine the relative importance of the major multiple factors that shape patterns of feeding specialization of insect herbivores. Predation is certainly one of these factors, but its role is still not clear.

Growth versus molting time of caterpillars as a function of temperature, nutrient concentration and the phenolic rutin

SummaryA factorial experiment tested the effects of varying nutrient concentration (normal versus diluted), presence or absence of the phenolic allelochemical rutin and daytime temperature (20, 25 and 30° C) on growth, molting and food utilization efficiencies of tobacco hornworms (Manduca sexta). Two of the utilization efficiencies (approximate digestibility and efficiency of conversion of ingested food) were unaffected by temperature; the third one, efficiency of conversion of digested food, was affected by temperature but there was no consistent effect. Lower temperatures significantly increased the proportion of the stadium spent molting, with larvae at a daytime temperature of 20° C spending 9% more of the stadium in molting than larvae at 30° C. Growth time was not influenced by nutrient concentration. When temperature was low and rutin absent, molt time and the proportion of the stadium spent molting were affected by nutrient concentration. Addition of the phenolic rutin did not have an appreciable effect on growth time or digestive processes. However, it increased molting time by 7 to 14% and thus increased the duration of the stadium and reduced relative growth rate. These results indicate that the effect of food quality on growth rate is a function of the thermal conditions of insect herbivores. At cooler temperatures, a disproportionate increase in time spent molting, rather than altered food utilization efficiencies, contributed to lower growth rates. The consequences for larval growth of fluctuating temperatures due to diurnal cycles and the presence of predators forcing larvae into thermally suboptimal microhabitats are discussed.

Response of Insect Herbivores to Multiple Allelochemicals Under Different Thermal Regimes

Temperature and food quality are well known to constrain the growth of insect herbivores, but less is known about how these factors may interact. We examined the effects of three tomato allelochemicals (chlorogenic acid, rutin, and tomatine) and two thermal regimes (210: 10?C and 26?: 15?C, representing spring and summer respectively) on three insect herbivores: a Solanaceae specialist, Manduca sexta, and the polyphagous Hel- icoverpa zea and Spodoptera exigua. The alkaloid tomatine slowed development of the polyphagous H. zea, but the two phenolics had little effect. Development of the polyphagous S. exigua was faster with tomatine at the cool thermal regime, and slower with the two phenolics together at the warm thermal regime. All three of the tomato allelochemicals slowed development of the specialist M. sexta. Values of approximate digestibility (or assimilation efficiency) for the two polyphagous species spanned a range twice that of the Solanaceae specialist, increasing when allelo- chemicals were added to diet. Our techniques permitted us to separate pre- and post-ingestive effects of allelochemicals on mass gained, but we found complex interaction of allelo- chemicals and of temperature and allelochemicals. The effects of an allelochemical on larval performance often depended on the combi- nation of other allelochemicals in the diet. But the allelochemicals did not always have a negative effect, and multiple allelochemicals did not always have a more negative effect than a single allelochemical. Interaction between thermal regime and the allelochemicals was also common. For example, whether rutin had a negative, neutral, or positive effect on relative growth rate of S. exigua depended on thermal regime and presence or absence of chlorogenic acid. Often the negative effect of an allelochemical was greater at the warm regime than at the cool regime, but not consistently so. Another experiment showed that thermal regime did not affect allelochemical concen- tration in the diets. Over time (7 d), only tomatine concentration declined. Therefore, the interactive effects of thermal regime and allelochemicals on the insect herbivores were not an artifact of the experimental procedures. Overall, the results indicated that insect performance is not a simple function of tem- perature and food quality because interactions occur between these factors. They also suggest that the array of chemical defenses exhibited by a particular plant species may be shaped in part by temperature altering the efficacy of allelochemicals.