Different photosynthesis-nitrogen relations in deciduous hardwood and evergreen coniferous tree species

Rapidly induced responses can alter host plant suitability for insect growth and survival. The effects of defoliation on the suitability of potted 5-year-old red pine, Pinus resinosa Ait., for the sawfly Neodiprion sertifer (Hymenoptera: Diprionidae), were measured in two experiments. In the first, overall larval growth rate increased on seedlings within 8 days of low (<15%) defoliation. Suitability varied among larval age groups: defoliation increased the performance of older larvae, but not that of young larvae. In the second experiment, larval survival and weight varied non-linearly with defoliation intensity 8 weeks after treatment. Similar responses were observed following artificial and natural defoliation, and on early- and late-season seedlings. These results suggest that some evergreen conifers can respond rapidly to defoliation injury, and that herbivores may simultaneously acclimate to the induced response as they develop. The relative importance of induced response rates to plant-insect interactions is discussed.     

Red foliage color reliably indicates low host quality and increased metabolic load for development of an herbivorous insect

Plant chemical defense and coevolved detoxification mechanisms in specialized herbivorous insects are fundamental in determining many insect–plant interactions. For example, Brassicale plants protect themselves from herbivory by producing glucosinolates, but these secondary metabolites are effectively detoxified by larvae of Pierid butterflies. Nevertheless, not all Brassicales are equally preferred by these specialist herbivores. Female Pieris butterflies avoid laying eggs on anthocyanin-rich red foliage, suggesting red color is a visual cue affecting oviposition behavior. In this study, we reared P. brassicae larvae on green and red cabbage leaves, to determine whether foliage color reliably indicates host plant quality. We did not find a difference in survival rates or maximal larval body mass in the two food treatments. However, larvae feeding on red cabbage leaves exhibited significantly lower growth rates and longer durations of larval development. Interestingly, this longer development was coupled with a higher consumption rate of dry food matter. The lower ratio of body mass gain to food consumption in larvae feeding on red cabbage leaves was coupled with significantly higher (ca. 10 %) larval metabolic rates. This suggests that development on red foliage may incur an increased metabolic load associated with detoxification of secondary plant metabolites. Energy and oxygen allocation to detoxification could come at the expense of growth and thus compromise larval fitness as a result of extended development. From an evolutionary perspective, red foliage color may serve as an honest defensive cue, as it reliably indicates the plant’s low quality as a substrate for larval development.     

Facilitative interactions between two lepidopteran herbivores of Asimina

Summary Insect herbivores that require young foliage for successful larval development are often restricted to a single generation during a year by the scarcity of suitable food over most of the growing season. The major specialist herbivore attacking shrubs in the genus Asimina in Florida, Eurytides marcellus, requires young foliage for successful larval development. Field manipulations were used to investigate the role of the young foliage produced by Asimina in response to defoliation by the late-season feeder Omphalocera munroei, a second specialist herbivore of Asimina in Florida, in maintaining Eurytides populations during the summer months when young foliage is otherwise scarce. Defoliation by Omphalocera proved to be the major inducer of young growth during the summer because Omphalocera defoliated Asimina shrubs so frequently and severely. When compared to young leaves produced in the absence of damage, the teaves produced by Asimina in response to defoliation were equally as suitable as food for Eurytides larvae and as acceptable as oviposition sites by Eurytides females. The availability of young foliage in an Asimina population was correlated with the size of the associated Eurytides population. The combination of regular, severe defoliation by Omphalocera and lack of a defensive response to damage by Asimina lead to a positive affect of Omphalocera on Eurytides population size, and may be central to other facilitative interactions between herbivores as well.     

Elevated CO2 levels and herbivore damage alter host plant preferences

Interactions between the moth Spodoptera littoralis and two of its host plants, alfalfa (Medicago sativa) and cotton (Gossypium hirsutum) were examined, using plants grown under ambient (350 ppm) and elevated (700 ppm) CO₂ conditions. To determine strength and effects of herbivore‐induced responses assays were performed with both undamaged (control) and herbivore damaged plants. CO₂ and damage effects on larval host plant preferences were determined through dual‐choice bioassays. In addition, larvae were reared from hatching to pupation on experimental foliage to examine effects on larval growth and development. When undamaged plants were used S. littoralis larvae in consumed more cotton than alfalfa, and CO₂ enrichment caused a reduction in the preference for cotton. With damaged plants larvae consumed equal amounts of the two plant species (ambient CO₂ conditions), but CO₂ enrichment strongly shifted preferences towards cotton, which was then consumed three times more than alfalfa. Complementary assays showed that elevated CO₂ levels had no effect on the herbivore‐induced responses of cotton, whereas those of alfalfa were significantly increased. Larval growth was highest for larvae fed undamaged cotton irrespectively of CO₂ level, and lowest for larvae on damaged alfalfa from the high CO₂ treatment. Development time increased on damaged cotton irrespectively of CO₂ treatment, and on damaged alfalfa in the elevated CO₂ treatment. These results demonstrate that elevated CO₂ levels can cause insect herbivores to alter host plant preferences, and that effects on herbivore‐induced responses may be a key mechanism behind these processes. Furthermore, since the insects were shown to avoid foliage that reduced their physiological performance, our data suggest that behavioural host plant shifts result in partial escape from negative consequences of feeding on high CO₂ foliage. Thus, CO₂ enrichment can alter both physiology and behaviour of important insect herbivores, which in turn may to impact plant biodiversity.     

Effect of nitrogen fertilization upon the secondary chemistry and nutritional value of quaking aspen (Populus tremuloides Michx.) leaves for the large aspen tortrix (Choristoneura conflictana (Walker))

Summary We investigated the effects of nitrogen fertilization upon the concentrations of nitrogen, condensed tannin and phenolic glycosides of young quaking aspen (Populus tremuloides) leaves and the quality of these leaves as food for larvae of the large aspen tortrix (Choristoneura conflictana), a Lepidopteran that periodically defoliates quaking aspen growing in North America. Nitrogen fertilization resulted in decreased concentrations of condensed tannin and phenolic glycosides in aspen leaves and an increase in their nitrogen concentration and value as food for the large aspen tortrix. These results indicate that plant carbon/nutrient balance influences the quality of aspen leaves as food for the large aspen tortrix in two ways, by increasing the concentrations of positive factors (e.g. nitrogen) and decreasing the concentrations of negative factors (eg. carbon-based secondary metabolites) in leaves. Addition of purified aspen leaf condensed tannin and a methanol extract of young aspen leaves that contained condensed tannin and phenolic glycosides to artificial diets at high and low levels of dietary nitrogen supported this hypothesis. Increasing dietary nitrogen increased larval growth whereas increasing the concentrations of condensed tannin and phenolic glycosides decreased growth. Additionally, the methanol extract prevented pupation. These results indicate that future studies of woody plant/insect defoliator interactions must consider plant carbon/nutrient balance as a potentially important control over the nutritional value of foliage for insect herbivores.     

Intra-plant host selection, oviposition preference and larval survival of Chrysophtharta agricola (Chapuis) (Coleoptera: Chrysomelidae: Paropsini) between foliage types of a heterophyllous host

Abstract 1 Paropsine chrysomelid beetles defoliate commercial eucalypt plantations in Australia. Adults and larvae feed on the same host, with the larval food source determined by the oviposition choice of females. Most eucalypt species are heterophyllous, with their foliage undergoing distinct morphological and chemical changes between adult and juvenile growth. 2 The intra‐plant foliage feeding and oviposition preference adults and the larval development of Chrysophtharta agricola were examined using adult and juvenile foliage of a heterophyllous plantation species, Eucalyptus nitens. The foliage types differ in chemistry, toughness, waxiness and timing of production. 3 In the field, feeding damage caused by adult beetles was 15% more frequent on adult foliage than on juvenile foliage; however, egg batches were three times more common on juvenile than on adult foliage. 4 Oviposition preference for juvenile foliage over adult foliage was confirmed in choice trials in the laboratory, with adult fecundity and longevity not significantly different between foliage types. 5 Larval survival, development time and subsequent pupal weight were also unaffected by foliage type, suggesting that neither foliage type is nutritionally superior for adults or for larvae. However, adult foliage was significantly thicker than juvenile foliage and this may prove a physical constraint to larval establishment. Biotic and abiotic factors (including interactions with natural enemies, competition, microclimate and mate location) that may affect patterns of host plant utilization are discussed.     

Interactions between above- and belowground insect herbivores as mediated by the plant defense system

Plants are frequently attacked by both above- and belowground arthropod herbivores. Nevertheless, studies rarely consider root and shoot herbivory in conjunction. Here we provide evidence that the root-feeding insect Agriotes lineatus reduces the performance of the foliage feeding insect Spodoptera exigua on cotton plants. In a bioassay, S. exigua larvae were allowed to feed on either undamaged plants, or on plants that had previously been exposed to root herbivory, foliar herbivory, or a combination of both. Previous root herbivory reduced the relative growth rates as well as the food consumption of S. exigua by more than 50% in comparison to larvae feeding on the undamaged controls. We found no effects in the opposite direction, as aboveground herbivory by S. exigua did not affect the relative growth rates of root-feeding A. lineatus . Remarkably, neither did the treatment with foliar herbivory affect the food consumption and relative growth rate of S. exigua in the bioassay. However, this treatment did result in a significant change in the distribution of S. exigua feeding. Plants that had been pre-exposed to foliar herbivory suffered significantly less damage on their young terminal leaves. While plant growth and foliar nitrogen levels were not affected by any of the treatments, we did find significant differences between treatments with respect to the level and distribution of plant defensive chemicals (terpenoids). Exposure to root herbivores resulted in an increase in terpenoid levels in both roots as well as in mature and immature foliage. Foliar damage, on the other hand, resulted in high terpenoid levels in young, terminal leaves only. Our results show that root-feeding herbivores may change the level and distribution of plant defenses aboveground. Our data suggest that the reported interactions between below- and aboveground insect herbivores are mediated by induced changes in plant secondary chemistry.     

A plant pathogen modulates the effects of secondary metabolites on the performance and immune function of an insect herbivore

Host plant chemical composition critically shapes the performance of insect herbivores feeding on them. Some insects have become specialized on plant secondary metabolites, and even use them to their own advantage such as defense against predators. However, infection by plant pathogens can seriously alter the interaction between herbivores and their host plants. We tested whether the effects of the plant secondary metabolites, iridoid glycosides (IGs), on the performance and immune response of an insect herbivore are modulated by a plant pathogen. We used the IG‐specialized Glanville fritillary butterfly Melitaea cinxia, its host plant Plantago lanceolata, and the naturally occurring plant pathogen, powdery mildew Podosphaera plantaginis, as model system. Pre‐diapause larvae were fed on P. lanceolata host plants selected to contain either high or low IGs, in the presence or absence of powdery mildew. Larval performance was measured by growth rate, survival until diapause, and by investment in immunity. We assessed immunity after a bacterial challenge in terms of phenoloxidase (PO) activity and the expression of seven pre‐selected insect immune genes (qPCR). We found that the beneficial effects of constitutive leaf IGs, that improved larval growth, were significantly reduced by mildew infection. Moreover, mildew presence downregulated one component of larval immune response (PO activity), suggesting a physiological cost of investment in immunity under suboptimal conditions. Yet, feeding on mildew‐infected leaves caused an upregulation of two immune genes, lysozyme and prophenoloxidase. Our findings indicate that a plant pathogen can significantly modulate the effects of secondary metabolites on the growth of an insect herbivore. Furthermore, we show that a plant pathogen can induce contrasting effects on insect immune function. We suspect that the activation of the immune system toward a plant pathogen infection may be maladaptive, but the actual infectivity on the larvae should be tested.     

Clutch size in a larval-pupal endoparasitoid

Clutch size decisions by Aphaereta minuta (Nees) (Hymenoptera: Braconidae), a polyphagous, gregarious, larval-pupal endoparasitoid, were studied under laboratory conditions. This parasitoid attacks larvae of Diptera inhabiting ephemeral microhabitats such as decaying plant and animal material. Females oviposit in young larval stages, but the eventual size of the host pupa determines host food availability for competing offspring. The size of the pupa can differ greatly between host species. We questioned how A. minuta females deal with this delay between the moment of oviposition and eventual host food availability, and whether they make clutch size decisions that benefit their fitness. It was shown that females indeed vary their clutch size considerably and in an adaptive way: (1) females lay larger clutches in larvae of host species that produce larger pupae, even when the larvae are the same size at the moment of oviposition, and (2) females lay larger clutches in larger larvae than in smaller larvae of the same host species. The latter seems functional as larvae parasitized at an older stage indeed developed into larger pupae compared to larvae parasitized at a younger stage. Furthermore, mortality of parasitized young host larvae was greater than that of both unparasitized larvae and parasitized older larvae. Under field conditions the risk of mortality of young host larvae is expected to be even higher due to the limited period of microhabitat (host food) availability, strong scramble type competition between the host larvae, and the longer period of being exposed to predation.     

Feeding patterns of monophagous,oligophagous, and polyphagous insect herbivores: The effect of resource abundance and plant chemistry

Summary Leaf tissue preferences of monophagous, oligophagous, and polyphagous insect herbivores were determined using young and mature leaf tissue abundances and herbivore feeding observations. Larvae of monophagous and oligophagous herbivores preferred young leaf tissues while, overall, larvae of polyphagous species preferred mature leaves of their various host plants. Even though a species is often polyphagous over its geographical range, larvae from local populations may be very specialized in their diet. When this occurs these specialized larvae prefer the more nutritious and perhaps more toxic young leaves of some of their host plants. Resource abundance and plant chemistry are discussed as major factors influencing herbivore feeding patterns.     

Movement and disappearance of mountain birch defoliators are influenced by the interactive effects of plant architecture and induced resistance

Abstract. 1. Changes in herbivore movement and feeding behaviour may determine the efficacy of induced plant resistance by affecting the location of damage within the foliage and by modifying the vulnerability of herbivores to predators. 2. Observations of larval feeding sites were used to test whether induced resistance increased the movement of free‐living Epirrita autumnata Borkh. (Lepidoptera, Geometridae) larvae feeding on mountain birch [Betula pubescens ssp. czerepanovii (Orlova) Hämet‐Ahti]. The amount of defoliation at different canopy parts was measured to test the associated changes in the spread of damage within the foliage. 3. The architectural complexity of trees was measured to test its association with the disappearance of larvae from their hosts. The underlying hypothesis was that the architectural traits of the host plant could affect disappearance by influencing the frequency of herbivores encountering predators. 4. Distance between the consecutive feeding positions, the number of leaves damaged, and consumption of long shoot leaves all increased in trees with induced resistance. 5. Disappearance of larvae depended on the architectural complexity of trees. The effect of complexity differed between defoliation treatments, and may depend on the activity and number of predators in relation to the canopy size. 6. Accordingly, this study suggested that the interactive effects of plant architecture, induced resistance, and herbivore behaviour can determine the performance of herbivores on their host plant.     

Characterization of induced resistance in tomato plants

We have characterized, using several types of bioassays, the resistance induced in young tomato plants by feeding of the corn earworm, Helicoverpa zea. Beet armyworm larvae, Spodoptera exigua, and leafminers, Liriomyza trifolii, were used to assay the induced resistance. In whole-plant experiments, damage localized to a single leaflet of fourleaf tomato plants induced a systemic increase in resistance such that beet armyworm larvae confined to previously damaged (induced) plants grew at a rate about half that of larvae raised on control plants and consumed less leaf tissue from induced plants than from control plants. In experiments using excised leaves, beet armyworm larvae suffered increased mortality when reared on leaves from induced plants. The strength of this induced resistance varied spatially relative to the damaged position; moreover, the spatial distribution of induced resistance changed over a three-week period following damage. Other experiments demonstrated that the mechanisms of induced resistance in tomato foliage involves both a decrease in larval preference for and a decrease in the nutritional value of induced foliage. Induction also retarded the oviposition and/or early development of leafminers. Thus, induced resistance has relatively severe effects on the biology of subsequent herbivores. These data should allow us to begin to elucidate cause-effect relationships between induced resistance and induced chemistry in tomato plants.     

Effects of aphid-tending Argentine ants, nitrogen enrichment and early-season herbivory on insects hosted by a coastal shrub

The presence of the exotic Argentine ant, Linepithema humile Mayr (Hymenoptera: Dolichoderinae), nitrogen enrichment, and early-season herbivory by the specialist beetle Trirhabda bacharidis (Coleoptera: Chrysomelidae) have been shown, through separate experiments, to affect the densities of insect herbivores of the coastal shrub Baccharis halimifolia (Asteraceae), in Florida. Using a fully-factorial field experiment, we examined the relative importance of all three of these factors to the six most common insect herbivore species utilizing this host plant in a West Central Florida coastal habitat. The presence of ants affected more herbivore species than either early-season herbivory by larval T. bacharidis or nitrogen enrichment. Experimental reductions of L. humile resulted in reductions of an aphid, its coccinellid predators, and adult T. bacharidis, and increases of two species of leafminers and one species of stemborer. Due to the strong negative effects of stemborer herbivory on host plant survival, the increase in stemborer abundance led to increased host plant mortality. Early-season herbivory by larval T. bacharidis only affected the abundance of aphids and their predators, both of which were more abundant on trees with reduced early-season herbivory. Nitrogen fertilization had the most limited effects and only T. bacharidis larvae achieved higher densities on fertilized trees. Our results indicate that aphid tending by the exotic L. humile affects other insects on B. halimifolia more so than herbivory by the exploitative competitor T. bacharidis or nitrogen as a limiting nutrient.     

Increased temperature reduces herbivore host‐plant quality

Globally increasing temperatures may strongly affect insect herbivore performance, as their growth and development is directly linked to ambient temperature as well as host‐plant quality. In contrast to direct effects of temperature on herbivores, indirect effects mediated via thermal effects on host‐plant quality are only poorly understood, despite having the potential to substantially impact performance and thereby to alter responses to the changing climatic conditions. We here use a full‐factorial design to explore the direct (larvae were reared at 17 °C or 25 °C) and indirect effects (host plants were reared at 17 °C or 25 °C) of temperature on larval growth and life‐history traits in the temperate‐zone butterfly Pieris napi. Direct temperature effects reflected the common pattern of prolonged development and increased body mass at lower temperatures. At the higher temperature, efficiency of converting food into body matter was much reduced being accompanied by an increased food intake, suggesting compensatory feeding. Indirect temperature effects were apparent as reduced body mass, longer development time, an increased food intake, and a reduced efficiency of converting food into body matter in larvae feeding on plants grown at the higher temperature, thus indicating poor host‐plant quality. The effects of host‐plant quality were more pronounced at the higher temperature, at which compensatory feeding was much less efficient. Our results highlight that temperature‐mediated changes in host‐plant quality are a significant, but largely overlooked source of variation in herbivore performance. Such effects may exaggerate negative effects of global warming, which should be considered when trying to forecast species' responses to climate change.     

CO2 and O3 effects on host plant preferences of the forest tent caterpillar (Malacosoma disstria)

Elevated levels of CO₂ and O₃ affect plant growth and phytochemistry, which in turn can alter physiological performance of associated herbivores. Little is known, however, about how generalist insect herbivores respond behaviorally to CO₂‐ and O₃‐mediated changes in their host plants. This research examined the effects of elevated CO₂ and O₃ levels on host plant preferences and consumption of forest tent caterpillar (FTC, Malacosoma disstria Hbn.) larvae. Dual choice feeding assays were performed with foliage from birch (Betula papyrifera Marsh.) and aspen (Populus tremuloides Michx., genotypes 216 and 259). Trees were grown at the Aspen Free Air CO₂ Enrichment (FACE) facility near Rhinelander, WI, USA, and had been exposed to ambient or elevated concentrations of CO₂ and/or O₃. Levels of nutritional and secondary compounds were quantified through phytochemical analyses. The results showed that elevated O₃ levels increased FTC larval preferences for birch compared with aspen, whereas elevated CO₂ levels had the opposite effect. In assays with the two aspen genotypes, addition of both CO₂ and O₃ caused a shift in feeding preferences from genotype 259 to genotype 216. Consumption was unaffected by experimental treatments in assays comparing aspen and birch, but were increased for larvae given high O₃ foliage in the aspen genotype assays. Elevated levels of CO₂ and O₃ altered tree phytochemistry, but did not explain shifts in feeding preferences. The results demonstrate that increased levels of CO₂ and O₃ can alter insect host plant preferences both between and within tree species. Also, consequences of altered host quality (e.g., compensatory consumption) may be buffered by partial host shifts in situations when alternative plant species are available. Environmentally induced changes in host plant preferences may have the potential to alter the distribution of herbivory across plant genotypes and species, as well as competitive interactions among them.     

Foliage of oaks grown under elevated CO2 reduces performance of Antheraea polyphemus (Lepidoptera: Saturniidae)

To understand how the increase in atmospheric CO2 from human activity may affect leaf damage by forest insects, we examined host plant preference and larval performance of a generalist herbivore, Antheraea polyphemus Cram., that consumed foliage developed under ambient or elevated CO2. Larvae were fed leaves from Quercus alba L. and Quercus velutina Lam. grown under ambient or plus 200 microl/liter CO2 using free air carbon dioxide enrichment (FACE). Lower digestibility of foliage, greater protein precipitation capacity in frass, and lower nitrogen concentration of larvae indicate that growth under elevated CO2 reduced the food quality of oak leaves for caterpillars. Consuming leaves of either oak species grown under elevated CO2 slowed the rate of development of A. polyphemus larvae. When given a choice, A. polyphemus larvae preferred Q. velutina leaves grown under ambient CO2; feeding on foliage of this species grown under elevated CO2 led to reduced consumption, slower growth, and greater mortality. Larvae compensated for the lower digestibility of Q. alba leaves grown under elevated CO2 by increasing the efficiency of conversion of ingested food into larval mass. Despite equivalent consumption rates, larvae grew larger when they consumed Q. alba leaves grown under elevated compared with ambient CO2. Reduced consumption, slower growth rates, and increased mortality of insect larvae may explain lower total leaf damage observed previously in plots in this forest exposed to elevated CO2. By subtly altering aspects of leaf chemistry, the ever-increasing concentration of CO2 in the atmosphere will change the trophic dynamics in forest ecosystems.     

Ecological and Evolutionary Interactions between Wild Crucifers and Their Herbivorous Insects

Abstract Insects feeding on ten species of wild crucifer were investigated. Differences in host plant range and insect community structure were examined with regard to anti-herbivore defense mechanisms. Most of the crucifer species deterred insect herbivory by disappearing in the summer or by lowering their intrinsic quality as food for insects. Species with these defense mechanisms were exploited by only a few specialized herbivorous insects that seemed to have counter defenses. The plants without these defense mechanisms were used by many herbivorous insect species. Rorippa indica lacked direct defenses, but supported a low total density of herbivore individuals. This crucifer has an indirect defense mechanism: ants attracted to floral nectar defended the plant from deleterious herbivores. Crucifers that disappeared seasonally lacked other anti-herbivore defense mechanisms. This suggests that the phonological response is an alternative other responses to herbivore attack.     

Crowding-triggered phenotypic responses alleviate consequences of crowding inEpirrita autumnata (Lep., Geometridae)

Summary CrowdedEpirrita larvae had shorter larval periods than, and similar pupal masses to, their solitary siblings when reared on low quality diets. When fed on high quality diets, pupal masses of crowded larvae were lower than in singletons, and there was no difference in larval period. Because changes in food availability (absolute shortage, induced resistance in foliage) are caused by high larval densities in the field, crowding-triggered phenotypic changes may helpEpirrita to overcome detrimental consequences of high larval density. Pupal period was longer in crowded larvae than in singletons and crowded adults emerged later than their solitary siblings. Eggs of late emerging moths eclosed late in the ensuing spring, which coincides with delayed leaf flush in the year after defoliation. The reason for the faster growth of crowded individuals on poor diets was higher intake albeit less thorough processing of food in crowded, but not in solitary, larvae. On good diets solitary individuals tended to consume more than crowded larvae but there was no difference in processing. Predicted differences of host plant use between stealthy and opportunistic types of herbivores (sensu Rhoades 1985) were generally found between solitary and aggregated larvae on poor but not on good diets. The group response could not be explained by benefits to the group although the assumptions of Wilson's model of group selection were satisfied.     

Secretory avocado idioblast oil cells: evidence of their defensive role against a non-adapted insect herbivore

We tested the hypothesis that avocado idioblast oil cells play a defensive role against herbivorous insects. Toxicities of the intact avocado idioblast oil cells and the extracted idioblast oil were compared for three insect herbivores. Spodoptera exigua (Hübner) larvae are generalists that do not feed on avocados. By contrast, Sabulodes aegrotata (Guenée) and Pseudoplusia includens (Walker) larvae are generalist herbivores that readily feed on avocados. All bioassays were performed at a naturally occurring concentration of idioblast oil cells (2% w/w). Choice experiments showed that S. exigua larvae avoided diet treated with avocado idioblast oil cells and consume more control than treated diet. In contrast, idioblast oil cells had no significant antifeedant effects on the adapted S. aegrotata and P. includens larvae. Subsequent experiments designed to assess resistance mechanisms separated pre-ingestive (behavioral) and post-ingestive (physiological) effects of the avocado idioblast oil cells, and the extracted idioblast oil, on the two adapted herbivores. Post-ingestive adaptation was the mechanism that allows feeding. Because the impact of the avocado idioblast oil cells was greatest on the performance of non-adapted S. exigua, additional experiments determined that larvae fed diet containing the oil cells had higher mortality and reduced larval growth compared to controls. Developmental times were significantly prolonged for the survivors. Thus, increased mortality, reduced developmental rates, and antifeedant activity in the non-adapted insect indicate that defense against non-adapted herbivores may be an important function of idioblast cells in avocados.