Root damage to apple plants by cockchafer larvae induces a change in volatile signals below‐ and above‐ground

Volatile organic compounds (VOCs) mediate communication between plants and insects. Plants under insect herbivore attack release VOCs either at the site of attack or systemically, indicating within‐plant communication. Some of these VOCs, which may be induced only upon herbivore attack, recruit parasitoids and predatory insects to feed on the attacking insects. Moreover, some plants are able to ‘eavesdrop’ on herbivore‐induced plant volatiles (HIPVs) to prime themselves against impending attack; such eavesdropping exemplifies plant–plant communication. In apple orchards, the beetle Melolontha melolontha L. (Coleoptera: Scarabaeidae) is an important insect pest whose larvae live and feed on roots for about 4 years. In this study, we investigated whether the feeding activity of M. melolontha larvae (1) alters the volatile profile of apple roots, (2) induces the release of HIPVs systemically in the leaves, and (3) whether infested plants communicate to neighbouring non‐infested conspecifics through HIPVs. To answer these questions, we collected constitutive VOCs from intact M9 roots as well as M. melolontha larvae‐damaged roots using a newly designed ‘rhizobox’, to collect root‐released volatiles in situ, without damaging the plant root system. We also collected VOCs from the leaf‐bearing shoots of M9 whose roots were under attack by M. melolontha larvae and from shoots of neighbouring non‐infested conspecifics. Gas chromatography‐mass spectrometry analysis showed that feeding activity of M. melolontha larvae induces the release of specific HIPVs; for instance, camphor was found in the roots only after larvae caused root damage. Melolontha melolontha also induced the systemic release of methyl salicylate and (E,E)‐α‐farnesene from the leaf‐bearing shoots. Methyl salicylate and (E,E)‐α‐farnesene were also released by the shoots of non‐infested neighbouring conspecifics. These phenomena indicate the induction of specific VOCs below‐ and above‐ground upon M. melolontha larvae feeding on apple roots as well as plant–plant communication in apple plants.     

Odor Uniformity among Tomato Individuals in Response to Herbivore Depends on Insect Species

Plants produce specific volatile organic compound (VOC) blends in response to herbivory. Herbivore-induced blends may prime the plant for future attack or attract carnivorous insects; these responses have been considered adaptive for plants. If herbivores differentially modify the VOC emission among individuals within a group of plants they feed upon, then plant responses to herbivores will not only produce specific blends but also variation in odor among individuals, i.e. individuals smell the same, then having a uniform odor. We investigated the VOC emission variation or uniformity among tomato individuals (Solanum lycopersicum L. cv. Castlemart) in response to moderate wounding by (1) nymphs of the psyllid Bactericera cockerelli (Sulc.) (TP); (2) Lepidoptera chewing-feeding larvae of Fall Armyworm (Spodoptera frugiperda Smith) (FAW) and (3) of Cabbage Looper (Trichoplusia ni Hübner) (CL), and (4) mechanical damage (MD). We used a ratio-based analysis to compare the fold-change in concentration from constitutive to induced VOC emission. We also used size and shape analysis to compare the emission of damaged and non-damaged individuals. Aside of finding herbivore-specific blends in line with other studies, we found patterns not described previously. We detected constitutive and induced odor variation among individuals attacked by the same herbivore, with the induced odor uniformity depending on the herbivore identity. We also showed that the fold-change of VOCs from constitutive to induced state differed among individuals independently of the uniformity of the blends before herbivore attack. We discuss our findings in the context of the ecological roles of VOCs in plant-plant and plant-carnivore insects’ interactions.     

Fitness costs of jasmonic acid-induced defense in tomato, Lycopersicon esculentum

The resource allocation hypothesis is based on the assumption that defenses are costly, but relatively few studies have quantified the reproductive price of induced defenses, which represent the best means of measuring such costs in isolation from the genotypic costs that confound research involving constitutive defenses. Jasmonic acid (JA) is a plant signal molecule involved in the defensive responses of plants. It induces many of the same chemicals that are associated with herbivore damage, and thus offers a means of inducing plants without the removal of leaf area, which incurs its own costs. In tomato plants, JA induced resistance to Manduca sexta and increased levels of two defensive enzymes, polyphenol oxidase and peroxidase. We measured the impact of JA-induced defenses in tomato, Lycopersicon esculentum (Solanaceae), on several variables associated with reproductive success: fruit number, fruit weight, ripening time, time of fruit-set, number of seeds per fruit, total seeds per plant, the relationship between fruit weight and seed number, and germination success. Plants were grown in a pest-free greenhouse and treated biweekly with solvent or with JA at either of two concentrations: 10 mM or 1 mM. The high concentration of JA led to fewer but larger fruits, longer ripening time, delayed fruit-set, fewer seeds per plant, and fewer seeds per unit of fruit weight. The reproductive impact of induction was reduced at the lower dose, but still significant; 1 mM JA resulted in delayed fruit-set and fewer seeds per unit of fruit weight, compared to control plants. Our research indicates that JA-induced defenses impose significant costs on tomato plants.     

Release of lipoxygenase products and monoterpenes by tomato plants as an indicator of Botrytis cinerea-induced stress

Changes in emission of volatile organic compounds (VOCs) from tomato induced by the fungus Botrytis cinerea were studied in plants inoculated by spraying with suspensions containing B. cinerea spores. VOC emissions were analysed using on-line gas chromatography–mass spectrometry, with a time resolution of about 1 h, for up to 2 days after spraying. Four phases were delimited according to the starting point and the applied day/night rhythm of the experiments. These phases were used to demonstrate changes in VOC flux caused by B. cinerea infestation. Tomato plants inoculated with B. cinerea emitted a different number and amount of VOCs after inoculation compared to control plants that had been sprayed with a suspension without B. cinerea spores. The changes in emissions were dependent on time after inoculation as well as on the severity of infection. The predominant VOCs emitted after inoculation were volatile products from the lipoxygenase pathway (LOX products). The increased emission of LOX products proved to be a strong indicator of a stress response, indicating that VOC emissions can be used to detect plant stress at an early stage. Besides emission of LOX products, there were also increases in monoterpene emissions. However, neither increased emission of LOX products nor of monoterpenes is specific for B. cinerea attack. The emission of LOX products is also induced by other stresses, and increased emission of monoterpenes seems to be the result of mechanical damage induced by secondary stress impacts on leaves.     

Priming of Production in Maize of Volatile Organic Defence Compounds by the Natural Plant Activator cis-Jasmone

cis-Jasmone (CJ) is a natural plant product that activates defence against herbivores in model and crop plants. In this study, we investigated whether CJ could prime defence in maize, Zea mays, against the leafhopper, Cicadulina storeyi, responsible for the transmission of maize streak virus (MSV). Priming occurs when a pre-treatment, in this case CJ, increases the potency and speed of a defence response upon subsequent attack on the plant. Here, we tested insect responses to plant volatile organic compounds (VOCs) using a Y-tube olfactometer bioassay. Our initial experiments showed that, in this system, there was no significant response of the herbivore to CJ itself and no difference in response to VOCs collected from unexposed plants compared to CJ exposed plants, both without insects. VOCs were then collected from C. storeyi-infested maize seedlings with and without CJ pre-treatment. The bioassay revealed a significant preference by this pest for VOCs from infested seedlings without the CJ pre-treatment. A timed series of VOC collections and bioassays showed that the effect was strongest in the first 22 h of insect infestation, i.e. before the insects had themselves induced a change in VOC emission. Chemical analysis showed that treatment of maize seedlings with CJ, followed by exposure to C. storeyi, led to a significant increase in emission of the defensive sesquiterpenes (E)-(1R,9S)-caryophyllene, (E)-α-bergamotene, (E)-β-farnesene and (E)-4,8-dimethyl-1,3,7-nonatriene, known to act as herbivore repellents. The chemical analysis explains the behavioural effects observed in the olfactometer, as the CJ treatment caused plants to emit a blend of VOCs comprising more of the repellent components in the first 22 h of insect infestation than control plants. The speed and potency of VOC emission was increased by the CJ pre-treatment. This is the first indication that CJ can prime plants for enhanced production of defensive VOCs antagonist towards herbivores.     

Electroculture of tomato plants in a commercial hydroponics greenhouse

An experiment was conducted to evaluate the effects of air ion treatment on tomato plants (Lycopersicon esculentum P. Miller) in terms of: (1) growth and health; (2) fruit yield and quality; and (3) economic factors. The plants were grown by a commercial greenhouse (G.H.) grower employing soilless culture techniques. An air ion generator and emitters were installed in such fashion that 864 plants were exposed to a high negative air ion density flux, while 576 plants grew in an area which received relatively few ions. Normal operational procedures, with certain modifications, were employed for plant culture, feed/irrigation, and environmental control. Plants responded vigouously to air ion stimulation, which equated to shortening of the seeding-to-harvest time period by two weeks as measured by vine growth, main stem height, time to blossoming, fruit set, and fruit yield. Throughout the first four-month growth period plant growth was good and no serious physiological disorders nor insect damage were observed. During the sixth harvest week a virus infection appeared in both control and ion-treated plants, but was not of sufficient severity to ruin the experimnent. Foliage and fruit samples were subjected to laboratory analyses. In general, the stimulated plants contained higher percentages of mineral elements than those of the controls. Fruit from ion-treated plants has more ascorbic and citric acid than that from control plants. Although there were no wide differences in fruit texture or flavor, a taste panel verdict indicated that fruit from the stimulated plants tasted better. An unexpected benefit was marked decrease in white fly infestation. All these factors combined with the low cost of air-ion treatment suggest that this modality offers potential for greenhouse cultivation of garden crops.     

Are insect frugivores always plant pests? The impact of fruit fly (Diptera: Tephritidae) larvae on host plant fitness

Herbivory is generally regarded as negatively impacting on host plant fitness. Frugivorous insects, which feed directly on plant reproductive tissues, are predicted to be particularly damaging to hosts. We tested this prediction with the fruit fly, Bactrocera tryoni, by recording the impact of larval feeding on two direct (seed number and germination) and two indirect (fruit decay rate and attraction/deterrence of vertebrate frugivores) measures of host plant fitness. Experiments were done in the laboratory, glasshouse and tropical rainforest. We found no negative impact of larval feeding on seed number or germination for three test plants: tomato, capsicum and eggplant. Further, larval feeding accelerated the initiation of decay and increased the final level of fruit decay in tomatoes, apples, pawpaw and pear, a result considered to be beneficial to the fruit. In rainforest studies, native rodents preferred infested apple and pears compared to uninfested control fruit; however, there were no differences observed between treatments for tomato and pawpaw. For our study fruits, these results demonstrate that fruit fly larval infestation has neutral or beneficial impacts on the host plant, an outcome which may be largely influenced by the physical properties of the host. These results may contribute to explaining why fruit flies have not evolved the same level of host specialization generally observed for other herbivore groups.     

Parasitoids select plants more heavily infested with their caterpillar hosts: a new approach to aid interpretation of plant headspace volatiles

Plants produce volatile organic compounds (VOCs) in response to herbivore attack, and these VOCs can be used by parasitoids of the herbivore as host location cues. We investigated the behavioural responses of the parasitoid Cotesia vestalis to VOCs from a plant-herbivore complex consisting of cabbage plants (Brassica oleracea) and the parasitoids host caterpillar, Plutella xylostella. A Y-tube olfactometer was used to compare the parasitoids' responses to VOCs produced as a result of different levels of attack by the caterpillar and equivalent levels of mechanical damage. Headspace VOC production by these plant treatments was examined using gas chromatography-mass spectrometry. Cotesia vestalis were able to exploit quantitative and qualitative differences in volatile emissions, from the plant-herbivore complex, produced as a result of different numbers of herbivores feeding. Cotesia vestalis showed a preference for plants with more herbivores and herbivore damage, but did not distinguish between different levels of mechanical damage. Volatile profiles of plants with different levels of herbivores/herbivore damage could also be separated by canonical discriminant analyses. Analyses revealed a number of compounds whose emission increased significantly with herbivore load, and these VOCs may be particularly good indicators of herbivore number, as the parasitoid processes cues from its external environment.     

Effects of cultural treatments on the incidence of, and damage done by, tomato brown root rot

In soils naturally infested with Pyrenochaeta lycopersici, which usually occurs as a grey sterile fungus (GSF), symptoms of brown root rot (BRR) developed sooner and more extensively in the second year of cropping than in the first. The amount of BRR attributable to corkiness increased as plants aged but, at comparable stages of cropping, decreased in the second and third seasons, an effect associated inversely with the severity of early GSF attack. The larger amount of corkiness, 33 compared with 18 %, on two batches of plants in 1965, each with 53 % end-of-season BRR, was also attributed to a less severe early GSF attack on the former than on the latter, growing in soils unsteamed for 1 and 3 years respectively. The incidence of BRR decreased with increasing depth in infested soils but increased on plants grown in plots with partially sterilized topsoil. Partially sterilizing soils at the G.C.R.I. and Fairfield E.H.S. decreased the incidence of BRR and increased crop weights from about the second month of picking, but fruit quality was poorer. Seasonal yields from plants in untreated soil progressively decreased relatively to those from repeatedly steamed plots, from 93 to 65 % in successive years at Fairfield and from 65 to 56 and 43 % at the G.C.R.I. Steaming done in 1963 and 1964 temporarily retarded GSF attack in 1965 with corresponding yield increases. Increasing the amounts of sterilized propagating soil surrounding roots at planting from 0·4 to 1·01 per plant increased yields by c. 0·4 kg/plant, this being a relatively large increase for plants in infested soil, where this treatment significantly delayed the early incidence of BRR near stem bases. Grafting commercially acceptable scions to rootstocks that tolerated colonization by GSF (‘resistant’ rootstocks), temporarily checked growth, delayed picking and decreased fruit quality. Usually grafted plants, irrespective of soil treatment, yielded at least as much fruit as ungrafted plants in steamed soil. In one of five comparisons, soil steaming increased yields of grafted plants. When testing the effects of previous cropping it seemed that populations of GSF increased similarly in soils planted with grafted and ungrafted plants. In addition to GSF attack, roots at Fairfield E.H.S. were often colonized by Colletotrichum coccodes, microsclerotia (= black dots) being more numerous as plants aged. Although significantly more black dot developed on GSF-resistant rootstocks grown in untreated soil than on those grown in steamed soil, the differences were not associated with effects on yield. C. coccodes colonized GSF-susceptible and -resistant roots equally.     

L-DOPA functions as a plant pheromone for belowground anti-herbivory communication

While mechanisms of plant–plant communication for alerting neighbouring plants of an imminent insect herbivore attack have been described aboveground via the production of volatile organic compounds (VOCs), we are yet to decipher the specific components of plant–plant signalling belowground. Using bioassay-guided fractionation, we isolated and identified the non-protein amino acid l -DOPA, released from roots of Acyrtosiphon pisum aphid-infested Vicia faba plants, as an active compound in triggering the production of VOCs released aboveground in uninfested plants. In behavioural assays, we show that after contact with l -DOPA, healthy plants become highly attractive to the aphid parasitoid (Aphidius ervi), as if they were infested by aphids. We conclude that l -DOPA, originally described as a brain neurotransmitter precursor, can also enhance immunity in plants.     

茉莉酸甲酯诱导邻近番茄植株抗斜纹夜蛾的浓度和距离效应

茉莉酸甲酯(Methyl jasmonate, MeJA)与植物间的防御通讯密切相关,其挥发性强,可以在空气中传播而诱导邻近植物产生防御反应,为探明MeJA诱导邻近植株抗虫性的浓度和距离效应,本研究在大棚(长4 m×宽1 m×高1 m)进口处喷以不同浓度MeJA,检测离气味源60 cm、120 cm、180 cm、240 cm和300 cm远的番茄植株叶片多酚氧化酶(Polyphenol oxidase, PPO),过氧化物酶(Peroxidase, POD)和脂氧合酶(Lipoxygenase, LOX)的活性以及斜纹夜蛾幼虫取食不同处理番茄植株叶片后的体重增长率,研究结果表明1 mmol/L和10 mmol/L MeJA挥发物可以显著诱导增强邻近番茄植株叶片PPO,POD和LOX酶的活性,以离10 mmol/L MeJA气味源60 cm处植株叶片的酶活性最高,并随着距离的增加而呈降低的趋势;取食60 cm和300 cm对照组植株叶片的斜纹夜蛾幼虫其体重增长率没有显著差异,然而1 mmol/L和10 mmol/L MeJA处理下,取食300 cm处植株叶片的斜纹夜蛾幼虫其体重增长率显...     

Do Leaf Cutting Ants Cut Undetected? Testing the Effect of Ant-Induced Plant Defences on Foraging Decisions in Atta colombica

Leaf-cutting ants (LCAs) are polyphagous, yet highly selective herbivores. The factors that govern their selection of food plants, however, remain poorly understood. We hypothesized that the induction of anti-herbivore defences by attacked food plants, which are toxic to either ants or their mutualistic fungus, should significantly affect the ants' foraging behaviour. To test this "induced defence hypothesis," we used lima bean (Phaseolus lunatus), a plant that emits many volatile organic compounds (VOCs) upon herbivore attack with known anti-fungal or ant-repellent effects. Our results provide three important insights into the foraging ecology of LCAs. First, leaf-cutting by Atta ants can induce plant defences: Lima bean plants that were repeatedly exposed to foraging workers of Atta colombica over a period of three days emitted significantly more VOCs than undamaged control plants. Second, the level to which a plant has induced its anti-herbivore defences can affect the LCAs' foraging behaviour: In dual choice bioassays, foragers discriminated control plants from plants that have been damaged mechanically or by LCAs 24 h ago. In contrast, strong induction levels of plants after treatment with the plant hormone jasmonic acid or three days of LCA feeding strongly repelled LCA foragers relative to undamaged control plants. Third, the LCA-specific mode of damaging leaves allows them to remove larger quantities of leaf material before being recognized by the plant: While leaf loss of approximately 15% due to a chewing herbivore (coccinelid beetle) was sufficient to significantly increase VOC emission levels after 24 h, the removal of even 20% of a plant's leaf area within 20 min by LCAs did not affect its VOC emission rate after 24 h. Taken together, our results support the "induced defence hypothesis" and provide first empirical evidence that the foraging behaviour of LCAs is affected by the induction of plant defence responses.     

Plant defence induced by PGPR against Spodoptera litura in tomato (Solanum lycopersicum L.)

• The research conducted including its rationale: Spodoptera litura is the major pest of tomato causing significant reduction in tomato yield. Application of Plant growth promoting rhizobacteria(PGPR) prevent use of chemical fertilizer and synthetic pesticides through enhancement of plant growth and yield and induction of systemic resistance. Present investigation is an attempt to evaluate the role of PGPR, Pseudomonas putida and Rothia sp. on the physiology and yield of tomato fruit infested with the S. litura.
• Central methods applied : The surface sterilized seeds of tomato were inoculated with 48 h culture of P. putida and Rothia sp. At 6–7 branching stage of the plant, the larvae of S. litura at 2nd in star was used to infect the tomato plant leaves.
• Key results: The S. litura infestation decreased dry weight of shoots and roots by 46% and 22%, and significant reduction was recorded in tomato fruit yield. The P. putida and Rothia sp. inoculations alleviated the adverse effects of insect infestation and resulted in 60% increase in plant biomass and 40% increase in yield over infested plants.
• Main conclusions including key points of discussion: PGPR: Defense appears to be mediated via increase in proline production, enhanced activities of antioxidant enzymes, stimulation in the activities of protease and polyphenol oxidases, increased contents of phenolics, protein and chlorophyll. The formulation of biopesticide involving PGPR comprise an environment friendly and sustainable approach to overcome insect infestation.

     

Induced carbon reallocation and compensatory growth as root herbivore tolerance mechanisms

Upon attack by leaf herbivores, many plants reallocate photoassimilates below ground. However, little is known about how plants respond when the roots themselves come under attack. We investigated induced resource allocation in maize plants that are infested by the larvae Western corn rootworm Diabrotica virgifera virgifera. Using radioactive ¹¹CO₂, we demonstrate that root‐attacked maize plants allocate more new ¹¹C carbon from source leaves to stems, but not to roots. Reduced meristematic activity and reduced invertase activity in attacked maize root systems are identified as possible drivers of this shoot reallocation response. The increased allocation of photoassimilates to stems is shown to be associated with a marked thickening of these tissues and increased growth of stem‐borne crown roots. A strong quantitative correlation between stem thickness and root regrowth across different watering levels suggests that retaining photoassimilates in the shoots may help root‐attacked plants to compensate for the loss of belowground tissues. Taken together, our results indicate that induced tolerance may be an important strategy of plants to withstand belowground attack. Furthermore, root herbivore‐induced carbon reallocation needs to be taken into account when studying plant‐mediated interactions between herbivores.     

Attraction of the stink bug egg parasitoid Telenomus podisi to defence signals from soybean activated by treatment with cis-jasmone

After herbivore attack or chemical activation, plants release a blend of volatile organic compounds (VOCs) that is qualitatively or quantitatively different to the blend emitted by an undamaged plant. The altered blend of VOCs is then usually attractive to the herbivore's natural enemies. Soybean, Glycine max (L.) (Fabaceae), when damaged by stink bug herbivory, has been shown to emit a blend of VOCs that attracts the stink bug egg parasitoid Telenomus podisi (Ashmead) (Hymenoptera: Scelionidae) to the plant. In this study, our aim was to investigate changes in the VOC profile of soybean (var. BR16) elicited by the naturally occurring plant activator cis -jasmone, and to determine whether these changes elicited the attraction of T. podisi . cis -Jasmone elicited chemical defence in soybean similar to that previously reported for stink bug damage. The main components induced by cis- jasmone were camphene, myrcene, ( E )-ocimene, methyl salicylate, and ( E , E )-4,8,12-trimethyltrideca-1,3,7,11-tetraene. In Y-tube behavioural bioassays, T. podisi preferred cis -jasmone treated plants over untreated plants. Thus, cis -jasmone appears to induce defence pathways in soybean similar to those induced by stink bug damage, and this phenomenon appears to be a promising tool for the manipulation of beneficial natural enemies in future sustainable stink bug control strategies. The delay in response demonstrates that cis -jasmone treatment is not directly causing the response, but, more importantly, that it is causing activation of induced defence, long after initial treatment.     

Editorial note

Generalist herbivores can face many challenges when choosing their host plant. This can be particularly difficult if their choice and performance are affected by host experience. Greenhouse whitefly, Trialeurodes vaporariorum Westwood (Hemiptera: Aleyrodidae), is an invasive generalist herbivore, which has established in year‐round greenhouses at northern latitudes where it cannot overwinter outdoors. It mainly uses crops such as cucumber (Cucumis sativus L.), tomato (Solanum lycopersicum L.), and ornamentals as host plants. However, every summer the insect escapes greenhouses and is exposed to natural vegetation. We evaluated the performance of T. vaporariorum on diverse vegetation outside greenhouses after prolonged experience of greenhouse crops. First, we surveyed the vegetation near infested greenhouses. Development success of the insect differed among wild hosts. We identified five new hosts among 12 plant species that bore pupae and were thus considered suitable as the insect's host plants. Members of the Urticaceae and Onagraceae were the most preferred and frequently inhabited by all insect life stages. The highest abundance of insects occurred in plots with low plant species richness, independent of plant family in these habitats. We then studied experimentally the impact of 1 year of preconditioning to one of three common greenhouse crops, cucumber, tomato, or poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch), on the performance of the preconditioned adults and their progeny on four wild plants. Adults from tomato and poinsettia preferred the novel host species over the species to which they were preconditioned. The whitefly population preconditioned to cucumber was the most fecund on all offered hosts. We conclude that generalist herbivores can have large variation in performance, despite polyphagy, on novel hosts as shown by the variable abundance of T. vaporariorum pupae among outdoor hosts. Furthermore, performance of whiteflies on natural vegetation was affected by experience on greenhouse crops. Based on our observations, we provide insights and recommendations for pest management.     

Genetic variation in plant volatile emission does not result in differential attraction of natural enemies in the field

Volatile organic chemical (VOC) emission by plants may serve as an adaptive plant defense by attracting the natural enemies of herbivores. For plant VOC emission to evolve as an adaptive defense, plants must show genetic variability for the trait. To date, such variability has been investigated primarily in agricultural systems, yet relatively little is known about genetic variation in VOCs emitted by natural populations of native plants. Here, we investigate intraspecific variation in constitutive and herbivore-induced plant VOC emission using the native common milkweed plant (Asclepias syriaca) and its monarch caterpillar herbivore (Danaus plexippus) in complementary field and common garden greenhouse experiments. In addition, we used a common garden field experiment to gauge natural enemy attraction to milkweed VOCs induced by monarch damage. We found evidence of genetic variation in the total constitutive and induced concentrations of VOCs and the composition of VOC blends emitted by milkweed plants. However, all milkweed genotypes responded similarly to induction by monarchs in terms of their relative change in VOC concentration and blend. Natural enemies attacked decoy caterpillars more frequently on damaged than on undamaged milkweed, and natural enemy visitation was associated with higher total VOC concentrations and with VOC blend. Thus, we present evidence that induced VOCs emitted by milkweed may function as a defense against herbivores. However, plant genotypes were equally attractive to natural enemies. Although milkweed genotypes diverge phenotypically in their VOC concentrations and blends, they converge into similar phenotypes with regard to magnitude of induction and enemy attraction.     

Vigour of a dioecious shrub and attack by a galling herbivore

1. The pattern of attack by the leaf‐galling insect Neopelma baccharidis (Homoptera: Psyllidae) was studied in three populations of the dioecious shrub Baccharis dracunculifolia (Asteraceae) in south‐eastern Brazil. The plant vigour hypothesis, which predicts higher rates of attack and increased herbivore performance on the longest plant shoots, was tested. This work also provides further information for the study of differential herbivory in dioecious plants. 2. In total, 9200 shoots were collected randomly from 46 male and 47 female plants belonging to the three populations. Shoot length, number of leaves per shoot, rate of galling, and survival of psyllids did not differ between male and female plants. Another population on the Campus of the Federal University of Minas Gerais was used only to determine the pattern of shoot growth. 3. The hypothesis of sex‐mediated herbivory was not corroborated in this study. 4. The frequency of galling increased with increasing shoot length, as predicted by the plant vigour hypothesis. Nevertheless, the number of oviposition sites (leaf buds) increased with shoot length. 5. The performance of the galling herbivore was not related to shoot length in the plant populations studied. 6. In conclusion, Neopelma baccharidis did not select shoots based on length only.