Influence of cabbage resistance and colour upon the diamondback moth and its parasitoid Oomyzus sokolowskii

Host plant resistance and biological control are vital integrated pest management tools against the diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae), but to date no study has investigated this system including the DBM parasitoid Oomyzus sokolowskii (Kurdjumov) (Hymenoptera: Eulophidae). We examined oviposition and development of P. xylostella exposed to two commercial cabbage cultivars (green ‘Chato de quintal’ and red ‘Roxo’) and possible effects upon O. sokolowskii. Under free‐choice tests, DBM females laid significantly more eggs on plants of the green cabbage, even though several population growth parameters showed that DBM developed better on the red cabbage. Furthermore, a laboratory free‐choice test with artificially green‐ and red‐painted kale leaf discs demonstrated a similar oviposition preference pattern, with green colour being preferred over red colour. The preference was apparently visually mediated; olfactometer tests showed similar attraction of moths to both green and red cultivars in choice and non‐choice tests. Host plant cultivar had no statistically significant effect on female parasitoid behaviour towards DBM larvae, nor on parasitoid numbers or longevity. Moreover, wasps parasitizing DBM larvae reared on the green cultivar developed more quickly and in larger numbers per parasitized larva. Thus, feeding on green cabbage rather than red does not hinder, and potentially even enhances, control of DBM by O. sokolowskii. On a practical level, these results suggest that intercalating green cabbage cultivars as a trap crop might help protect more profitable red cultivars in growing fields.     

Diamondback moth oviposition: effects of host plant and herbivory

The oviposition behaviour of Plutella xylostella L. (Lepidoptera: Plutellidae) on Chinese cabbage (Brassica rapa L. Pekinensis, cv. Wombok), canola (Brassica napus L. cv. Thunder TT), and cabbage (Brassica oleracea L. Capitata, cv. sugarloaf) (Brassicaceae) was studied in the laboratory. In no‐choice experiments moths laid most eggs on the stems and lower three leaves of cabbage plants, the lower three leaves of canola plants, but on the upper three leaves of Chinese cabbage plants. The effects of conspecific herbivore damage to foliage could be replicated by mechanical damage. When foliage was damaged, injured cabbage and canola plants were preferred for oviposition over intact conspecifics, whereas injured Chinese cabbage plants were less preferred than intact conspecifics. However, when root tissue was damaged, intact cabbage and canola plants were preferred over injured conspecifics, whereas moths did not discriminate between root‐damaged and intact Chinese cabbage plants. Injury to upper leaves significantly affected the intra‐plant distribution of eggs. In cabbage and canola plants, injury to leaf 6 significantly increased the number of eggs laid on this leaf, resulting in a significant decrease in the number of eggs laid on the lower foliage/stem of plants, whereas in Chinese cabbage plants it significantly decreased the number of eggs laid on leaf 6. Following oviposition on intact plants, neonate larvae established the vast majority of feeding sites on leaves 5–8 in all three host plants, indicating that larvae moved a considerable distance from preferred oviposition sites in cabbage and canola plants. The growth rate of neonates fed on leaf‐6 tissue was significantly greater than that of those fed on leaf‐1 tissue; >90% of larvae completed development when fed exclusively on leaf‐6 tissue but no larvae completed development when fed exclusively on leaf‐1 tissue. The study demonstrates the complex and unpredictable interactions between P. xylostella and its host plants and provides a basis from which we can begin to understand observed distributions of the pest in Brassica crops.     

Temporal changes in olfactory and oviposition responses of the diamondback moth to herbivore‐induced host plants

Temporal changes in the pre‐ and post‐alighting responses of mated female diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), to two species of Brassica (Brassicaceae) host plants induced by larval feeding were studied using olfactometer and oviposition assays. Females displayed strong olfactory and oviposition preferences for herbivore‐induced common cabbage (Brassica oleracea var. capitata L. cv. sugarloaf) plants over intact plants; these preferences decreased with time and disappeared by the 7th day after induction. In herbivore‐induced common cabbage plants, eggs were clustered near feeding damage on the younger leaves (leaves 5–7), whereas in intact plants, eggs were clustered on the stem and lower leaves (leaves 1–4) . However, as the time interval between larval feeding and oviposition increased, more eggs were laid on the lower leaves of induced plants. This demonstrates a change in egg distribution from the pattern associated with induced plants to that associated with intact plants. In contrast, females displayed strong olfactory and oviposition preferences for intact Chinese cabbage [Brassica rapa ssp. pekinensis (Lour.) Hanelt cv. Wombok] plants over induced plants; these preferences decreased with time and disappeared by the 5th day after induction. More eggs were laid on the upper leaves (leaves 4–6) than on the lower leaves (leaves 1–3) of intact Chinese cabbage plants at first, but the distribution changed over time until there were no significant differences in the egg count between upper and lower leaves by the 4th day post induction. For both host plant species, pre‐alighting responses of moths were reliable indicators of post‐alighting responses on the first 2 days post induction. The results suggest that temporal changes in a plant's profile (chemical or otherwise) following herbivory may influence attractiveness to an insect herbivore and be accompanied by changes in olfactory and oviposition preferences.     

Contrary effects of jasmonate treatment of two closely related plant species on attraction of and oviposition by a specialist herbivore

Elevated jasmonic acid (JA) concentrations in response to herbivory can induce wounded plants to produce defences against herbivores. In laboratory and field experiments we compared the effects of exogenous JA treatment to two closely related cabbage species on the host‐searching and oviposition preference of the diamondback moth (DBM), Plutella xylostella. JA‐treated Chinese cabbage (Brassica campestris) was less attractive than untreated Chinese cabbage to ovipositing DBM, while JA‐treatment of common cabbage (B. oleracea) made plants more attractive than untreated controls for oviposition by this insect. Similar effects were observed when plants of the two species were damaged by DBM larvae. In the absence of insect‐feeding, or JA application, Chinese cabbage is much more attractive to DBM than common cabbage. Inducible resistance therefore appears to occur in a more susceptible plant and induced susceptibility appears to occur in a more resistant plant, suggesting a possible balance mechanism between constitutive and inducible defences to a specialist herbivore.     

Oviposition site preference and developmental performance of a gall‐inducing leafhopper on galled and non‐galled host plants

Oviposition preferences of herbivorous insects affect offspring performance. Both positive and negative links between oviposition preference and offspring performance have been reported for many species. A gall‐inducing leafhopper, Cicadulina bipunctata Melichar (Hemiptera: Cicadellidae), feeds on various Poaceae plants and induces galls of enhanced nutritional value for their offspring. Although gall induction by C. bipunctata improves nymphal performance, the oviposition preference of females between galled and non‐galled host plants is still unclear. In this paper, the nymphal performance and oviposition and feeding‐site preference of C. bipunctata were investigated using galled wheat, Triticum aestivum L., and non‐galled barley, Hordeum vulgare L., as host plants. The survival rate of C. bipunctata on wheat was significantly higher than on barley. In the choice test, significantly more eggs were laid into barley, whereas the number of eggs deposited on both hosts was not significantly different in the no‐choice test. The number of settling individuals per leaf area was not significantly different between wheat and barley, suggesting no clear preference for oviposition between these plants. Experience as a nymph with a growing host did not affect oviposition preference as adult female. The inconsistent correspondence between offspring performance and oviposition preference of C. bipunctata may reflect the high mobility of nymphs and/or differences in leaf area between host plants. The results indicate that the previous finding that oviposition preference and offspring performance are not always positively correlated in herbivorous insects is applicable to gall‐inducing insects.     

Preference of Bt‐resistant and susceptible Busseola fusca moths and larvae for Bt and non‐Bt maize

The sustainability of genetically engineered insecticidal Bacillus thuringiensis Berliner (Bt) maize, Zea mays L. (Poaceae), is threatened by the evolution of resistance by target pest species. Several Lepidoptera species have evolved resistance to Cry proteins expressed by Bt maize over the last decade, including the African maize stem borer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae). The insect resistance management (IRM) strategy (i.e., the high‐dose/refuge strategy) deployed to delay resistance evolution is grounded on certain assumptions about the biology and ecology of a pest species, for example, the interactions between the insect pest and crop plants. Should these assumptions be violated, the evolution of resistance within pest populations will be rapid. This study evaluated the assumption that B. fusca adults and larvae select and colonize maize plants at random, and do not show any preference for either Bt or non‐Bt maize. Gravid female B. fusca moths of a resistant and susceptible population were subjected to two‐choice oviposition preference tests using stems of Bt and non‐Bt maize plants. Both the number of egg batches as well as the total number of eggs laid on each stem were recorded. The feeding preference of Bt‐resistant and susceptible neonate B. fusca larvae were evaluated in choice test bioassays with whorl leaf samples of specific maize cultivars. Although no differential oviposition preference was observed for either resistant or susceptible female moths, leaf damage ratings indicated that neonate larvae were able to detect Bt toxins and that they displayed feeding avoidance behaviour on Bt maize leaf samples.     

Oviposition preference of Anthocoris nemoralis and A. nemorum (Heteroptera: Anthocoridae) on pear leaves affected by leaf damage, honeydew and prey

Anthocorids are important predators of insect pests in pome fruit. Their choice of oviposition site determines the later distribution of nymphs. In two-choice experiments it was tested whether A. nemorum and A. nemoralis would show oviposition preference with regard to simulated insect damage, mimicked by piercing leaves with a fine pin, and whether the oviposition preference of A. nemoralis was affected by the presence of honeydew or eggs of their prey C. pyri. Leaves with simulated damage were preferred by A. nemoralis, but this was not the case for A. nemorum. Honeydew-treated leaves attracted more oviposition than honeydew-free leaves. On honeydew-treated leaves significantly more eggs were laid on the surface where honeydew had been applied, rather than the opposite. When C. pyri eggs were placed along the abaxial midvein, prey infestation attracted more oviposition. On the infested leaf more eggs were laid near the prey along the abaxial midvein than along the adaxial midvein. In contrast, when prey was placed along the adaxial midvein, no overall preference was found for prey-infested leaves, but on infested leaves, more eggs were laid near the prey along the adaxial midvein than along the abaxial midvein. Results showed that prey cues and presence of prey guide predator oviposition, even within the single leaf. The perspectives for biological control in orchards are discussed.     

Resistance of cotton genotypes with different leaf colour and trichome density to Bemisia tabaci biotype B

The silverleaf whitefly (SLW), Bemisia tabaci biotype B, is considered one of the most serious pests in cotton worldwide. Its control is based on the use of pesticides, which are well‐known for their harmful effects on non‐target organisms and the environment. Here, we examined resistance of several cotton genotypes in terms of antixenosis and antibiosis against SLW. We also investigated the mechanisms of cotton genotypes involved in antixenosis against SLW by correlating with leaf trichome density and leaf colour parameters. In choice tests, we selected resistant cotton genotypes, Gossypium hirsutum palmeri, Aubum 56‐7, G. hirsutum morrilli, Mocó, Nu‐16 (B2 B3 B6)‐ 78/658, Paymaster 53‐816 and ‘IAC 23’, based on SLW oviposition and host preference. No‐choice tests with these genotypes confirmed antixenosis effect on SLW; however, no antibiosis resistance was detected in terms of developmental period and adult emergence rate. We found SLW oviposition preference was positively correlated with trichome density in the abaxial leaf surface, while adult host preference was mostly explained by high levels of green intensity of adaxial surface. Nevertheless, both SLW oviposition and host preference were negatively correlated with the lightness level of adaxial leaf surface. Probably, the underlying mechanism of cotton resistance is based on more than just these two parameters, but our study will contribute to selecting resistant cotton genotypes and helps to understand the mechanisms involved in cotton resistance against SLW. To our best knowledge, this is the first work showing that cotton leaf colour parameters are related to resistance against SLW.     

Leaf surface preference in the cabbage worm,Pieris rapae crucivora,and parasitism by the gregarious parasitoid Cotesia glomerata

Leaf surface preference of the cabbage worm, Pieris rapae crucivora Boisduval (Lepidoptera: Pieridae), for cabbage, Brassica oleracea L. var. capitata (Brassicaceae), and parasitism by the parasitoid Cotesia glomerata (L.) (Hymenoptera: Braconidae) were investigated experimentally in the laboratory. Female butterflies did not discriminate between the adaxial and abaxial surfaces of cabbage leaves when laying eggs on a vertically placed leaf. Larvae also did not discriminate between the adaxial and abaxial surfaces throughout their larval life. However, second and third instars preferred the lower surface of horizontally placed leaves to the upper surface, irrespective of whether they had hatched on the upper or lower side; other instars showed no preference for the lower surface. Parasitism rates of first and second instars on the upper surface were higher than those of larvae on the lower surface. Egg distribution on leaf surfaces and the leaf surface preference by young larvae are discussed in terms of avoidance of parasitism by the parasitoid C. glomerata.     

Experience-induced preference for oviposition repellents derived from a non-host plant by a specialist herbivore

Foraging adults of phytophagous insects are attracted by host‐plant volatiles and supposedly repelled by volatiles from non‐host plants. In behavioural control of pest insects, chemicals derived from non‐host plants applied to crops are expected to repel searching adults and thereby reduce egg laying. How experience by searching adults of non‐host volatiles affects their subsequent searching and oviposition behaviour has been rarely tested. In laboratory experiments, we examined the effect of experience of a non‐host‐plant extract on the oviposition behaviour of the diamondback moth (DBM), Plutella xylostella, a specialist herbivore of cruciferous plants. Naive ovipositing DBM females were repelled by an extract of dried leaves of Chrysanthemum morifolium, a non‐host plant of DBM, but experienced females were not repelled. Instead they were attracted by host plants treated with the non‐host‐plant extract and laid a higher proportion of eggs on treated than on untreated host plants. Such behavioural changes induced by experience could lead to host‐plant range expansion in phytophagous insects and play an important role in determining outcome for pest management of some behavioural manipulation methods.     

Preference for plants damaged by conspecific larvae in ovipositing cabbage root flies: influence of stimuli from leaf surface and roots

In laboratory dual-choice assays females of the cabbage root fly, Delia radicum, prefer for oviposition plants with roots damaged by conspecific larvae to undamaged controls. Cauliflower and kale plants were inoculated with root fly eggs (25 per plant) and the hatching larvae were allowed to feed on the roots for various periods of time (1–17 days). After 4 (cauliflower) or 5 (kale) days of larval feeding the oviposition preference was most pronounced and flies laid between 64% and 68% of their eggs near plants with damaged roots. Later, with increasing damage but fewer surviving, and thus actively feeding, larvae, the magnitude of the preference declined. The preference for plants already damaged by conspecific larvae may contribute to the previously observed aggregated distribution of D. radicum eggs in Brassica crop fields.Further experiments revealed that the sensory cues inducing this oviposition preference originate from the complex consisting of the damaged roots, the surrounding substrate (soil) and associated microbes, rather than from the aerial plant parts. In choice assays using the root-substrate complex of damaged and control plants (aerial parts removed), the observed preference for damaged roots was similar to that found for the entire plant but was more pronounced. The damaged roots alone, compared to control roots, received up to 72% (cauliflower) and 75% (kale) of the eggs. By contrast, surrogate leaves sprayed with methanolic leaf surface extracts from the most preferred plants which had been damaged were not discriminated from surrogate leaved sprayed with extracts of the respective control plants. Analysis of glucosinolate levels in methanolic leaf surface extracts revealed that root damage resulted in enhanced concentrations of indole-glucosinolates on the leaf surface in kale but not in cauliflower. Although indole-glucosinolates are oviposition stimulants for the cabbage root fly, the induced changes were apparently too small to influence oviposition behaviour.     

A molecular mechanism that confines the activity pattern of miR165 in Arabidopsis leaf primordia

In Arabidopsis leaf primordia, the expression of HD‐Zip III, which promotes tissue differentiation on the adaxial side of the leaf primordia, is repressed by miRNA165/166 (miR165/166). Small RNAs, including miRNAs, can move from cell to cell. In this study, HD‐Zip III expression was strikingly repressed by miR165/166 in the epidermis and parenchyma cells on the abaxial side of the leaf primordia compared with those on the adaxial side. We also found that the MIR165A locus, which was expressed in the abaxial epidermis, was sufficient to establish the rigid repression pattern of HD‐Zip III expression in the leaf primordia. Ectopic expression analyses of MIR165A showed that the abaxial‐biased miR165 activity in the leaf primordia was formed neither by a polarized distribution of factors affecting miR165 activity nor by a physical boundary inhibiting the cell‐to‐cell movement of miRNA between the adaxial and abaxial sides. We revealed that cis‐acting factors, including the promoter, backbone, and mature miRNA sequence of MIR165A, are necessary for the abaxial‐biased activity of miR165 in the leaf primordia. We also found that the abaxial‐determining genes YABBYs are trans‐acting factors that are necessary for the miR165 activity pattern, resulting in the rigid determination of the adaxial–abaxial boundary in leaf primordia. Thus, we proposed a molecular mechanism in which the abaxial‐biased patterning of miR165 activity is confined.     

Insect oviposition preference between Epichloë‐symbiotic and Epichloë‐free grasses does not necessarily reflect larval performance

Variation in plant communities is likely to modulate the feeding and oviposition behavior of herbivorous insects, and plant‐associated microbes are largely ignored in this context. Here, we take into account that insects feeding on grasses commonly encounter systemic and vertically transmitted (via seeds) fungal Epichloë endophytes, which are regarded as defensive grass mutualists. Defensive mutualism is primarily attributable to alkaloids of fungal origin. To study the effects of Epichloë on insect behavior and performance, we selected wild tall fescue (Festuca arundinacea) and red fescue (Festuca rubra) as grass–endophyte models. The plants used either harbored the systemic endophyte (E+) or were endophyte‐free (E?). As a model herbivore, we selected the Coenonympha hero butterfly feeding on grasses as larvae. We examined both oviposition and feeding preferences of the herbivore as well as larval performance in relation to the presence of Epichloë endophytes in the plants. Our findings did not clearly support the female's oviposition preference to reflect the performance of her offspring. First, the preference responses depended greatly on the grass–endophyte symbiotum. In F. arundinacea, C. hero females preferred E+ individuals in oviposition‐choice tests, whereas in F. rubra, the endophytes may decrease exploitation, as both C. hero adults and larvae preferred E? grasses. Second, the endophytes had no effect on larval performance. Overall, F. arundinacea was an inferior host for C. hero larvae. However, the attraction of C. hero females to E+ may not be maladaptive if these plants constitute a favorable oviposition substrate for reasons other than the plants' nutritional quality. For example, rougher surface of E+ plant may physically facilitate the attachment of eggs, or the plants offer greater protection from natural enemies. Our results highlight the importance of considering the preference of herbivorous insects in studies involving the endophyte‐symbiotic grasses as host plants.     

Differences in host selection and performance between B and Q putative species of Bemisia tabaci on three host plants

B and Q are two putative species of the Bemisia tabaci complex (Hemiptera: Aleyrodidae), and are among the most invasive and destructive pests of crops and horticultural plants worldwide. In China, Q predominates and is displacing B. Although researchers have proposed that the higher capacity of Q to utilize host plants plays an important role in its replacement of B, there are few relevant field surveys and experimental studies. The difference in host assessment between B and Q in multiple‐choice rather than in no‐choice situations may be essential to understanding the displacement. Here, we compared settling and oviposition preferences, and adult and nymph performance, for the putative species B and Q of the B. tabaci complex on three common host species: poinsettia [Euphorbia pulcherrima Wild. ex Klotsch (Euphorbiaceae)], cotton [Gossypium hirsutum L. (Malvaceae)], and cabbage [Brassica oleracea L. (Brassicaceae)]. Although the preferred hosts for settling and oviposition were the same as those that supported maximum fitness (adult longevity, fecundity, and nymph survivorship), these hosts differed between B and Q. When given a choice, B preferred to settle and oviposit on cabbage over poinsettia and cotton, whereas Q preferred to settle and oviposit on poinsettia and cotton over cabbage. In a no‐choice experiment, adult longevity, fecundity, and nymphal survival for B were greater on cabbage than on poinsettia and cotton, but the opposite was true for Q.     

Cabbage waxes affect Trissolcus brochymenae response to short‐range synomones

We show that induced synomones, emitted as a consequence of Murgantia histrionica activity on Brassica oleracea, are adsorbed by the epicuticular waxes of leaves and perceived by the egg parasitoid Trissolcus brochymenae. Leaves were exposed to M. histrionica females placed on the abaxial leaf surface. After 24 h, the leaves were treated mechanically using gum arabic, or chemically using chloroform, on the adaxial surface, and finally the adaxial surface was assayed with T. brochymenae by two‐choice tests in a closed arena. Wasp females responded to mechanically dewaxed cabbage leaf portions with feeding punctures and footprints (Ff) and with feeding punctures, oviposition and footprints (FOf), showing no effect of wax removal. In contrast, the removal of the epicuticular waxes from leaf portions close to FOf, and from leaves with oviposition and footprints (Of), determined the lack of responses by T. brochymenae. Solvent extracts of different treatments were bioassayed, but only FOf triggered parasitoid response. Thus the detection of oviposition‐induced synomones by the parasitoid depends on their adsorption by the epicuticular waxes. Mechanical wax removal from leaf portions contaminated with host footprints (f) also determined a lack of wasp responses, suggesting that the footprints might trigger the induction of a “footprint‐induced synomone” adsorbed onto the epicuticular waxes and exploited by the parasitoid. Leaf portions with the abaxial lamina previously dewaxed and then contaminated by footprints (D+f) of M. histrionica did not affect the parasitoid response, indicating that the abaxial epicuticular waxes are not directly involved in the chemicals induced by M. histrionica footprints.     

Valuable New Resistances Ensure Improved Management of Sclerotinia Stem Rot (Sclerotinia sclerotiorum) in Horticultural and Oilseed Brassica Species

Field resistances against Sclerotinia rot (SR) (Sclerotinia sclerotiorum) were determined in 52 Chinese genotypes of Brassica oleracea var. capitata, 14 Indian Brassica juncea genotypes carrying wild weedy Brassicaceae introgression(s) and four carrying B‐genome introgression, 22 Australian commercial Brassica napus varieties, and 12 B. napus and B. juncea genotypes of known resistance. All plants were individually inoculated by securing an agar disc from a culture of S. sclerotiorum growing on a glucose‐rich medium to the stem above the second internode with Parafilm tape. Mean stem lesion length across tested genotypes ranged from <1 to >68 mm. While there was considerable diversity within the germplasm sets from each country, overall, 65% of the B. oleracea var. capitata genotypes from China showed the highest levels of stem resistance, a level comparable with the highest resistance ever recorded for oilseed B. napus or B. juncea from China or Australia. One Indian B. juncea line carrying weedy introgression displayed a significant level of both stem and leaf resistance. However, the vast majority of commercial Australian oilseed B. napus varieties fell within the most susceptible 40% of genotypes tested for stem disease. There was no correlation between expressions of stem versus leaf resistance, suggesting their independent inheritance. A few Chinese B. oleracea var. capitata genotypes that expressed combined extremely high‐level stem (≤1 mm length) and leaf (≤0.5 mean number of infections/plant) resistance will be particularly significant for developing new SR‐resistant horticultural and oilseed Brassica varieties.     

Mediation of host selection and oviposition behavior in the diamondback moth Plutella xylostella and its predator Chrysoperla carnea by chemical cues from cole crops

Host plant-mediated orientation and oviposition by diamondback moth (DBM) Plutella xylostella (L.) (Lepidoptera: Yponomeutidae) and its predator Chrysoperla carnea Stephens (Neuroptera: Chrysopidae) were studied in response to four different brassica host plants: cabbage, (Brassica oleracea L. subsp. capitata), cauliflower (B. oleracea L. subsp. botrytis), kohlrabi (B. oleracea L. subsp. gongylodes), and broccoli (B. oleracea L. subsp. italica). Results from laboratory wind tunnel studies indicated that orientation of female DBM and C. carnea females towards cabbage and cauliflower was significantly greater than towards either broccoli or kohlrabi plants. However, DBM and C. carnea males did not orient towards any of the host plants. In no-choice tests, oviposition by DBM did not differ significantly among the test plants, while C. carnea layed significantly more eggs on cabbage, cauliflower, and broccoli than on kohlrabi. However, in free-choice tests, oviposition by DBM was significantly greater on cabbage, followed by cauliflower, broccoli, and kohlrabi, while C. carnea preferred to oviposit on cabbage and cauliflower, followed by broccoli and kohlrabi. The predation rates of DBM by C. carnea on kohlrabi and broccoli were not significantly different from one another, but were significantly higher than that on cabbage and cauliflower. When two types of plant, intact and injured, were available to adult DBM, female oviposition was significantly greater on injured plant leaves than on intact plants leaves. Similarly, C. carnea oviposition was significantly greater on injured plant leaves than on intact leaves. Implications regarding the possible role of green leaf volatiles in host selection/preference, as well as in tritropic interactions, are discussed.     

Plant‐mediated interactions between two herbivores differentially affect a subsequently arriving third herbivore in populations of wild cabbage

• Plants are part of biodiverse communities and frequently suffer from attack by multiple herbivorous insects. Plant responses to these herbivores are specific for insect feeding guilds: aphids and caterpillars induce different plant phenotypes. Moreover, plants respond differentially to single or dual herbivory, which may cascade into a chain of interactions in terms of resistance to other community members. Whether differential responses to single or dual herbivory have consequences for plant resistance to yet a third herbivore is unknown.
• We assessed the effects of single or dual herbivory by Brevicoryne brassicae aphids and/or Plutella xylostella caterpillars on resistance of plants from three natural populations of wild cabbage to feeding by caterpillars of Mamestra brassicae. We measured plant gene expression and phytohormone concentrations to illustrate mechanisms involved in induced responses.
• Performance of both B. brassicae and P. xylostella was reduced when feeding simultaneously with the other herbivore, compared to feeding alone. Gene expression and phytohormone concentrations in plants exposed to dual herbivory were different from those found in plants exposed to herbivory by either insect alone. Plants previously induced by both P. xylostella and B. brassicae negatively affected growth of the subsequently arriving M. brassicae. Furthermore, induced responses varied between wild cabbage populations.
• Feeding by multiple herbivores differentially activates plant defences, which has plant‐mediated negative consequences for a subsequently arriving herbivore. Plant population‐specific responses suggest that plant populations adapt to the specific communities of insect herbivores. Our study contributes to the understanding of plant defence plasticity in response to multiple insect attacks.

     

Quantification of plant surface metabolites by matrix‐assisted laser desorption–ionization mass spectrometry imaging: glucosinolates on Arabidopsis thaliana leaves

The localization of metabolites on plant surfaces has been problematic because of the limitations of current methodologies. Attempts to localize glucosinolates, the sulfur‐rich defense compounds of the order Brassicales, on leaf surfaces have given many contradictory results depending on the method employed. Here we developed a matrix‐assisted laser desorption–ionization (MALDI) mass spectrometry protocol to detect surface glucosinolates on Arabidopsis thaliana leaves by applying the MALDI matrix through sublimation. Quantification was accomplished by spotting glucosinolate standards directly on the leaf surface. The A. thaliana leaf surface was found to contain approximately 15 nmol of total glucosinolate per leaf with about 50 pmol mm^?2 on abaxial (bottom) surfaces and 15–30 times less on adaxial (top) surfaces. Of the major compounds detected, 4‐methylsulfinylbutylglucosinolate, indol‐3‐ylmethylglucosinolate, and 8‐methylsulfinyloctylglucosinolate were also major components of the leaf interior, but the second most abundant glucosinolate on the surface, 4‐methylthiobutylglucosinolate, was only a trace component of the interior. Distribution on the surface was relatively uniform in contrast to the interior, where glucosinolates were distributed more abundantly in the midrib and periphery than the rest of the leaf. These results were confirmed by two other mass spectrometry‐based techniques, laser ablation electrospray ionization and liquid extraction surface analysis. The concentrations of glucosinolates on A. thaliana leaf surfaces were found to be sufficient to attract the specialist feeding lepidopterans Plutella xylostella and Pieris rapae for oviposition. The methods employed here should be easily applied to other plant species and metabolites.     

Sooty Mould on Hibiscus rosa‐sinensis Caused by Leptoxyphium kurandae is Associated with Extrafloral Nectaries

In August 2013, sooty mould was observed on Chinese hibiscus (Hibiscus rosa‐sinensis) in a propagation nursery in Seoul, Korea. The sooty mould initially developed at the junction between the leaf blade and leaf petiole and then dispersed along the vein on the abaxial surface. The fungal growth pattern on the plants was quite different from general sooty moulds growing on honeydew secreted by insects on the plants. On the basis of the morphological characteristics and phylogenetic analysis using the internal transcribed spacer rDNA, this fungus was identified as Leptoxyphium kurandae. A pathogenicity test was carried out to fulfil Koch's postulates. Through field observation and a pathogenicity test, we found an association between the sooty mould and extrafloral nectaries. To our knowledge, this is the first report of sooty mould caused by L. kurandae on the extrafloral nectaries of H. rosa‐sinensis.