Managing lepidopteran pests in cabbage with herbicide-induced resistance, in combination with a pyrethroid insecticide

S-ethyldipropylthiocarbamate (EPTC) applied as a soil treatment or over-the-top spray on cabbage plants (Brassica oleracea L.) caused the leaves to turn ‘glossy’ for as long as 30 days. EPTC-induced glossy plants were damaged significantly less than untreated plants by diamondback moth,Plutella xylostella (L.), imported cabbage worm,Pieris rapae (L.), and cabbage looper,Trichoplusia ni (Hbn.). Reductions in damage were equivalent to those obtained from treatment with permethrin. When used in combination with permethrin, EPTC provided additive control of damage by these pests. Our calculations show EPTC-induced resistance to be cost-effective. This use of EPTC has several limitations, however. Younger plants (<9 leaves) were killed or injured by the herbicide. The growth of older plants was not affected, but plants did not become glossy for ca. 10 days after they were treated with EPTC. The crop must be protected with insecticides until the plants are mature enough to treat with EPTC, and until treated plants become glossy. In addition, since the glossy trait is only effective against first instar larvae, populations of later instars on glossy plants must be reduced with an application of insecticide. Finally, EPTC formulations are water-soluble and can be washed away from the plants by heavy rains and irrigation, which may make this use of EPTC impractical in some situations. Where its use is practical, and the indicated precautions are taken, EPTC-induced resistance could reduce dependence on chemical insecticides and reduce selection for insecticide resistance in diamondback moth.     

Predators mediate host plant resistance to a phytophagous pest in cabbage with glossy leaf wax

This study examined the role of generalist predators in producing higher mortality ofPlutella xylostella L. (Plutellidae) larvae on glossy vs. normal-wax cabbage,Brassica oleracea var.capitata L. To test this, survival and feeding ofP. xylostella were measured on individually caged glossy and normal-wax plants with and without each of three generalist predators,Chrysoperla carnea (Stephens) (Chrysopidae),Orius insidiosus (Say) (Anthocoridae), andHippodamia convergens Guerin-Meneville (Coccinellidae). In the greenhouse, predators always significantly reduced survival ofP. xylostella larvae on glossy plants, but never on normal-wax plants. In the field, predators significantly reducedP. xylostella survival on glossy plants, but onlyC. carnea was effective on normal-wax plants. In similar experiments with excised leaves,O. insidiosus andC. carnea were more effective predators on the glossy leaves, whileH. convergens was equally effective on both kinds of leaves. Patterns for feeding were similar, but significance levels differed from those forP. xylostella survival. The greater effectiveness of predators on glossy plants is apparently due to the reported improved mobility of these animals on glossy leaf surfaces. The data also suggest that reduced mining byP. xylostella exposes the larvae to more predation on glossy plants and contributes some to the resistance. Regardless of the mechanism, resistance toP. xylostella on glossyB. oleracea appears to depend on the action of generalist predators for its full expression. This dependence on predation must be considered in the development and deployment of glossy insect-resistantB. oleracea.     

Mobility of three generalist predators is greater on cabbage with glossy leaf wax than on cabbage with a wax bloom

Adult coccinellids Hippodamia convergens Guerrin-Menneville, adult minute pirate bugs Orius insidiosus (Say), and larval lacewings Chrysoperla carnea (Stephens) have been reported to more effectively reduce populations of Plutella xylostella (L.) (Plutellidae: Lepidoptera) larvae on a cabbage with glossy surface waxes than on a standard cabbage variety. To examine the mechanisms, the behavior of each predator species was quantified on the two cabbage types. All three predators spent less time walking on the standard variety and more time in other activities, including grooming and scrambling (ineffective forward locomotion). In addition, walking by each predator was distributed more evenly among the parts of the leaf on the glossy cabbage than on the standard variety. In small enclosures, C. carnea and O. insidiosus found and attacked individual first instar P. xylostella more quickly on the glossy cabbage. Scanning electron micrographs showed debris, probably wax, on the tarsae of C. carnea and O. insidiosus that had walked on the standard variety, but not on those that had walked on glossy cabbage. H. convergens tarsae did not accumulate debris on either cabbage type. Predator mobility and effectiveness were apparently impeded by wax crystals, which are present only on the standard variety. This mechanism can explain the greater effectiveness of these generalist predators against P. xylostella larvae on glossy vs normal-wax cabbage.     

The responses of some Lepidoptera to labiate herb and white clover extracts

Herbs have been traditionally used as intercrops with crop plants on the assumption that their odour repels pest species. Alcohol extracts and essential oils of labiate herbs were tested in the laboratory for deterrent/repellent responses to ovipositing Plutella xylostella (L.) and feeding larvae of P. xylostella and Pieris brassicae L.Alcohol extracts of hyssop, rosemary, sage, thyme and white clover reduce oviposition by P. xylostella on pieces of brassica leaf in dual-choice tests. Essential oils of sage and thyme reduce oviposition on pieces of brassica leaf. Feeding, in dual-choice tests, by final instar P. xylostella and P. brassicae larvae is reduced by application of alcohol extracts of herbs.

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Résumé Des plantes médicinales ont traditionnellement été utilisées en association avec des plantes cultivées dans l'espoir que leur odeur repousserait les insectes nuisibles. Des extraits à l'alcool et des essences volatiles de labiées médicinales ont été essayés au laboratoire pour examiner leur effets répulsif et dissuadant pour la ponte de Plutella xylostella et l'alimentation des chenilles de P. xylostella et Pieris brassicae. Les extraits alcooliques d'hysope, de romarin, de sauge, de thym et de trèfle blanc réduisent la ponte de P. xylostella sur des morceaux de feuilles de chou traitées. Les essences volatiles de sauge et de thym réduisent la ponte de P. xylostella sur des morceaux de feuilles de chou dans des expériences de choix. L'alimentation des chenilles de dernier stade de P. xylostella et P. brassicae dans des expériences de choix est réduite par l'application d'extraits alcooliques des labiées médicinales.

     

Tritrophic Choice Experiments with Bt Plants,the Diamondback Moth (Plutella xylostella) and the parasitoid Cotesia plutellae

Parasitoids are important natural enemies of many pest species and are used extensively in biological and integrated control programmes. Crop plants transformed to express toxin genes derived from Bacillus thuringiensis (Bt) provide high levels of resistance to certain pest species, which is likely to have consequent effects on parasitoids specialising on such pests. A better understanding of the interaction between transgenic plants, pests and parasitoids is important to limit disruption of biological control and to provide background knowledge essential for implementing measures for the conservation of parasitoid populations. It is also essential for investigations into the potential role of parasitoids in delaying the build-up of Bt-resistant pest populations. The diamondback moth (Plutella xylostella), a major pest of brassica crops, is normally highly susceptible to a range of Bt toxins. However, extensive use of microbial Bt sprays has led to the selection of resistance to Bt toxins in P. xylostella. Cotesia plutellae is an important endoparasitoid of P. xylostella larvae. Although unable to survive in Bt-susceptible P. xylostella larvae on highly resistant Bt oilseed rape plants due to premature host mortality, C. plutellae is able to complete its larval development in Bt-resistant P. xylostella larvae. Experiments of parasitoid flight and foraging behaviour presented in this paper showed that adult C. plutellae females do not distinguish between Bt and wildtype oilseed rape plants, and are more attracted to Bt plants damaged by Bt-resistant hosts than by susceptible hosts. This stronger attraction to Bt plants damaged by resistant hosts was due to more extensive feeding damage. Population scale experiments with mixtures of Bt and wildtype plants demonstrated that the parasitoid is as effective in controlling Bt-resistant P. xylostella larvae on Bt plants as on wildtype plants. In these experiments equal or higher numbers of parasitoid adults emerged per transgenic as per wildtype plant. The implications for integrated pest management and the evolution of resistance to Bt in P. xylostella are discussed.     

Parasitism rates and sex ratios of a parasitoid wasp: effects of herbivore and plant quality

Summary We studied interactions among collards, Brassica oleracea var. acephala, the diamondback moth (DBM), Plutella xylostella (Lepidoptera: Yponomeutidae) and its parasitoid Diadegma insulare (Hymenoptera: Ichneumonidae) by manipulating plant nitrogen (N) concentrations in field and laboratory experiments. Parasitoid abundance strongly reflected DBM abundance and was related to total leaf N. Parasitism rates were high (70.7%) and density-independent. Wasp sex ratios varied markedly (3–93% female) in response to the herbivores, the plants, or both. Higher proportions of female wasps emerged from DBM larvae on plants with high leaf N than on unfertilized plants. More female wasps also emerged from larvae parasitized as larger instars. We suggest that wasps have the potential to control DBM populations through long-term numerical responses mediated by variable sex ratios.     

Two sides of a leaf blade: <Emphasis Type="Italic">Blumeria graminis</Emphasis> needs chemical cues in cuticular waxes of <Emphasis Type="Italic">Lolium perenne</Emphasis> for germination and differentiation

Plant surface characteristics were repeatedly shown to play a pivotal role in plant–pathogen interactions. The abaxial leaf surface of perennial ryegrass (Lolium perenne) is extremely glossy and wettable compared to the glaucous and more hydrophobic adaxial surface. Earlier investigations have demonstrated that the abaxial leaf surface was rarely infected by powdery mildew (Blumeria graminis), even when the adaxial surface was densely colonized. This led to the assumption that components of the abaxial epicuticular leaf wax might contribute to the observed impairment of growth and development of B. graminis conidia on abaxial surfaces of L. perenne. To re-assess this hypothesis, we analyzed abundance and chemical composition of L. perenne ab- and adaxial epicuticular wax fractions. While the adaxial epicuticular waxes were dominated by primary alcohols and esters, the abaxial fraction was mainly composed of n-alkanes and aldehydes. However, the major germination and differentiation inducing compound, the C₂₆-aldehyde n-hexacosanal, was not present in the abaxial epicuticular waxes. Spiking of isolated abaxial epicuticular Lolium waxes with synthetically produced n-hexacosanal allowed reconstituting germination and differentiation rates of B. graminis in an in vitro germination assay using wax-coated glass slides. Hence, the absence of the C₂₆-aldehyde from the abaxial surface in combination with a distinctly reduced surface hydrophobicity appears to be primarily responsible for the failure of normal germling development of B. graminis on the abaxial leaf surfaces of L. perenne.     

Factors affecting entomopathogenic nematode infection ofPlutella xylostella on a leaf surface

We investigated the ability of entomopathogenic nematodes to infect diamondback moth (DBM),Plutella xylostella (L.) (Lepidoptera: Plutellidae) on a leaf surface. In a leaf disk assay, mortality of late stage DBM larvae ranged from <7% caused bySteinernema kushidai Mamiya to >95% caused byS. carpocapsae (Weiser) All strain. LC₅₀ values forS. carpocapsae, S. riobravis Cabanillas, Poinar & Raulston, andHeterorhabditis bacteriophora Poinar NC1 strain were 14.6, 15.4, and 65.4 nematodes/larva, respectively.S. carpocapsae, S. riobravis, andH. bacteriophora caused 29%, 33%, and 14% mortality of DBM pupae, respectively. DBM mortality caused byS. carpocapsae on radish declined at low (<76%) to moderate (76–90%) RH, because nematode survival and infectivity declined at low (<76%) to moderate (76–90%) RH. However, DBM mortality caused byS. riobravis did not decline with RH.S. riobravis survival declined with RH, but infectivity did not. Overall, nematode survival and infectivity to DBM larvae were lower forS. riobravis than forS. carpocapsae. In addition, DBM mortality was higher on radish plants (pubescent leaves) than on cabbage plants (glaborous leaves).     

Parasitoid Preference for Host-Infested Plants Is Affected by the Risk of Intraguild Predation

Flowering Rorippa indicaplants are attended by ants that collect nectar and, at the same time, prey on herbivorous insects, including larvae of the diamondback moth, Plutella xylostella.Here, we showed that P. xylostellalarvae suffered higher predation on R. indicawhose flowers were accessible by ants than on plants those whose flowers were inaccessible. Ants showed equal predation preference between unparasitized and larvae parasitized by Cotesia plutellae,a dominant specialist parasitic wasp of P. xylostellalarvae. C. plutellaepreferred non-flowering, host-infested R. indicato flowering, host infested R. indica.Based on these results, we infer that the preference of C. plutellaefor non-flowering, host-infested plants is in part explained by the avoidance of intraguild predation by attending ants.     

Impact of flowering buckwheat on Lepidopteran cabbage pests and their parasitoids at two spatial scales

We assessed the potential of annual buckwheat, Fagopyrum esculentum Moench, to lead to improved parasitism of lepidopteran cabbage pests over four years. Pest, parasitism, and hyperparasitism rates were monitored in replicated cabbage plots (12 × 20 m) with or without 3 m wide buckwheat borders from 2000 to 2003. Floral borders did not significantly increase egg, larval, or pupal densities of cabbage looper, Trichoplusia ni (Hübner), imported cabbageworm, Pieris rapae (L.), or diamondback moth, Plutella xylostella (L.). Buckwheat increased parasitism rates by Voria ruralis (Fallen) on T. ni larvae and Cotesia rubecula (Marshall) on P. rapaelarvae over four years. Parasitism by Diadegma insulare (Cresson) on P. xylostella larvae was higher in buckwheat than control plots in the first year, and parasitism by Euplectrus plathypenae (Howard) on T. ni larvae was lower in buckwheat than control plots in the second year. The hyperparasitoid Conura side (Walker) attacked D. insulare all four years, but buckwheat did not affect hyperparasitism rates. The effect of spatial scale on pest densities and parasitism in 2001 was evaluated by comparing plots separated at least 67 m (nearby) versus 800 m apart (isolated). T. ni pupae and P. rapae eggs and pupae were more abundant in plots in closer proximity, whereas P. xylostella densities did not vary by the spatial separation of plots. Tachinids and Pteromalus puparum (L.) attacked more P. rapae in nearby plots. E. plathypenae responded to the treatment × scale interaction, parasitizing more in control than buckwheat when plots were isolated but not when plots were nearby.     

Effects of wax bloom variation in Brassica oleracea on foraging by a vespid wasp

Foraging by individual Polistes dominulus (Christ) (Hymenoptera: Vespidae) was observed and compared on Brassica oleracea L. plants that differed in surface wax bloom. Twenty-six wasps, previously trained to forage on plants for 4th instar Trichoplusia ni (Walker), were presented in the greenhouse with mixtures of plants with normal wax bloom or genetically reduced wax bloom, and on which T. ni caterpillars had been placed. During foraging, the wasps were observed to slip significantly more frequently from the leaf surfaces of normal-wax bloom plants than from reduced-wax bloom plants (129 vs 63 occurrences) and to retrieve significantly fewer pieces of caterpillar after attack on normal-wax bloom plants (151 vs 223 pieces). Altogether the wasps retrieved 333 caterpillar pieces from reduced-wax bloom plants and 248 pieces from normal-wax bloom plants. Despite these differences, the number of caterpillars attacked and killed did not differ between the two wax bloom types (116 vs 121), nor did handling time for individual attacks (time from contact with prey until prey piece was carried to the nest) (170±12.5 s vs 180±10.5 s). Thus, in contrast with previous reports for smaller predators, wax bloom variation in B. oleracea did not influence the effectiveness of P. dominulus as a predator of T. ni.     

Artichoke Leaf Morphology and Surface Features in Different Micropropagation Stages

Artichoke (Cynara scolymus L.) leaf size and shape, glandular and covering trichomes, stomatal density, stomata shape, pore area and epicuticular waxes during micropropagation stages were studied by scanning electron microscopy (SEM) and morphometric analysis with the aim to improve the survival rate after transfer to greenhouse conditions. Leaves from in vitro shoots at the proliferation stage showed a spatular shape, ring-shaped stomata, a large number of glandular trichomes and juvenile covering hairs, but failed to show any epicuticular waxes. Leaves from in vitro plants at the root elongation stage showed a lanceolated elliptic shape with a serrated border, elliptical stomata, decreased pore area percentage, stomatal density, and mature covering trichomes. One week after transfer to ex vitro conditions, epicuticular waxes appeared on the leaf surface and stomata and pore area were smaller as compared to in vitro plants. Artichoke acclimatization may be improved by hormonal stimulation of root development, since useful morphological changes on leaves occurred during root elongation.     

Effect of spring flooding on endophyte differentiation,nitrogenase activity,root growth and shoot growth inMyrica gale

Summary Spring flooding was investigated as a possible limiting factor in the development of nitrogenase activity, root growth, and shoot growth inMyrica gale. Dormant, one year oldMyrica gale plants were placed in a greenhouse in early April and given three treatments: control (not flooded), flooded-water (flooded with water to 2.5 cm above the soil level) and flooded-peat (flooded with water-saturated peat to 4.0 cm above the soil level). Nitrogenase activity was absent at budbreak but appeared concurrently with the differentiation of vesicles by theFrankia sp. endophyte. Flooding delayed the onset of nitrogenase activity, substantially reduced the specific nitrogenase activity of the nodules, and also severely limited the production of the new nodule biomass. Consequently by 67 days past budbreak nitrogenase activity was much greater in the control plants (5.55±0.42 mol C₂H₄/plant.h; ± SE; N=9) than in the flooded-water (1.18±0.29) and flooded-peat (0.15±0.05) plants. Production of new secondary roots was substantially reduced in the flooded plants but adventitious roots were rapidly produced along the flooded portion of the stem in the better aerated zone near the surface. New nodules formed on several adventitious roots by 67 days indicating that the plants are able to replace their largely nonfunctional deeply flooded nodules with new nodules in the aerobic zone. Initially shoot growth was unaffected by flooding but by 67 days the flooded plants had substantially less leaf biomass, lower leaf and stem nitrogen concentrations, and less total shoot nitrogen content than the control plants.     

Natural enemies ofPlutella xylostella (Lep.: Plutellidae) on crucifers in Honduras

Three primary parasitoids in three genera were reared fromPlutella xylostella (L.) larvae and pupae collected in various crucifer producing regions of Honduras. The ichneumonidDiadegma insulare (Cresson) was by far the most abundant species. TwoSpilochalcis species, facultative hyperparasitoids attackingP. xylostella andD. insulare, were encountered as well as eleven species in nine genera of obligate hyperparasitoids attackingD. insulare. Three vespid predators are noted as predating on larvae.      

Surface wax, a new taxonomic feature in Plagiochilaceae

A scanning electron microscope study of 81 species of Plagiochilaceae revealed the presence of superficial waxes on the leaves and stems ofPlagiochilion mayebarae and 5 species ofPlagiochila. The waxes are not visible in the light microscope and were unknown in Plagiochilaceae.Plagiochila fuscolutea andP. longiramea (=P. sect.Fuscoluteae) are characterised by the predominant occurrence of membraneous wax platelets;Plagiochila aerea, P. rudischusteri andP. tabinensis(=P. sect.Bursatae) predominately form various types of wax rodlets. Our findings show for the first time the systematic usefulness of leaf surface waxes in the liverworts.P. tabinensis contains surface waxes in amounts of ca. 1.4% dry weight composing of steryl esters, triacylglycerols and free fatty acids.     

Host plant nutritional quality affects the performance of the parasitoid Diadegma insulare

The well documented biochemical profile of Brassicaceae, oligophagy of the herbivore Plutella xylostella (L.) (Lepidoptera: Plutellidae), and host specialization of the parasitoid Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae) provide an ideal system for investigating tritrophic interactions mediated by nutritional quality of plants. We evaluated the bottom-up effects of five soil fertility regimes on nutritional quality of canola (Brassica napus L.) and then on several fitness correlates of female and male D. insulare as mediated through P. xylostella. Variation in soil fertility influenced the nutritional quality of host plants and this in turn affected the performance of D. insulare. In general, D. insulare performed best on plants grown with 3.0 g fertilizer pot⁻¹; these plants had 2.06-, 3.77-, and 1.02-fold more nitrogen, phosphorous and potassium, respectively than ones grown without any added fertilizer. P. xylostella escape from D. insulare was highest (32%) on plants grown at 1.0 g fertilizer, and this could be attributed to both physical and physiological defense mechanisms mediated by host plant nutritional quality. Plant stress and plant vigor are competing paradigms pertaining to the performance of herbivorous insects on their host plants. These hypotheses were originally proposed to predict responses of herbivores, but may also explain the effects of plant quality on koinobiont parasitoids, such as D. insulare.     

Nitrate and ammonium absorption by plants growing at a sufficient or insufficient level of phosphorus in nutrient solutions

Summary Absorption of nitrate and ammonium was studied in water culture experiments with 4 to 6 weeks old plants of barley (Hordeum vulgare L.), buckwheat (Fagopyrum esculentum L. Moench) and rape (Brassica napus L.). The plants were grown in a complete nutrient solution with nitrate (5.7±0.2 mM) or nitrate (5.6±0.2 mM) + ammonium (0.04±0.02 mM). The pH of the nutrient solution was kept at 5.0 using a pH-stat. It was found that phosphorus deficiency reduced the rate of nitrate uptake by 58±3% when nitrate was the sole N source and by 83±1% when both nitrate and ammonium were present. The reduction occurred even before growth was significantly impeded by P deficiency. The inhibition of the uptake of ammonium was less,i.e. ammonium constituted 10±1% of the total N uptake in the P sufficient plants and 30±5% in the P deficient plants. The reduction of nitrate absorption greatly decreased the difference between the uptake of anions and cations. It is suggested that P deficiency reduced the assimilation of NO ₃ ^– into the proteins, which might cause a negative feedback on NO ₃ ^– influx and/or stimulate NO ₃ ^– efflux.     

Tri-trophic consequences of UV-B exposure: plants,herbivores and parasitoids

In this paper we demonstrate a UV-B-mediated link between host plants, herbivores and their parasitoids, using a model system consisting of a host plant Brassica oleracea, a herbivore Plutella xylostella and its parasitoid Cotesia plutellae. Ultraviolet-B radiation (UV-B) is a potent elicitor of a variety of changes in the chemistry, morphology and physiology of plants and animals. Recent studies have demonstrated that common signals, such as jasmonic acid (JA), play important roles in the mechanisms by which plants respond to UV-B and to damage by herbivores. Plant responses elicited by UV-B radiation can affect the choices of ovipositing female insects and the fitness of their offspring. This leads to the prediction that, in plants, the changes induced as a consequence of UV damage will be similar to those elicited in response to insect damage, including knock-on effects upon the next trophic level, predators. In our trials female P. xylostella oviposited preferentially on host plants grown in depleted UV-B conditions, while their larvae preferred to feed on tissues from UV-depleted regimes over those from UV-supplemented ones. Larval feeding patterns on UV-supplemented tissues met the predictions of models which propose that induced defences in plants should disperse herbivory; feeding scars were significantly smaller and more numerous – though not significantly so – than those on host plant leaves grown in UV-depleted conditions. Most importantly, female parasitoids also showed a clear pattern of preference when given the choice between host plants and attendant larvae from the different UV regimes; however, in the case of the female parasitoids, the choice was in favour of potential hosts foraging on UV-supplemented tissues. This study demonstrates the potential for UV-B to elicit a variety of interactions between trophic levels, most likely mediated through effects upon host plant chemistry.     

Relative activity of baculoviruses of the diamondback moth, <Emphasis Type="Italic">Plutella xylostella</Emphasis> (L.) (Lepidoptera: Plutellidae)

The relative activities of the granulovirus (PxGV) and the nucleopolyhedrovirus (PxMNPV) of the diamondback moth, Plutella xylostella (L.), against this insect were evaluated in the laboratory. Similar numbers of occlusion bodies (OBs) of PxGV and PxMNPV were required to kill P. xylostella. However, cadavers of insects infected with PxGV contained more OBs per unit weight than did cadavers infected with PxMNPV. Less cadaver material was thus required to kill insects with PxGV. Larvae that were killed by PxGV survived approximately 2 d longer than did those killed by PxMNPV. When P. xylostella larvae were fed both viruses together, mortality was higher than that caused by either virus alone. The virulence of PxMNPV produced in P.␣xylostella did not differ from that of PxMNPV produced in the beet armyworm, Spodoptera exigua (Hübner). S. exigua was less susceptible to PxMNPV than to its homologous nucleopolyhedrovirus, SeMNPV. Use of trade names does not imply endorsements by USDA-ARS of products named nor criticism of similar ones not mentioned.