Oviposition by Plutella xylostella (Lepidoptera: Plutellidae) and effects of phylloplane waxiness

Three approaches were used to investigate effects of host plant epicuticular waxes on oviposition site selection by Plutella xylostella (L.). In the first approach, oviposition on canola (Brassica napus L.) that had epicuticular wax reduced by application of a carbamate herbicide (S-ethyl dipropylthiocarbamate) was compared with oviposition on untreated control plants. A second approach compared oviposition on sibling strains of B. napus with different wax blooms (glossy and waxy), and a third approach compared oviposition by P. xylostella on parafilm that had been applied to glossy and waxy B. napus strains for transfer of leaf components. Significantly more eggs were deposited on herbicide-treated plants (with reduced epicuticular wax) than on untreated controls. Similarly, more eggs were deposited on glossy than on waxy sibling strains of B. napus. In parafilm assays significantly more eggs were deposited on treated than on untreated parafilm. Several mechanisms could explain the differences in attractiveness of surfaces with varying wax content as oviposition sites for P. xylostella, including visual, chemical, and tactile differences between substrates. These mechanisms are discussed.     

Management of diamondback moth,Plutella xylostella (Lepidoptera: Plutellidae) by mating disruption

Abstract Field trials were conducted in China in 2008 and 2009 to evaluate the efficacy of mating disruption (MD) on diamondback moth, Plutella xylostella, in cabbage, Brassica oleracea var. capitata. Effectiveness was positively correlated with the MD dispenser density in the field. A density of 167 MD dispensers per ha produced an average population decrease of about 50% compared to the conventional‐practice field. Significant fewer males were captured in pheromone‐treated and conventional‐practice fields than in the blank control field, but the difference was not significant between the pheromone‐treated and conventional‐practice fields. In addition, fewer eggs and larvae were observed in pheromone‐treated fields. Our results suggest mating disruption coupled with minimal insecticidal supplements is a promising solution for resistance management and control of diamondback moth infestation.     

Evaluating trap crops for diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae)

Potential trap crops for the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), were evaluated through a series of ovipositional preference and larval survival experiments in outdoor screenhouses in 2002 and 2003. Hosts examined as trap crops were glossy and waxy collards, Brassica oleracea L. variety acephala; Indian mustard, Brassica juncea (L.) Czern; and yellow rocket, Barbarea vulgaris (R. Br.) variety arcuata. More eggs were laid on the potential trap crops, with the exception of waxy collards, than on cabbage. When P. xylostella was offered multiple hosts at the same time, numbers of eggs laid on glossy collards, Indian mustard, and yellow rocket were 3, 18, and 12 times greater than on cabbage, respectively. Similarly, when P. xylostella was offered a single trap crop host and cabbage, numbers of eggs laid on glossy collards, Indian mustard, and yellow rocket were 300, 19, and 110 times greater than on cabbage, respectively. Our studies suggest differences in oviposition between the potential trap crops and cabbage were likely due to host volatiles, leaf morphology and color, or a combination of these factors, rather than to total leaf areas, leaf shape, or plant architecture. Two-choice tests with a Y-tube olfactometer indicated that plant volatiles were major factors in P. xylostella host preference. The percentage larval survival from egg to pupation was 22.2% on cabbage, 18.9% on waxy collards, and 24.4% on Indian mustard, whereas survival was significantly lower on glossy collards (6.7%) and yellow rocket (0%). Based on our tests, it seems that yellow rocket may be the best candidate for use as a trap crop for P. xylostella because it is highly attractive for oviposition, but larvae do not survive on it.     

Nickel hyperaccumulation by Streptanthus polygaloides protects against the folivore Plutella xylostella (Lepidoptera: Plutellidae)

We determined the effectiveness of Ni as an elemental defence of Streptanthus polygaloides (Brassicaceae) against a crucifer specialist folivore, diamondback moth (DBM), Plutella xylostella. An oviposition experiment used arrays of S. polygaloides grown on Ni-amended (high-Ni) soil interspersed with plants grown on unamended (low-Ni) soil and eggs were allowed to hatch and larvae fed freely among plants in the arrays. We also explored oviposition preference by allowing moths to oviposit on foil sheets coated with high- or low-Ni plant extract. This was followed by an experiment using low-Ni plant extract to which varying amounts of Ni had been added and an experiment using sheets coated with sinigrin (allyl glucosinolate) as an oviposition stimulant. Diamondback moths laid 2.5-fold more eggs on low-Ni plants than on high-Ni plants and larval feeding was greater on low-Ni plants. High-Ni plants grew twice as tall, produced more leaves, and produced almost 3.5-fold more flowers. Low-Ni plants contained more allyl glucosinolate than high-Ni plants and moths preferred to oviposit on foil sheets dipped in low-Ni plant extract. Moths showed no preference when Ni concentration of low-Ni extract was varied and overwhelmingly preferred sinigrin coated sheets. We conclude that Ni hyperaccumulation is an effective elemental defence against this herbivore, increasing plant fitness through a combination of toxicity to DBM larvae and decreased oviposition by adults.     

Effects of temperature and humidity on emergence dynamics of Plutella xylostella (Lepidoptera: Plutellidae)

The effects of temperature and humidity on the emergence patterns of diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), were studied at four temperatures (20, 25, 28, and 33 degrees C),three relative humidities (50, 70, and 90%) and a photoperiod of 14:10 (L:D) h. Both sexes emerged primarily in the late afternoon to early morning, and the peaks of emergence varied depending on temperature and humidity. Females emerged significantly earlier than males. Incubation at 33 degrees C and 90% RH had a significant effect on the emergence rate, but no significant interaction was found between temperature and humidity. Emergence duration was 25.3 h for both sexes at 90% RH, with emergence duration at 33 degrees C lower than the other treatments. The sex ratios of diamondback moth under different temperature and humidity treatments were approximately 1:1, and there were no significant effects of temperature and humidity or interactions between these two factors.     

Effect of electron beam irradiation on developmental stages of Plutella xylostella (Lepidoptera: Plutellidae)

The effect of electron beam irradiation on each developmental stage of diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), was examined. Eggs, larvae, pupae, and adults were irradiated at target doses of 30, 50, or 100 Gy or they were left untreated as controls in replicated experiments. When eggs and pupae were irradiated with 100 Gy, emergence rates greatly decreased and, although some adults laid eggs, they did not hatch. Egg hatching of irradiated larvae and adult decreased with increasing irradiation doses. However, electron beam irradiation did not kill P. xylostella directly. Adult longevity was not affected. Reciprocal crosses between irradiated and unirradiated moths demonstrated that females were more radiosensitive than males in hatchability; however, the difference was not significant. In addition, electron beam-irradiated larvae showed typical DNA fragmentation in a dose-dependent manner compared with cells from unirradiated larvae. Our findings suggest that electron beam irradiation induces abnormal development and reproduction of P. xylostella; therefore, it may contribute to effective disinfestation and quarantine treatments of P. xylostella.     

Oviposition site selection by the diamondback moth,Plutella xylostella (L.) (Lepidoptera: Plutellidae)

AlthoughPlutella xylostella (L.) is a worldwide pest of cruciferous crops, relatively little is known about its oviposition behaviour. This study was undertaken to provide necessary information about mechanisms involved inP. xylostella host selection. Four oviposition behaviours were described. Moths were given artificial substrates treated with water, sinigrin, orBrassica napus (cv. Westar) squashes, combined withB. napus volatiles and/or grooves in the substrate. No eggs were deposited in the absence of olfactory and gustatory stimuli. Moths given gustatory but not olfactory stimuli deposited similar numbers of eggs but spent significantly more time performing olfactory-related behaviours. Conversely, moths given olfactory but not gustatory stimuli did not oviposit.SSubstrate grooves did not influence egg numbers but appeared to influence egg location. The order in which oviposition behaviours occurs and the relative importance of stimuli type are discussed.     

Evidence for resistance to pyrethroids and organophosphates in Plutella xylostella (Lepidoptera: Plutellidae) from Pakistan

The susceptibility of representative pyrethroid (cypermethrin, deltamethrin, lambdacyhalothrin, bifenthrin), organophosphate (chlorpyriphos, triazophos, profenophos) and new chemistry insecticides (spinosad, indoxacarb and emamectin) was investigated for 18 field populations of Plutella xylostella (Linnaeus) from three different zones in Pakistan. The LC(50) (mg ml(-1); 48 h) values of pyrethroids for various populations ranged from 0.19-1.88 for cypermethrin, 0.31-2.64 for deltamethrin, 0.08-1.16 for lambdacyhalothrin and 0.07-0.88 for bifenthrin. The LC(50) (mg ml(-1); 48 h) of organophosphates ranged from 0.52-5.67 for chlorpyriphos, 0.37-4.14 for triazophos and 0.03-2.65 for profenophos. The most probable reason for low toxicity of organophosphates and pyrethroids is the evolution of multiple resistance mechanisms; however, further studies are required to establish these mechanisms. When these same products were tested against a susceptible laboratory population (Lab-Pak), the new chemistry compounds were significantly more toxic than pyrethroids and organophosphates. The results are discussed in relation to integrated pest management and insecticide resistance management strategies for P. xylostella.     

Exploring Valid Reference Genes for Quantitative Real-time PCR Analysis in Plutella xylostella (Lepidoptera: Plutellidae)

Abstract: Quantitative real-time PCR (qRT-PCR), a primary tool in gene expression analysis, requires an appropriate normalization strategy to control for variation among samples. The best option is to compare the mRNA level of a target gene with that of reference gene(s) whose expression level is stable across various experimental conditions. In this study, expression profiles of eight candidate reference genes from the diamondback moth, Plutella xylostella, were evaluated under diverse experimental conditions. RefFinder, a web-based analysis tool, integrates four major computational programs including geNorm, Normfinder, BestKeeper, and the comparative ΔCt method to comprehensively rank the tested candidate genes. Elongation factor 1 (EF1) was the most suited reference gene for the biotic factors (development stage, tissue, and strain). In contrast, although appropriate reference gene(s) do exist for several abiotic factors (temperature, photoperiod, insecticide, and mechanical injury), we were not able to identify a single universal reference gene. Nevertheless, a suite of candidate reference genes were specifically recommended for selected experimental conditions. Our finding is the first step toward establishing a standardized qRT-PCR analysis of this agriculturally important insect pest.     

Susceptibility of Plutella xylostella (L.) (Lepidoptera: Plutellidae) populations in Mexico to commercial formulations of Bacillus thuringiensis

Populations of diamondback moth, Plutella xylostella (L.), sampled from commercial fields of crucifers in three states of Mexico, were tested for susceptibility to commercial formulations of Bacillus thuringiensis subsp. kurstaki (Berliner) (Dipel 2X), B. thuringiensis subsp. aizawai (XenTari), delta endotoxin Cry 1C (MC), and CryIA(c) (MVP), and a mixture of B. thuringiensis subsp. kurstaki and subsp. aizawai (Agree). Leaf-dip bioassays confirmed variation in susceptibility of up to 13-fold for MVP, 12-fold for Dipel 2X, sevenfold for XenTari, fivefold for Agree, and less than fivefold for MC. Comparisons with previously published data indicate that at least the 12-fold variation in Dipel 2X would result in significant differences in control in the field. Based on the LC99 values observed for the products, we propose discriminating concentrations for each product. To ensure continued performance in the field we suggest that a resistance monitoring program be implemented to detect any changes in susceptibility to B. thuringiensis products and specific toxins and that their use be restricted to one generation per crop and that they be rotated with other groups of insecticides. Furthermore, we suggest enforcement of a crucifer host-free period and the development and implementation of cultural and biological control strategies to reduce overall population pressure so that fewer insecticidal treatments will be needed.     

Genetic, biochemical, and physiological characterization of spinosad resistance in Plutella xylostella (Lepidoptera: Plutellidae)

Bioassays (at generation G2) with a newly collected field population (designated MN) of Plutella xylostella (L.) (Lepidoptera: Plutellidae) from Multan, Pakistan, indicated resistance to spinosad, indoxacarb, deltamethrin, abamectin, and acetamiprid. At G2 the field-derived population was divided into two subpopulations, one was selected (G2 to G11) with spinosad (Spino-SEL), whereas the second was left unselected (UNSEL). A significant reduction in the resistance ratio for each compound was observed in UNSEL at G12, indicating that the observed resistance to each insecticide was unstable. For Spino-SEL, bioassays at G12 found that selection with spinosad gave a resistance ratio of 283 compared with MN at G2. The resistance to indoxacarb and acetamiprid in the Spino-SEL population increased to 13- and 67-fold, respectively, compared with MN at G2. The toxicity of deltamethrin to Spino-SEL was similar to its toxicity to the MN population at G2. This suggests that spinosad selection maintained the otherwise unstable resistance to the compound. In contrast, resistance to abamectin decreased significantly from G2 to G12 in Spino-SEL. Logit regression analysis of F1 reciprocal crosses between Spino-SEL and the susceptible Lab-UK indicated that resistance to spinosad was inherited as an autosomal, incompletely recessive trait. The spinosad resistance allele significantly delays the developmental time, reduced pupal weight, number of eggs laid, and number of eggs hatched compared with Lab-UK. Further analysis suggests Spino-SEL exhibited a significantly lower intrinsic rate of population increase (r(m)) to all other populations tested.     

玫烟色棒束孢诱导的小菜蛾免疫响应表达谱分析

【目的】本研究旨在筛选小菜蛾Plutella xylostella应对玫烟色棒束孢Isaria fumosorosea侵染的免疫应答及其网络调控基因,以进一步探讨小菜蛾对玫烟色棒束孢的防御机制。【方法】采用第二代高通量测序技术RNA-seq,对处理后12 h的感染玫烟色棒束孢和健康(平行对照)的小菜蛾4龄幼虫转录组进行了测序,通过生物信息学分析对得到的差异基因进行了功能注释和分类,对差异基因参与的信号通路进行了分析。【结果】在感菌和健康小菜蛾幼虫转录组两个表达谱里,分别获得了12 346 987和12 315 210个clean reads,有60.93%和61.26%的reads分别能比对到参考基因库里,其中完美匹配(perfect match)的比例分别为32.15%和32.73%。共得到351个显著差异表达基因(differentially expressed unigenes,DEUs),上调表达基因有275个,下调表达基因有76个,与免疫防御反应潜在相关的基因有156个。GO(Gene Ontology)富集分析有102个DEUs分布到46个GO term里,KEGG(Kyoto Encyclopedia of Genes and Genomes)pathway富集分析结果显示,有132个DEUs显著富集在13个代谢通路(pathway)里。【结论】这些差异表达基因中,大部分编码潜在的与免疫识别及调控相关的基因,集中在能量代谢、疾病反应和防御反应等相关通路。研究结果为挖掘与玫烟色棒束孢侵染相关的小菜蛾免疫应答候选基因提供了重要数据库,也为阐明小菜蛾对玫烟色棒束孢的免疫机制奠定理论基础。     

Enhanced attraction of Plutella xylostella (Lepidoptera: Plutellidae) to pheromone-baited traps with the addition of green leaf volatiles

Diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is one of the most serious pests of Brassicaceae crops worldwide. Electrophysiological and behavioral responses of P. xylostella to green leaf volatiles (GLVs) alone or together with its female sex pheromone were investigated in laboratory and field. GLVs 1-hexanol and (Z)-3-hexen-1-ol elicited strong electroantennographic responses from unmated male and female P. xylostella, whereas (Z)-3-hexenyl acetate only produced a relatively weak response. The behavioral responses of unmated moths to GLVs were further tested in Y-tube olfactometer experiments. (E)-2-Hexenal, (Z)-3-hexen-1-ol, and (Z)-3-hexenyl acetate induced attraction of males, reaching up to 50%, significantly higher than the response to the unbaited control arm. In field experiments conducted in 2008 and 2009, significantly more moths were captured in traps baited with synthetic sex pheromone with either (Z)-3-hexenyl acetate alone or a blend of (Z)-3-hexenyl acetate, (Z)-3-hexen-1-ol, and (E)-2-hexenal compared with sex pheromone alone and other blend mixtures. These results demonstrated that GLVs could be used to enhance the attraction of P. xylostella males to sex pheromone-baited traps.     

Insecticidal effects of selected biological control agents on the larvae of Plutella xylostella (Lepidoptera: Plutellidae)

To identify a more effective and safe biological control agent against a common cabbage pest, Plutella xylostella (L.) (Lepidoptera: Plutellidae), the insecticidal effects of selected biological agents were evaluated. The highest insecticidal effects determined were 100, 73.5, 45.5, 47 and 55.3% using toxin HD‐1 (isolated from the Harry Dumagae strain of Bacillus thuringiensis), toxin BTS‐1 (isolated from the tenebrionis strain of B. thuringiensis), B. thuringiensis Berliner, B. thuringiensis israelensis and B. thuringiensis kurstaki, respectively.     

Evaluation of fixed sample-size plans for Plutella xylostella (Lepidoptera: Plutellidae) on broccoli crops in Australia

Fixed sample-size plans for monitoring Plutella xylostella (L.) (Lepidoptera: Plutellidae) on broccoli and other Brassica vegetable crops are popular in Australia for their simplicity and ease of application. But the sample sizes used are often small, approximately 10-25 plants per crop, and it may be that they fail to provide sufficient information upon which to base pest control decisions. We tested the performance of seven fixed sample-size plans (10, 15, 20, 30, 35, 40, and 45 plants) by resampling a large data set on P. xylostella in commercial broccoli crops. For each sample size, enumerative and presence-absence plans were assessed. The precision of the plans was assessed in terms of the ratio of the standard error to the mean; and at least 45 and 35 samples were necessary for the enumerative and presence-absence plans, respectively, to attain the generally accepted benchmark of < or = 0.3. Sample sizes of 10-20 were highly imprecise. We also assessed the consequences of classifications based on action thresholds (ATs) of 0.2 and 0.8 larvae per plant for the enumerative case, and 0.15 and 0.45 proportion of plants of infested for the presence-absence case. Operating characteristic curves and investigations of the frequency of correct decisions suggest improvements in the performance of plans with increased sample size. In both the enumerative and presence-absence cases, the proportion of incorrect decisions was much higher for the lower of the two ATs assessed, and type II errors (i.e., failure to suggest pest control upon the AT is exceeded) generally accounted for the majority of this error. Type II errors are the most significant from a producer's standpoint. Further consideration is necessary to determine what is an acceptable type II error rate.     

Synergism between Bacillus thuringiensis and Xenorhabdus nematophila against resistant and susceptible Plutella xylostella (Lepidoptera: Plutellidae)

The diamondback moth (DBM) Plutella xylostella (L.) is one of the most destructive pests of crucifer crops in many parts of the world. Inappropriate applications have caused DBM to become resistant to most insecticides. We tested the combined effects of the bacteria Bacillus thuringiensis (Bt) and Xenorhabdus nematophila (Xn) against DBM. Populations of Bt-resistant moths (DBM-R), selected repeatedly with B. thuringiensis subsp. kurstaki HD-73 (Bt HD-73), showed resistance rates of 105.43 and 0.92 for Bt HD-73 and X. nematophila HB310 (Xn HB310), respectively. These results not only indicated that DBM-R is highly resistant to Bt HD-73 and only slightly resistant to Xn HB310, but also showed that there is no interactive resistance between the two bacteria. For the third-instar larvae of DBM-R and DBM-S, no significant differences were found in the oral insecticidal activities of Bt HD-73?+?Xn HB310 combination treatments or Bt HD-73 alone. However, mortality rates were significantly increased in response to Xn HB310?+?Bt HD-73 when compared with the Xn HB310-alone treatment. These results demonstrate a synergism of Bt on Xn, but no such effect of Xn on Bt. Therefore, our findings highlight the potential for both bacteria to be used in a mixture for field applications.     

Bottom-up effect of different host plants on Plutella xylostella (Lepidoptera: Plutellidae): a life-table study on canola

The effects of 10 commercial canola, Brassica napus L., cultivars widely grown in Iran--'SLM(046),' 'Opera,' 'Okapi,' 'RGS(003),' 'Modena,' 'Sarigol,' 'Zarfam,' 'Licord,' 'Hayula(420),' and 'Talaye'--on the demographic parameters of diamondback moth, Plutella xylostella L. (Lepidoptera-Plutellidae), were determined. The experiments were conducted in a growth chamber at 25 +/- 1 degrees C, 65 +/- 2% RH, and a photoperiod of 16:8 (L:D) h. The comparison of intrinsic rate of natural increase (r(m)), net reproductive rate (R0), and the survival rate of adult stage of P. xylostella on 10 canola cultivars suggested that this pest performed best on SLM(046). The r(m) value of P. xylostella ranged between 0.241 on RGS(003) and 0.304 on SLM(046). The R0, finite rate of increase (lambda), mean generation time (T), and doubling time (DT) values of P. xylostella on SLM(046) were 52, 1.35, 13.4, and 2.35 and on RGS(003) were 31, 1.27, 14.4, and 2.94, respectively. The Weibull model adequately described the shape of the survivorship curve of adult P. xylostella from life-table data. A significant fit was obtained with the Weibull model for P. xylostella in all experimental canola cultivars. As a result, SLM(046), Opera, and Hayula(420) were the most suitable hosts and had least negative impact on life-history statistics of the pest.     

Behavioural responses of diamondback moth Plutella xylostella (Lepidoptera: Plutellidae) to extracts derived from Melia azedarach and Azadirachta indica

The impact of three different doses of botanical insecticide derived from the syringa tree, Melia azedarach and the neem tree, Azadirachta indica was tested on the behaviour of the diamondback moth, Plutella xylostella (Linnaeus). Both botanical insecticides had a significant impact on larval behaviour. At higher doses the extracts showed feeding deterrent activity, with larvae preferring the untreated sides of cabbage leaves and consuming less of the treated half of cabbage leaves. The botanical insecticides had less of an effect on the oviposition behaviour of P. xylostella moths. In laboratory and glasshouse trials, significantly fewer eggs were oviposited on the plants that had been treated with syringa extracts. Therefore, the syringa extracts appear to have a repellent effect. In contrast, when exposed to the neem extracts the moths did not discriminate between control plants and treated plants. Behavioural observation indicated that, despite the lower number of eggs oviposited on cabbage treated with syringa extracts, the moths chose cabbage treated with the highest dose of syringa more often than they chose control cabbage plants. Similar observations were found in cabbage plants treated with neem, moths chose the medium dose more often than they chose the control. Oviposition and feeding deterrent properties are important factors in pest control, and results from this study indicate that botanical insecticides have the potential to be incorporated into control programmes for P. xylostella in South Africa.