Temperature,deltamethrin-resistance status and performance measures of Plutella xylostella: complex responses of insects to environmental variables

1. Worldwide, the excessive use of insecticides has resulted in field-evolved insecticide-resistant populations of diamondback moth (DBM), Plutella xylostella. A deltamethrin-resistant DBM population from the field was divided into two subpopulations in the laboratory. One population (S-strain) was maintained with no further exposure to insecticides, whereas the other population (R-strain) was maintained under a regime of intermittent selection with deltamethrin. 2. Individuals from both strains were reared at constant temperatures in the range 10–35 °C in the absence of deltamethrin, and the effects of rearing temperature on various traits were investigated. At the time of experimentation, the R-strain was 20-fold more resistant to deltamethrin than the S-strain. 3. Temperature differentially affected developmental time, adult life span, pupal weight, and fecundity of both strains. Although both strains laid eggs after being reared at 10 °C, few of these eggs were fertile. The R-strain developed significantly faster than the S-strain. The integrated performance of the S-strain and R-strain was greatest at 25 and 15 °C, respectively. 4. The present study provides important information on the complexities of the outcomes of the interactions between ectotherms and temperature. Specifically, temperature-trait relationships may not be unimodal, and ectotherm genotypes (in this case insecticide-resistance status) and abiotic stresses can interact with unpredictable outcomes. 5. Current models predicting DBM population dynamics and relative abundance in different locations do not consider different thermal biologies of different genotypes. The present study shows the dramatic effects of environment on many parameters used in these models and will help to enhance their accuracy, and thus their utility.     

Mode of action of a surfactant–polymer formulation to support performance of the entomopathogenic nematode Steinernema carpocapsae for control of diamondback moth larvae (Plutella xylostella)

The use of entomopathogenic nematodes on cabbage leaves against larvae of the diamondback moth (DBM) Plutella xylostella requires the addition of formulation adjuvants to achieve satisfying control. Without adjuvants nematodes settle in the tank mix of backpack sprayers causing uneven distribution. The polymers arabic and guar gum, alginate and xanthan were used in concentrations between 0.05 and 0.3% to retard sedimentation of Steinernema carpocapsae. Arabic gum had no effect, guar gum prevented sedimentation at 0.3% but the effect dropped significantly at lower concentration. At 0.05%, xanthan prevented nematode sedimentation better than alginate. Deposition of nematodes on the leaves was significantly increased by the addition of any of the polymers. Spraying nematodes on leaves with an inclination of 45° without the addition of any formulation resulted in 70% run-off. Adding 0.2% alginate or xanthan reduced the losses to <20%. The use of a surfactant–polymer formulation significantly reduced defoliation by DBM larvae. Visual examinations provided evidence that nematodes are not ingested by DBM larvae. Invasion of S. carpocapsae is an active process via the anus. The function of the formulation is not to prolong nematode survival, but to provide environmental conditions which enable rapid invasion of the nematodes. Nematode performance was improved by selection of the best surfactant in combination with xanthan and by optimisation of the concentrations of the surfactant Rimulgan® and the polymer xanthan. The best control results were achieved with Rimulgan® at 0.3% together with 0.3% xanthan, causing DBM mortality of >90% at 80% relative humidity and >70% at 60%. The formulation lowered the LC₅₀ from 12 to 1 nematode/larva. The viscosity of the surfactant–polymer formulations correlated well with nematode efficacy, prevention of sedimentation and adherence to the leave. This physical parameter can therefore be recommended for improvement of nematode formulations to be used for foliar application against DBM.     

Heritability estimates of phenthoate resistance in the diamond-back moth

Realized heritabilities were estimated for the character of phenthoate resistance in two local strains of the diamond-back moth, Plutella xylostella L. (Lepidoptera: Yponomeutidae), by performing artificial laboratory selection for resistance and susceptibility to phenthoate. Heritability estimates indicated that such traits are moderately heritable ( ²=0.42 and 0.41 in the resistant selection and ²=0.31 and 0.21 in the susceptible selection), and give an experimental basis accounting for rapid evolutionary changes of phenthoate resistance observed in field populations of this insect.The observed changes in variances of phenthoate susceptibility are discussed in relation to the additive genetic variance eliminated by directional selection. The explanation stresses the importance on the underlying genetic system.

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Résumé L'héritabilité de la résistance au phenthoate obtenue chez deux souches locales de P. xylostella L. (Lep.; Yponomeutidae) a été calculée en procédant au laboratoire à des sélections artificielles pour la résistance et pour la sensibilité à cet insecticide. Les calculs de l'héritabilité ont montré que de tels caractères sont moyennement héritables ( ²=0,42 et 0,41 lors de la sélection pour la résistance, et ²=0,31 et 0,21 lors de la sélection pour la sensibilité), et ont fourni la base expérimentale rendant compte des changements évolutifs rapides observés pour la résistance au phenthoate chez des populations naturelles de cet insecte.Les changements observés de la variance de la sensibilité au phenthoate sont discutés en fonction de la variance génétique additive éliminée par la sélection orientée. L'explication insiste sur l'importance du système génétique souligné.

     

TWO HERBIVORES AND CONSTRAINTS ON SELECTION FOR RESISTANCE IN BRASSICA RAPA

Although most plants experience herbivory by several insect species, there has been little empirical work directed toward understanding plant responses to these simultaneous selection pressures. In an experiment in which herbivory by flea beetles (Phyllotreta cruciferae) and diamondback moths (Plutella xylostella) was manipulated in a factorial design, I found that selection for resistance to these herbivores is not independent in Brassica rapa. Specifically, the effect of flea beetle damage on B. rapa fitness depends on the amount of diamondback moth damage a plant experiences: damage by these herbivores has a nonadditive effect on plant fitness. When diamondbacks are abundant, plants that sustain high levels of damage by flea beetles are favored by natural selection, but when diamondbacks are rare, a low level of damage by flea beetles is favored. However, resistance to the later-feeding diamondback moth is not affected by the presence or absence of damage by early-feeding flea beetles. Thus, there are no plant-mediated ecological interactions between these herbivores that affect the outcome of selection for resistance. Because these herbivores do not independently affect plant fitness, neither is likely to develop a pairwise coevolutionary relationship with its host. Instead, coevolution is diffuse.     

Waxes enhance Plutella xylostella oviposition in response to sinigrin and cabbage homogenates

Oviposition by the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae), on substrates treated with host stimuli (cabbage homogenate or sinigrin) and/or waxes (paraffin or a mixture of 10 single chain n-alkanes) was quantified using continuous observations and endpoint bioassays. Paraffin or an n-alkane mixture applied over cabbage homogenate or sinigrin caused an increase in oviposition compared to that on any single stimulus in choice tests. Sinigrin alone at 10^–5 M to 10^–2 M is an ovipositional stimulant; addition of alkane over sinigrin made all sinigrin concentrations (10^–6 M to 10^–2 M) significantly more stimulatory than controls. Waxes alone do not stimulate oviposition. In choice tests, insect movement between sinigrin/alkane treatment combinations was random, however, once encountered, visit duration was significantly longer on sinigrin with alkane than on sites treated with either stimulus alone. Given the ubiquity of waxes on plant surfaces and the interaction between waxes and host-specific chemical stimuli, waxes should be included when considering factors that significantly influence herbivore host acceptance.     

Preliminary caged‐field trial,using the fungal pathogen Zoophthora radicans brefeld (zygomycetes: entomophthorales) against the diamondback moth Plutella xylostella L. (lepidoptera: Yponomeutidae) in the UK

Zoophthora radicans Brefeld was tested for control of Plutella xylostella L. in a caged‐field trial. No infection was seen in control cages, but between 36% and 68% of larvae in live samples, and 38% and 55% of larvae at the final harvest, were infected in inoculated cages, suggesting the biological control potential of this fungus.     

艾氏剂环氧化酶及细胞色素P－450对小菜蛾抗药性发展的影响

本文对室内长期饲养的小菜蛾（Ｐｌｕｔｅｌｌａ　ｘｙｌｏｓｔｅｌｌａ　Ｌ．）敏感品系和田间采集的抗性种群体内的艾氏剂平氧化酶及细胞色素Ｐ－４５０进行了比较研究。结果证明,艾氏剂环氧化酶在感性和抗性小菜蛾间存在着量及质的差异。抗性种群的艾氏剂环氧化酶的Ｖｍａｘ和Ｋｍ值分别为感性品系的５．４％和６．５倍。抗性种群的细胞色素Ｐ－４５０的含量是感性品系的１．１—１．３倍。艾氏剂环氧化酶在量上及质上的差异及细胞色素Ｐ－４５０含量的提高是导致小菜蛾抗药性发生与发展的重要机制之一。而且质的差异较之量的差异可能起着更为重要的作用．     

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.