Development of a bioassay to assess resistance to Fusarium oxysporum (Schlecht.) in asparagus (Asparagus officinalis L.)

Fusarium oxysporum is one of the major pathogens causing root and crown rot in asparagus. Breeding of cultivars resistant to F. oxysporum would be the most efficient strategy for pathogen control. In this study, a bioassay was developed for screening seedling resistance. The non‐destructive bioassay comprises inoculation with a highly aggressive F. oxysporum isolate, incubation in a climate chamber and quantification of disease symptoms by a digital image analysing system and a PTA‐ELISA. This bioassay is simple to implement and demonstrated high reproducibility. Subsequently, it was used to determine the resistance behaviour of 16 asparagus genotypes to F. oxysporum. The asparagus cultivars revealed different levels of susceptibility, whereas the wild relative A. densiflorus was confirmed to be resistant.     

Evaluation of organic insecticides for management of spotted‐wing drosophila (Drosophila suzukii) in berry crops

Spotted‐wing drosophila, Drosophila suzukii (Matsumura), is an invasive pest affecting fruit production in many regions of the world. Insecticides are the primary tactic for controlling D. suzukii in organic as well as conventional production systems. Organic growers have a greater challenge because fewer insecticides are approved for use in organic agriculture. The most effective organically approved product is spinosad, but alternatives are needed because of label restrictions limiting the number of applications per year, toxicity to beneficial arthropods and the risk of developing resistance. We evaluated several organically approved insecticides against D. suzukii in laboratory assays and field trials conducted on organic blueberry and raspberry farms. Spinosad was consistently the most effective insecticide, but a few other insecticides such as azadirachtin + pyrethrins, Chromobacterium subtsugae and sabadilla alkaloids showed moderate activity. None of the treatments had long residual activity. Mortality started to decline by 3 days after treatment, and by 5 days after application, the treatments were not different from the controls. These products may be useful in rotation programmes, necessary for reducing reliance on spinosad and mitigating resistance. Cultural and biological control approaches are needed in fruit production for D. suzukii management, but insecticides will likely continue to be the dominant management tactic while these other approaches are being optimized and adopted.     

The potential of novel African isolates of Phthorimaea operculella granulovirus for the control of Tuta absoluta

Phthorimaea operculella granulovirus (PhopGV) is infectious for larvae of different Gelechiidae insect species, including Tuta absoluta and Phthorimaea operculella. As these are major economic pests in North and sub‐Saharan Africa as well as in the Mediterranean area, the development of locally suitable biocontrol agents is essential. We have studied five isolates of PhopGV from Tunisia (Tns16, Tu1.11 and Tu2.11), Kenya (Ken13) and Yemen (Ym14) for their biological activity and the sequence polymorphism of their granulin and ecdysteroid UDP‐glucosyltransferase (egt) genes and allocated the isolates to two different egt types. Infection experiments with neonate larvae of T. absoluta and P. operculella demonstrated their pathogenicity to both host species. The isolate PhopGV Tu1.11 was the most virulent one for T. absoluta but had a relatively low infectivity to two P. operculella populations originating from Italy and Tunisia.     

不同地理种群小菜蛾对甲胺磷的抗药性

采用浸叶法研究了福州建新镇、福建农林大学校园内、闽侯县上街镇和闽侯县甘蔗镇等地田间小菜蛾种群对甲胺磷的抗性水平及抗性机制。结果表明,与敏感小菜蛾相比,上述4个田间小菜蛾种群已对甲胺磷产生了34-82倍的抗性水平。敏感小菜蛾乙酰胆碱酯酶(AChE)的Ki值是田间抗性小菜蛾AChE的Ki值的17.3-39.1倍。不同小菜蛾种群对甲胺磷的抗性水平也存在着差异,小菜蛾种群AChE对甲胺磷的敏感性大小与其对甲胺磷的抗性水平高低显著相关。     

Effects of Phosphorus Removal on the Maximal Algal Growth in Bioassay Experiments with Water from Four Dutch Lakes

This paper reports the effects of phosphorus removal at three sewage wastewater treatment plants on the state of eutrophication of four shallow lakes in the south-eastern part of the Rijnland Waterboard area during the years 1980–1982. With chemical analyses and bioassay experiments using the natural phytoplankton population no significant lowering could be detected of respectively the phosphate concentration and the maximal algal growth potential. All lakes proved to be principally nitrogen limited except the Reeuwijk Lakes, which showed clearly, after a primary nitrogen limitation, a secondary phosphorus limitation. Therefore the main attention with respect to phosphorus reduction should be concentrated on the Reeuwijk Lakes in the first place. For the other lakes in the investigated area phosphorus removal will, when it is the only measure taken, presumably not lead at short notice to a decrease of the algal biomass.     

Community structure of ectomycorrhizal fungi in a Pinus muricata forest: minimal overlap between the mature forest and resistant propagule communities

We have investigated colonization strategies by comparing the abundance and frequency of ectomycorrhizal fungal species on roots in a mature Pinus muricata forest with those present as resistant propagules colonizing potted seedlings grown in the same soil samples. Thirty-seven fungal species were distinguished by internal transcribed spacer (ITS) restriction fragment length polymorphisms (RFLPs); most were identified to species level by sporocarp RFLP matches or to genus/family level by using sequence databases for the mitochondrial and nuclear large-subunit rRNA genes. The below-ground fungal community found in the mature forest contrasted markedly with the resistant propagule community, as only four species were found in both communities. The dominant species in the mature forest were members of the Russulaceae, Thelephorales and Amanitaceae. In contrast, the resistant propagule community was dominated by Rhizopogon species and by species of the Ascomycota. Only one species, Tomentella sublilacina (Thelephorales), was common in both communities. The spatial distribution of mycorrhizae on mature roots and propagules in the soil differed among the dominant species. For example, T. sublilacina mycorrhizae exhibited a unique bias toward the organic horizons, Russula brevipes mycorrhizae were denser and more clumped than those of other species and Cenococcum propagules were localized, whereas R. subcaerulescens propagules were evenly distributed. We suggest that species differences in resource preferences and colonization strategies, such as those documented here, contribute to the maintenance of species richness in the ectomycorrhizal community.     

A novel test system for detection of tick repellents

A bioassay was developed to examine the response of ticks towards potential repellents that may protect vertebrates against tick bites. Such tick repellents must be effective despite the unavoidable presence of various attractive host-derived stimuli. Therefore, a moving-object-bioassay (MO-bioassay) was developed that mimicks body warmth and movement of vertebrates by a rotating and heated drum. Compounds which were tested for their effects on ticks were applied onto a small elevated area of the drum. Ticks were allowed to approach the drum by walking on a glass rod which ended 1 mm away from the local elevation. Ticks could cling to this elevation that intermittently passed by, whereas the remaining drum surface was too far away from the tip of the rod to be contacted by the ticks.Without the presence of any repellents, 85.5% of 600 hungry, field-collected Ixodes ricinus nymphs moved to the heated, rotating drum within 2 min. Further experiments with unfed I. ricinus nymphs were performed to test whether one established and two potential tick repellents elicit an avoidance reaction in the ticks despite the proven attractiveness of the drum. Freshly applied DEET (N,N-diethyl-m-toluamide) at a concentration of 0.11 mg cm^–2 proved active as a repellent in the MO-bioassay over a distance of a few mm as well as by direct contact. A similar repellent effect was observed with (–)-myrtenal at 1 mg cm^–2, but not at 0.1 mg cm^–2, indicating that this terpenaldehyde is a weaker repellent for I. ricinus nymphs than DEET. No repellent effect was observed with camphor (0.1 mg cm^–2).The MO-bioassay thus is a rapid, simple and low-cost test method allowing the investigation of tick host-contact behaviour as well as the screening of candidate repellents which are either perceived as volatiles or via contact chemoreception.     

Effects of Brassica oleracea waxblooms on predation and attachment by Hippodamia convergens

Four mutations that reduce waxbloom in Brassica oleracea L. were examined for their effects on predation, mobility, and adhesion to the plant surface by the general predator Hippodamia convergens (Guérin-Menéville) (Coleoptera: Coccinellidae). The mutations reduce waxbloom to different degrees, but all produce a glossy phenotype. Plants tested were inbred lines, near isogenic lines, or segregating F₂ populations, depending on the mutation. In an experiment on caged leaves, predation of Plutella xylostella L. larvae by H. convergens adult females was significantly greater on glossy types as compared with normal-wax or wild-type counterparts. Although the trend was the same for each mutation, individual comparisons between glossy and normal-wax lines or segregants were only significant for two of them, those producing mutant alleles gl_a and gl_d. Individual H. convergens were observed to spend more time walking on leaf edges and less time walking on leaf surfaces of normal-wax plants than glossy plants. Hippodamia convergens also obtained better adhesion to the surfaces of glossy plants than to normal-wax plants when tested using a centrifugal device. Two of the mutations produced similarly strong effects on predation, behaviour, and adhesion by H. convergens. These two are the same previously determined by us to provide the strongest similar effects on another generalist predator, Chrysoperla plorabunda (Fitch). The results indicate that waxbloom variation in nature could affect herbivore populations through its effects on generalist predators.     

Predation, behavior, and attachment by Chrysoperla plorabunda larvae on Brassica oleracea with different surface waxblooms

Predation by Chrysoperla plorabunda (Fitch) (Chrysopidae) first instars on Plutella xylostella (L.) (Plutellidae) neonates was measured on nine Brassica oleracea L. types with different surface wax crystal morphology. Forty-eight-hour survival of P. xylostella caged on leaves was significantly reduced by C. plorabunda on plants expressing glossy mutations that reduce surface waxbloom, but not on plants with normal waxbloom. During 5-min observations, C. plorabunda spent more time walking and less time scrambling (moving the legs with no locomotion) on glossy types than on the normal-wax types. Adhesive force produced by C. plorabunda on plant surfaces was 20 to 200-fold greater on glossy types than on normal-wax types. All the glossy types provided an advantage to C. plorabunda over normal-wax types by increasing the force of attachment to the leaf suface by the larvae, which in turn increased time allocated to walking, leading to greater predation of P. xylostella larvae. Among glossy and normal wax types together, attachment force and time allocated to walking were significantly correlated with predation by C. plorabunda. Within glossy or normal-wax types, however, these correlations were not significant. Neither time allocated to walking, nor attachment to the leaf surface was a predictor of predation by C. plorabunda within glossy or normal-wax types. Although diverse mechanisms therefore contribute to differences in predation, the results show that reduction in waxbloom can substantially affect the mobility and effectiveness of this generalist predator.     

Effects of selected insecticides on Cotesia plutellae, endoparasitoid of Plutella xylostella

Effects of field dosages ofselected insecticides to Cotesiaplutellae (Kurdjumov) (Hymenoptera: Braconidae), a larval endoparasitoidof Plutella xylostella L. (Lepidoptera: Plutellidae), wereinvestigated under laboratory conditions.Emergence of adult C. plutellae frominsecticide-treated pupae was not significantlydifferent from the control treatment. Contacttoxicity to C. plutellae adults variedgreatly among the insecticides in a paperresidue contact bioassay. Threeazadirachtin-based insecticides, Agroneem(4.8 mg a.i.liter^–1), Neemix (20 mga.i.liter^–1) and Ecozin (20 mgai.liter^–1) caused 11.1, 16.7 and 5.6%adult mortality, respectively. Of fourcommercial Bacillus thuringiensis (Bt)insecticides examined (all at 1.2 mga.i.liter^–1), Crymax and Xentari had noeffect on adult parasitoids, whereas Mattchcaused 5.6% mortality, and Dipel caused 11.1%mortality. Indoxacarb (53 mg a.i.liter^–1),-cyhalothrin (28 mg a.i.liter^–1) andspinosad (53 mg a.i.liter^–1) caused 100,88.5 and 50% adult mortalities, respectively.Low adult mortality (0–5.6%) was recorded fromingestion of azadirachtin-based, Btinsecticides and indoxacarb, compared with100% adult mortality in treatments of spinosador -cyhalothrin. Compared with the watercontrol, ingestion of azadirachtin-basedinsecticides significantly reduced parasitismby 50–57%, and Bt insecticides by 8–25%.However, ingestion of these insecticides didnot affect longevity of male and femaleparasitoid adults with one exception; femalelongevity was significantly reduced in theindoxacarb treatment. Insecticide residuescaused considerable mortality of C.plutellae adults, 39 and 44% mortality causedby 10 d old indoxacarb and -cyhalothrin,respectively, and 24 and 0% mortality causedby 7 and 10 d old residues of spinosad,respectively.