Microscopical studies of the effect of metalaxyl on the interaction between sunflower,Helianthus annuus L. and downy mildew,Plasmopara halstedii

The mode of action of the fungicide metalaxyl againstPlasmopara halstedii, the causal agent of sunflower downy mildew, was studied following its application before, during and after artificial contamination of seedlings. The development of the fungus within the treated seedlings was examined microscopically and compared to that occuring in untreated genetically susceptible or resistant genotypes. Hypersensitive-like reactions and necrotic zones leading to the inhibition of fungus growth within the hypocotyl were observed for the three modes of application. This suggests that sunflower defence mechanisms activated by genetical resistance are also involved in the control of downy mildew by metalaxyl.     

Biological activity of neem seed kernel extracts and synthetic azadirachtin against larvae of Plutella xylostella L

The activity of two neem extracts, AZT and NEEM-AZAL (containing 30 and 3 mg azadirachtin ml^?1 respectively) and synthetic azadirachtin (AZ) against second-instar larvae (L2) of Plutella xylostella L. was examined using leafdip bioassays. On Chinese cabbage, AZ was significantly (P <0.05) less toxic (3 to 4-fold; LC₅₀ 0.54 μg AZ ml^?1) than either neem extract against a laboratory strain of P. xylostella (FS). The LC₅₀ values for AZT against the FS and another laboratory strain (Wellcome) were not significantly different on Chinese cabbage. The activity of AZT against the FS and Wellcome strains was similar on Chinese cabbage and Brussels sprout. AZT was significantly less toxic (3-fold) on Brussels sprout against an acylurea-resistant field strain (Sawi) when compared with the FS strain on Chinese cabbage. Larval mortality (at day 13) was found to increase with increasing exposure time of P. xylostella (FS) larvae to AZT-treated Chinese cabbage, although there was little difference in mortality between 48 and 120 h exposure. When AZT, NEEM-AZAL and AZ were applied at a dose (1 μg AZ ml^?1) which gave end-point mortalities between 50 and 90% (at day 13), all treatments delayed the development of a proportion of surviving larvae but no morphogenetic abnormalities were observed in larvae which reached pupation. Evidence for antifeedant (reduced weight gain) and repellant effects (choicechamber) for AZT were observed with L2 P. xylostella (Wellcome) on Chinese cabbage. AZT was also shown to have ovicidal activity against P. xylostella (Wellcome) at relatively high dose ranges (10-1000 μg AZ ml^?1) as well as some contact activity (FS strain) in topical bioassays. In residual bioassays on glass with adults of the hymenopteran endo-larval parasitoid of P. xylostella, Diadegma semiclausum (Ichneumonidae), AZT showed little or no activity at rates up to 1000 μg AZ ml^?1. In medium-volume (MV, 200 litre ha^?1) and ultra-low-volume (c. 1 litre ha^?1) spray bioassays on Brussels sprout, AZT gave 16-92% and 88-100% mortality respectively (Wellcome strain) at rates approximating to 1-20 g AZ ha^?1. The residual activity of AZT and NEEM-AZAL against P. xylostella (FS) on Brussels sprout (MV spray) was observed to decrease appreciably after three days, the decline in activity being particularly marked for NEEM-AZAL.     

Differential effects of the acylurea insect growth regulator teflubenzuron on the adults of two endolarval parasitoids of Plutella xylostella,Cotesia plutellae and Diadegma semiclausum

The acylurea chitin synthesis inhibitor, teflubenzuron (‘Nomolt’® SC) showed no apparent activity against adult males of Diadegma semiclausum Hellén or Cotesia plutellae Kurdj. when applied to foliage at a rate ( = 50 g a.i. ha^?1) approximating to 2 × field usage. In contrast, the pyrethroid cypermethrin (‘Cymbush’® EC) showed appreciable foliar activity (at rates = 10–50 ga.i. ha^?1) to D. semiclausum but showed low activity against C. plutellae. When single adult female D. semiclausum were pre-treated with teflubenzuron (= 50 g a.i. ha^?1) and then exposed to 5. 10. 25, 50 or 100 second-instar host larvae, the number of purasitoid cocoons formed was significantly reduced compared with controls except at the lowest host density (P 0-05). However, there was no evidence that teflubenzuron had a host density-dependent effect on parasitism by D. semiclausum. Teflubenzuron had no significant effect on host parasitism in an equivalent experiment with C. plutellae (P > 0-05) except at the median host density. When batches of ten host larvae were introduced at hourly intervals to female D. semiclausum pre-treated with teflubenzuron (= 50 g a.i. ha^?1), the number of cocoons formed in the first two batches was not significantly different from untreated controls (P > 0-05), whereas teflubenzuron significantly reduced the number of cocoons in the next three batches of host larvae (P < 0-05). This suggests that teflubenzuron may affect chitin synthesis in developing eggs within female D. semiclausum but has little or no effect on pre-formed eggs which are more likely to be oviposited first. Teflubenzuron had no significant (P > 0-05) effect on parasitism by C. plutellae when presented with batches of host larvae. When female D. semiclausum and C. plutellae were placed in the same host arena, the former showed a 3-fold greater rate of parasitism. Pre-treatment with teflubenzuron significantly reduced parasitism by D. semiclausum (P > 0-05) but had no significant effect with C. plutellae (P > 0-05) although the latter species failed to exploit the reduction in parasitism by D. semiclausum. The differential response of D. semiclausum and C. plutellae to insecticides is discussed in relation to the relative rates of establishment of these species in Malaysia.     

Recovery of floristic diversity,composition and structure of regrowth forests on fallow lands: implications for conservation and restoration of degraded forest lands in Laos

New Forests - In Laos, there are extensive shifting cultivation areas and regrowth forests spontaneously established on fallow lands. The aims of the study were to conduct a comparative study of...     

Cross-resistance between a Bacillus thuringiensis Cry toxin and non-Bt insecticides in the diamondback moth

BACKGROUND: Bacillus thuringiensis Berliner (Bt) crystal (Cry) toxins are expressed in various transgenic crops and are also used as sprays in integrated pest management and organic agricultural systems. The diamondback moth (Plutella xylostella L.) is a major worldwide pest of crucifer crops and one that has readily acquired field resistance to a broad range of insecticides.RESULTS: Selection of a subpopulation of the P. xylostella SERD4 population with the pyrethroid deltamethrin increased resistance to both deltamethrin and Cry1Ac relative to an unselected subpopulation. Selection of a second subpopulation with the Bt toxin Cry1Ac also increased resistance to both Cry1Ac and deltamethrin. A complementation test between the Cry1Ac-selected and deltamethrin-selected subpopulations suggested the presence of a common genetic locus or loci that control resistance to both insecticides. A piperonyl butoxide analogue with potent inhibitory activity against insect esterases significantly increased the toxicity of Cry1Ac and deltamethrin against the respective resistant subpopulations, but showed no such synergism with the unselected subpopulation of SERD4.CONCLUSION: Selection of one resistance phenotype resulted in the simultaneous selection of the other. This phenomenon could be due to a single mechanism acting against both classes of insecticide or to genetically linked, but separate, mechanisms.     

Improving models of forest nutrient export with equations that predict the nutrient concentration of tree compartments

? The objective of this study was to explore the distribution of major nutrients (N, P, K, Ca and Mg) in the aboveground compartments of an intensively managed tree species (Pinus pinaster Ait.). A total of 53 trees were cut down in even-aged stands respectively 8, 16, 26, 32 and 40 years old. The nutrient concentrations of the aboveground compartments were analysed.

? Nutrient concentrations of foliage did not vary with any of the variables used, except needle age. Nutrient concentrations of living branches, stem bark, stem sapwood, stem heartwood, stemwood and stem decreased with increasing branch diameter, bark thickness, sapwood thickness and heartwood thickness, respectively. Beyond a certain value of the predictive variable (stem diameter ≈ 15 cm; branch diameter ≈ 2.5 cm), the concentration of all the nutrients stabilised.

? A 50 year-old pine stand was used to obtain a validation dataset for nitrogen concentration. For this nutrient, the regression relationships gave satisfactory estimates for most compartments (mean error = 12–25%) and particularly for the stem.

? A procedure is proposed to estimate the nutrient exports associated with harvests of Pinus pinaster biomass.

     

Temperature Dependent Seed Transmission of Stagonospora nodorum in Wheat

The transmission of Stagonospora nodorum from four naturally infected winter wheat seedlots was quantified in controlled environment germination chambers at 9, 13, 17, 21, and 25 °C. Seedlings were harvested when the second leaf began to emerge. Coleoptiles and first seedling leaves were examined for the presence of lesions caused by S. nodorum. First leaves were incubated on Bannon's medium for 2 weeks, after which they were examined for pycnidia of S. nodorum. Transmission to the coleoptile occurred at all temperatures, but decreased from 100% to 72% as temperature increased from 9 to 25 °C. Transmission to the first leaf was less, dropping from 37% to 2% as temperature increased from 9 to 25 °C. At least 44% of infected first leaves were symptomless at all temperatures, with 96% of infected leaves showing no symptoms at 25 °C. Transmission to seedling leaves occurred over a broad temperature range. Under the high densities at which wheat is sown, a significant number of infected seedlings per unit area may originate from relatively low initial seed infection levels and transmission efficiencies.     

Fatty Acid Composition of Muscle,Liver, and Adipose Tissue of Freshwater Fish from Lake Victoria,Uganda

Fish oils may differ in fatty acid (FA) composition depending on diet. Oils extracted from muscle, liver, and adipose tissue of Nile perch (Lates niloticus), Nile tilapia (Oreochromis niloticus), silver fish (Rastrineobola argentea), lungfish (Protopterus aethiopicus), Victoria squeaker (Synodontis victoriae), and two catfishes (Clarias gariepinus and Bagrus docmac) from Lake Victoria, a tropical freshwater lake, were evaluated for FA composition. Oil contents of muscles, livers, and adipose tissues were in the range of 3.16 to 13.8%, 3.62 to 53.4%, and 28.8 to 42.4%, respectively. Omega-3 polyunsaturated FA, particularly alpha-linolenic (ALA), eicosapentaenoic (EPA), docosapentaenoic (DPA), and docosahexaenoic (DHA) acids, were found to be in substantial amounts in oils from all seven fish species. Ratios of polyunsaturated FA to saturated FA (0.79 to 1.18) were in the range considered adequate for normal health. Overall, the results show that the fish species studied are a rich source of omega-3 polyunsaturated FA.