Reversal of resistance to pyriproxyfen in the Q biotype of Bemisia tabaci (Hemiptera: Aleyrodidae)

Pyriproxyfen, a juvenile hormone (JH) mimic, is a biorational insecticide that disrupts insect development. It is one of the principal insecticides being used to control Bemisia tabaci (Gennadius) on cotton, and has many environmentally positive attributes that make it compatible with integrated pest management (IPM) programs. In Israel, a high level of resistance to pyriproxyfen has been observed in several isolated regions. Here, tests were conducted to establish whether temporal refuges from exposure to pyriproxyfen could be useful for restoring the effectiveness of the compound. Resistance was found to decrease by a factor of 8 when exposure to pyriproxyfen was ceased for 13 generations. Reversal of resistance was accompanied with increased biotic fitness of the revertant colony. By incorporating experimental estimates of nymph survival, sex ratio, fecundity, egg hatching rate and developmental time, the seasonal cost per generation for resistant insects was estimated to be 25%. A genetic simulation model, optimized by empirical data from bioassays, predicted fitness cost per generation of 19% for resistant homozygous (RR) females and hemizygous (R) males, and produced rates of reversal similar to the experimental results. The model also predicted that, even after 5 years ( approximately 55 generations) without pyriproxyfen treatments, the frequency of the resistance allele (R) will still remain high (0.02). It is therefore concluded, on the basis of experimental and modeling results, that the effectiveness of temporal refuges for reversing development of resistance to pyriproxyfen in B. tabaci may be limited.     

Effect of tillage practices on irrigation requirement,weed control and yield of lowland rice

Dry soil bulk density increased from 1.42 g/cm³ with ordinary ploughing to 1.69 g/cm³ with puddling twice and to 1.80 g/cm³ with soil compaction. Consequently, saturated hydraulic conductivity decreased from 113 mm/day with ordinary ploughing to 48 mm/day with puddling twice and to 29 mm/day with soil compaction. The irrigation requirement was 2295 mm with ordinary ploughing compared with 1350 mm with soil compaction. Dry weight of weeds was only 0.6 and 0.7 t/ha with puddling twice and soil compaction, respectively, and 3.9 t/ha with ordinary ploughing. The highest grain yield of rice (4.5 t/ha) was obtained with puddling twice, due to effective weed control. Weeds shared 57 and 35% of the nutrient (N+P+K) with ordinary ploughing and puddling once, respectively, as compared with only 8 and 15% with puddling twice and soil compaction, respectively. The ratio of total biological yield (grain + straw + weeds) to total nutrient (N+P+K) uptake was 59.0 with puddling twice and soil compaction as compared with 53.5 with puddling once and 50.0 with ordinary ploughing. This suggests that tillage practices may affect the efficiency of the use of soil and applied nutrients by the rice crop.     

Effect of nitrogen source and salinity level on salt accumulation of two chickpea genotypes

This study aimed at investigating mechanisms of salt tolerance and ionic relations of chickpea (Cicer arietinum L.) cultivars with different nitrogen (N) sources. Two resistant genotypes, ILC‐205 and ILC‐1919, were subjected to four levels of salinity (0.5, 3.0, 6.0, and 9.0 dS m^‐1). Nitrogen sources consisted of inoculation with two resistant Rhizobium strains, CP‐29 and CP‐32, mineral N additions, and no N application. Data was collected on root and shoot contents of sodium (Na⁺) chlorine, (Cl^‐,) and potassium (K⁺), and shoot to root Na⁺ratio, as well as shoot K⁺ to Na⁺ ratio. Salinity affected shoot Na⁺ and Cl^‐contents, but nodulating plants had higher shoot Na⁺ contents than plants supplied with mineral N. Shoot to root Na⁺ ratios were lower in the mineral N treatment than in nodulating treatments at 3.0 dS m^‐1, indicating that root compartmentalization and shoot exclusion were only possible at low salinities. Potassium levels of nodulating plant shoots were lower than those of non‐nodulating plants only at low salinities. N‐source significantly affected shoot K⁺/Na⁺ ratio, with nodulating plants having lower ratios than non‐nodulating plants, indicating that rhizobial infection or nodule formation may lead to salt entry curtailing the selective ability of chickpea roots.     

Camel trypanosomosis in Morocco 1: results of a first epidemiological survey

An epidemiological survey of camel trypanosomosis was conducted for the first time in Morocco in 1997 and 1998. Five provinces located in the south and southeast of the High Atlas mountain chain were included in this study. A total number of 1460 serum samples were collected and tested by a card agglutination test for trypanosomosis (CATT) and enzyme-linked immunosorbant assay (Ab-ELISA) to detect anti-Trypanosoma evansi antibodies. The overall seroprevalence was 14.1% by CATT and 18.2% by Ab-ELISA. Two very active foci were identified in Zagora and Merzouga with high Ab-ELISA prevalences of 43.3 and 35.4%, respectively, for each area. The concordance between the CATT and Ab-ELISA was 94.1%. The age-related distribution of seroprevalence showed a tendency for the infection rate to increase with age up to a maximum in the 7-10 year-old group. Clinical examination revealed that enlargement of lymph nodes was the most frequent sign in seropositive animals (79.7%). The comparison of haematocrit values of seropositive and seronegative animals showed a significant difference that indicated severe anaemia in infected animals. Isolation of the parasite by inoculation of mice allowed us to collect 15 T. evansi isolates from Zagora and three from Merzouga. The study showed that the Saharan provinces of Morocco were slightly affected by camel trypanosomosis but two hyperendemic foci, Zagora and Merzouga, were found to be sources of contamination for the surrounding regions.     

Dynamics of insecticide resistance in Bemisia tabaci: a case study with the insect growth regulator pyriproxyfen

The dynamics of pyriproxyfen resistance in Bemisia tabaci (Gennadius) have been studied intensively in cotton fields and greenhouses in Israel. High resistance to pyriproxyfen evolved one year after its introduction for use on flowers in greenhouses, after three successive applications. After ten years of pyriproxyfen use in cotton fields, with only one application per season, a high level of resistance was observed, but its rate of development differed among localities. It is hypothesized that these differences reflect temporal and spatial variations in the availability of alternative host plants for B tabaci, and that pyriproxyfen resistance evolved most readily in geographically isolated areas. It is also likely that the occurrence and development of resistance is partially biotype-related. Pyriproxyfen use has ceased in high-resistance localities, but in areas with low frequencies of resistance, cotton growers have continued to achieve acceptable control of B tabaci with pyriproxyfen. Due to the absence of applications of pyriproxyfen in some cotton fields, resistance levels tended to decline between 1998 and 2001. Laboratory experiments support the hypothesis that this decline reflects, in part, fitness costs associated with pyriproxyfen resistance.     

Rice Root Genetic Architecture: Meta-analysis from a Drought QTL Database

During the last 10 years, a large number of quantitative trait loci (QTLs) controlling rice root morphological parameters have been detected in several mapping populations by teams interested in improving drought resistance in rice. Compiling these data could be extremely helpful in identifying candidate genes by positioning consensus QTLs with more precision through meta-QTL analysis. We extracted information from 24 published papers on QTLs controlling 29 root parameters including root number, maximum root length, root thickness, root/shoot ratio, and root penetration index. A web-accessible database of 675 root QTLs detected in 12 populations was constructed. This database includes also all QTLs for drought resistance traits in rice published between 1995 and 2007. The physical position on the pseudo-chromosomes of the markers flanking each QTL was determined. An overview of the number of root QTLs in 5-Mb segments covering the whole genome revealed the existence of “hot spots,” The 32 trait × chromosome combinations comprising six or more QTLs were subjected to a meta-QTL analysis using the software package MetaQTL. The method enabled us both to determine the likely number of true QTLs in these areas using an Akaike information criterion and to estimate their position. The meta-QTL confidence intervals were notably reduced and, for the smallest ones, encompassed only a few genes.