Transmission of Iris yellow spot virus by Frankliniella fusca and Thrips tabaci (Thysanoptera: Thripidae)

Thrips-transmitted Iris yellow spot virus (IYSV) (Family Bunyaviridae, Genus Tospovirus) affects onion production in the United States and worldwide. The presence of IYSV in Georgia was confirmed in 2003. Two important thrips species that transmit tospoviruses, the onion thrips (Thrips tabaci (Lindeman)) and the tobacco thrips (Frankliniella fusca (Hinds)) are known to infest onion in Georgia. However, T. tabaci is the only confirmed vector of IYSV. Experiments were conducted to test the vector status of F. fusca in comparison with T. tabaci. F. fusca and T. tabaci larvae and adults reared on IYSV-infected hosts were tested with antiserum specific to the nonstructural protein of IYSV through an antigen coated plate ELISA. The detection rates for F. fusca larvae and adults were 4.5 and 5.1%, respectively, and for T. tabaci larvae and adults they were 20.0 and 24.0%, respectively, indicating that both F. fusca and T. tabaci can transmit IYSV. Further, transmission efficiencies of F. fusca and T. tabaci were evaluated by using an indicator host, lisianthus (Eustoma russellianum (Salisbury)). Both F. fusca and T. tabaci transmitted IYSV at 18.3 and 76.6%, respectively. Results confirmed that F. fusca also can transmit IYSV but at a lower efficiency than T. tabaci. To attest if low vector competency of our laboratory-reared F. fusca population affected its IYSV transmission capability, a Tomato spotted wilt virus (Family Bunyaviridae, Genus Tospovirus) transmission experiment was conducted. F. fusca transmitted Tomato spotted wilt virus at a competent rate (90%) suggesting that the transmission efficiency of a competent thrips vector can widely vary between two closely related viruses.     

Detection of Gene Flow from Sexual to Asexual Lineages in Thrips tabaci (Thysanoptera: Thripidae)

Populations of Thrips tabaci are known to have two sympatric but genetically isolated reproductive modes, arrhenotoky (sexual reproduction) and thelytoky (asexual reproduction). Herein, we report behavioral, ecological and genetic studies to determine whether there is gene flow between arrhenotokous and thelytokous T. tabaci. We did not detect significant preference by arrhenotokous males to mate with females of a particular reproductive mode, nor did we detect significant behavioral differences between arrhenotokous males mated with arrhenotokous or thelytokous females in their pre-copulation, copulation duration and mating frequency. Productive gene transfer resulting from the mating between the two modes was experimentally confirmed. Gene transfer from arrhenotokous T. tabaci to thelytokous T. tabaci was further validated by confirmation of the passage of the arrhenotokous male-originated nuclear gene (histone H3 gene) allele to the F₂ generation. These behavioral, ecological and genetic studies confirmed gene transfer from the sexual arrhenotokous mode to the asexual thelytokous mode of T. tabaci in the laboratory. These results demonstrate that asexual T. tabaci populations may acquire genetic variability from sexual populations, which could offset the long-term disadvantage of asexual reproduction.     

Resistance to Thrips palmi (Thysanoptera: Thripidae) in beans

Resistance in beans, Phaseolus vulgaris L., to the melon thrips Thrips palmi Karny was studied under field conditions at two sites in Colombia. Genotypes were rated for resistance on the basis of visual damage scores, bean production ratings (a visual estimate of pod and seed set), and grain yields. Of 1,138 genotypes tested, only 60 (5.3%) were rated as resistant. Repeated testing allowed us to identify potential sources of resistance in five germplasm accessions (G 02402, G 02852, G 03177, G 03569, and G 04055), one commercial variety ('Brunca'), six elite breeding lines (A 216, DOR 714, EMP 486, FEB 115, FEB 161, and FEB 162), 41 recombinant inbred lines derived from the BAT 881 x G 21212 cross, and seven recombinant inbred lines derived from a cross between DOR 364 and BAT 477. Resistance was not associated with maturity, growth habit, pubescence, and seed color or seed size. In general, correlations between visual damage scores and bean production ratings and between damage scores and yield were high and significant meaning that selection on damage rating is useful to identify genotypes that may have tolerance as a mechanism of resistance. The continuous distribution of damage scores of 139 recombinant inbred lines suggested that the inheritance of resistance to the melon thrips might be quantitative. Overall, resistance levels in beans can be considered as moderate, because none of the genotypes tested received damage scores of <3 on a 1-9 scale and none was ever rated as highly resistant in terms of bean production ratings.     

Polymorphic microsatellite markers in Thrips hawaiiensis (Thysanoptera: Thripidae)

Thrips hawaiiensis (Morgan, 1913) is an important insect pest of fruits and vegetables. At present, it is primarily distributed in tropical and subtropical area. For a better understanding of the genetic makeup and migration patterns of T. hawaiiensis throughout the world, we isolated 11 novel polymorphic microsatellite loci from an enriched genomic library based on a biotin/streptavidin capture protocol. Genetic parameters were estimated on 80 individual thrips from two natural populations. The results showed that all 11 loci were highly polymorphic; the number of alleles ranged from six to 37, and nine loci demonstrated polymorphic information content (PIC) > 0.5. The observed (H _O) and expected (H _E) heterozygosities ranged from 0.350 to 0.925 and 0.307 to 0.952, respectively. Furthermore, only four locus/population combinations significantly deviated from Hardy–Weinberg equilibrium (HWE). These microsatellite markers have potential utility in population structure and gene flow studies of this species.     

Discrimination of reproductive forms of Thrips tabaci (Thysanoptera: Thripidae) by PCR with sequence specific primers

In agriculture, although it is important to identify species of pest insects, the morphological identification is often difficult. DNA genotyping is useful for the identification of species in morphologically indiscriminable species. Thrips tabaci (Lindeman) can be divided into two reproductive forms (arrhenotoky and thelytoky, each of which different in pesticide resistance) but morphological discrimination is not possible. Here, we establish a simple method to discriminate the strains based on their mitochondrial DNA sequences. Phylogenetic analysis including the T. tabaci and congeneric species provided ancestor sequences of each strain of T tabaci. Based on the ancestor sequences, we developed a primer set that include strain specific primers on sense strand and common primer on anti sense strand. Using this primer set, the strains of 196 individuals of T. tabaci were successfully assigned to each ofgenotypic forms. As the phylogeny and ancestor sequences were based on worldwide samples, this method will work well on most populations around the world.     

An improved cage system for the bioassay of Metarhizium anisopliae on Thrips tabaci (Thysanoptera: Thripidae)

The ability of thrips and other minute insects to escape from conventional assay cups led to development of a successful assay chamber. In the present study, we evaluated three bioassay systems for thrips; Tashiro cage, microtube assay and leaf sandwich assay and introduced an improved thrips entomopathogenic bioassay system (TEBS). Our objective was to investigate the effect of Metarhizium anisopliae on Thrips tabaci using an easy-to-use and inexpensive holding chamber that would produce bioassay results with acceptable levels of accuracy and precision. In our assays, we found that escape of second-instar larvae from TEBS was significantly less than those of other methods which we tested. Our results indicated that our assay system was more suitable than the other conventional assays for the bioassay of entomopathogenic fungi on T. tabaci.     

The efficacy of two entomopathogenic biocontrol agents against adult Thrips palmi (Thysanoptera:Thripidae)

The efficacy of the two entomopathogenic biocontrol agents, Steinernema feltiae (Filipjev) and Lecanicillium muscarium (Petch), against juvenile Thrips palmi Karny in the laboratory situation is well documented. This study investigated the extrapolation of this knowledge to the control of adult stages. S. feltiae caused significantly higher mortality to juvenile T. palmi than to adult stages, whereas L. muscarium had a more significant impact on adult stages. The potential to develop an IPM strategy to control T. palmi utilising the two entomopathogens is discussed.     

Transmission of capsicum chlorosis virus by Thrips palmi (Thysanoptera: Thripidae)

Applied Entomology and Zoology - Transmission efficiencies of capsicum chlorosis virus (CaCV) were investigated for four thrips species (Thysanoptera: Thripidae), Frankliniella occidentalis...     

Genetic and host-associated differentiation within Thrips tabaci Lindeman (Thysanoptera: Thripidae) and its links to Tomato spotted wilt virus-vector competence

Of eight thelytokous populations of onion thrips (Thrips tabaci) collected from potato (three populations), onion (four) or Chrysanthemum (one) hosts from various regions of Australia, only those from potato were capable of transmitting Tomato spotted wilt virus (TSWV) in controlled transmission experiments. Genetic differentiation of seven of these eight populations, and nine others not tested for TSWV vector competence, was examined by comparison of the DNA sequences of mitochondrial cytochrome oxidase subunit 1 (COI) gene. All Australian populations of T. tabaci grouped within the European ‘L2'' clade of Brunner et al. (2004). Within this clade the seven populations from potato, the three from onion, and the four from other hosts (Chrysanthemum, Impatiens, lucerne, blackberry nightshade) clustered as three distinct sub-groupings characterised by source host. Geographical source of thrips populations had no influence on genetic diversity. These results link genetic differentiation of thelytokous T. tabaci to source host and to TSWV vector capacity for the first time.     

Preliminary evaluation of the effectiveness of a Verticillium lecanii isolate in the control of Thrips tabaci (Thysanoptera: Thripidae)

Thrips tabaci is a polyphagous pest that attacks many different crop and ornamental plant species. Recently, the need to reduce the use of insecticides in the control of thrips is more and more increasing. On that account, the possibility to make use of the entomopathogenic fungus Verticillium lecanii (DAOM198499) was evaluated in laboratory conditions. Suspension of 10(3) to 10(7) conidia/ml grown in liquid culture medium with a control (distilled water) treated on second larval stage of T. tabaci in controlled condition (16L: 8D photoperiod, 25 +/- 1 degrees C temperature, 98% RH in first 48h and 70% RH in next days). Preliminary results showed that mortality in lethal suspension (10(7) conidia/ml) of V. lecanii after 24 hours and 2-7 days in lower suspension appears. This suggests that strine DAOM 198499 of V. lecanii has the ability to control T. tabaci in greenhouse conditions.     

Antixenosis and antibiosis of common beans to Thrips palmi Karny (Thysanoptera: Thripidae)

Antixenosis and antibiosis in the resistance of common beans (Phaseolus vulgaris L.) to the melon thrips, Thrips palmi Karny, were investigated under laboratory and field conditions. Experiments were conducted for four moderately resistant genotypes ('Brunca', BH-130, EMP 486, and FEB 115) in comparison with one susceptible genotype (APN 18). Multiple-choice tests recorded most thrips on EMP 486 and least on FEB 115. Dual-choice tests conducted in both laboratory and field confirmed the antixenotic effect of FEB 115 and the attractant effect of EMP 486 on thrips. These results demonstrate the significance of antixenosis in the resistance of common beans to T. palmi. Life-table studies showed significant differences in egg duration, immature and adult survivorship, female body length and longevity, daily oviposition rate, and total fecundity among the bean genotypes. The intrinsic rate of natural increase (r(m)) and its associated population parameters varied significantly with the bean genotype on which T. palmi cohorts were reared. Based on mean r(m) values, the five bean genotypes can be divided into two groups, with BH-130 and 'Brunca' being less favorable for the population growth of thrips than EMP 486 and FEB 115; the latter were comparable to the susceptible genotype APN 18. These life-table results indicate the role of antibiosis in enhancing the resistance of common beans to T. palmi.     

Antagonism between Beauveria bassiana and imidacloprid when combined for Bemisia argentifolii (Homoptera: Aleyrodidae) control

Imidacloprid and the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin are both used to control the whitefly Bemisia argentifolii Bellows & Perring. We tested whether the two control strategies acted additively, synergistically, or antagonistically when combined for whitefly control. We found antagonism in that B. bassiana inhibited the effectiveness of imidacloprid. When B. bassiana was combined with imidacloprid, insect response was either less than or similar to (depending on B. bassiana rates) that when imidacloprid was used alone. Adding imidacloprid to B. bassiana treatments always increased mortality, but the increase was less than additive. Beauveria bassiana spore germination and colony formation were not inhibited by imidacloprid in vitro, and B. bassiana did not adsorb or degrade imidacloprid in a tank mix. We hypothesize that B. bassiana caused a behavioral response that reduced insect feeding and uptake of imidacloprid.     

Population ecology of Thrips palmi (Thysanoptera:Thripidae) in orchid farms in Thailand

The population dynamics and in-farm distribution of Thrips palmi Karny were evaluated in two orchid farms, one in the Bangkok metropolitan area and the other in Ratchaburi Province, Thailand. Yellow sticky traps were hung above the orchid canopy (three traps/220 m²), and the trapped adults were counted weekly from November 2009 to October 2010 at the first location and from January to December of 2012 at the second location. T. palmi nymphs and adults were both counted weekly on 300 orchid panicles in an area of 2,200 m² at both locations. We found that the T. palmi population dynamics at the first location was represented by a W-shaped curve, with the distribution clumped throughout the year, whereas the curve was J-shaped at the second location, with the distribution mostly clumped. Qualitative surveys of thrips species were conducted on ten different orchid genera in ten locations and on 20 different Dendrobium hybrids in 20 locations. Five random sampling plots (1 × 4 m²) were examined on each farm. The results indicated that only T. palmi was found on every farm. Moreover, quantitative comparison among five orchid types revealed that Dendrobium flowers with darker color are more attractive to T. palmi. However, this phenomenon does not coincide with Oncidium sp. and Mokara sp. because of their unfavorable morphology for thrips behavior.     

Temperature-dependent development and life table parameters of Thrips palmi (Thysanoptera: Thripidae) on eggplant

The life history of Thrips palmi Karny on eggplant (Solanum melongena L.) leaves was studied based on the age stage and two sex-life tables at 16, 19, 22, 25, and 31?°C. The intrinsic rate of increase (r) at these temperatures was 0.0427, 0.0566, 0.0979, 0.1738, and 0.2237?day^?1, respectively. The relationship among the gross reproductive rate (GRR), the net reproductive rate (R ₀), and the pre-adult survivorship (l _a) is consistent with R ₀?<?l _a?×?GRR?<?GRR for all results at different temperatures. The mean generation time was 47.52, 38.33, 29.52, 19.81, and 13.88?days, respectively. The developments of pre-adult and adult stages were faster in males than in females. The means of developmental periods for each developmental stage decreased with increases of temperature. The maximum life span of female adults was noted at 56.67?days, whereas that of males was 50.66?days at 16?°C. The maximum female fecundity (64.18?eggs/female) was recorded at 25?°C and the lowest (23.38 eggs/female) at 16?°C. Life table data could be used to project population growth, to design mass rearing programs, and to establish management tactics to control insect pests.     

Assessment of loci for DNA barcoding in the genus Thrips (Thysanoptera:Thripidae)

Identification of the juveniles of economically important thrips species on imports by morphology alone can be challenging and culturing is usually required. In the case of EU quarantine species such as Thrips palmi, rapid and accurate identification is essential. DNA barcoding using the Cytochrome oxidase I (COI) gene has become a popular technique for species identification; however, in some invertebrate genera COI has been shown to provide insufficient variability for species discrimination. This study presents a comparison of five different loci to investigate their ability to discriminate a small number of Thrips species. All five loci discriminated the species by neighbour-joining tree and varying degrees of discrimination were determined upon further investigation of the intraspecific and interspecific distances. Two distinct COI clades were observed for T. Palmi and judged to be COI haplotypes when data from the other four additional loci and geographical collection data were taken into consideration. COI was shown to provide sufficient variation to be used in future DNA barcoding efforts within the genus Thrips.     

Economic thresholds of Thrips palmi (Thysanoptera: Thripidae) for eggplants in a greenhouse

Thrips palmi Karny has recently been implicated in damage to eggplant fruits. This thrips causes cosmetic scars on and deforms fruits, thus lowering their market value. To investigate the relationship between the density of thrips and the resulting damage, experimental plots with initial release densities of 0, 20, 40, 70, and 100 adults thrips per plot were established under greenhouse conditions. Thrips density (for flower sampling = x ₁; for sticky trap = x ₂) was monitored using flower sampling and blue sticky trap counts, and was found to be directly related to the proportion of damaged-fruits (y) and the reduction in fruit yield: significant relationships were found for the flower samples (y = 1.2261x ₁ ? 0.6232, r ² = 0.8582) and for the trap catches (y = 11.667 ln(x ₂) ? 9.5, r ² = 0.8896). The proportion of damaged fruits that could be tolerated from an economic perspective, based on the cost of controlling the thrips population chemically and the market value of the fruit, was 6.67–11.76 %; this translated into economic thresholds of 1.05–1.5 adults and/or larvae of T. palmi per flower or 4.91–10.17 adults T. palmi per four-day sticky card count.     

Economic Thresholds of Western Flower Thrips (Thysanoptera: Thripidae) for Unripe Red Pepper in Greenhouse

This study was conducted to develop economic thresholds of western flower thrips (Frankliniella occidentalis Pergande) for unripe red pepper in greenhouses. To investigate the relationship between the density of thrips and resulting damages, experimental plots with five treatments (0, 4, 16, 48, 96 adults per plot) as initial release densities were established at the National Institute of Agricultural Science and Technology, Suwon, Korea, in 2004. Western flower thrips density was monitored using flower samplings and yellow sticky trap (8×13 cm) counts. Western flower thrips density was directly related to increased numbers of damaged fruits and reduced fruit yield. The number of marketable fruits produced decreased as the thrips densities increased. The major damage to pepper fruits caused by thrips was cosmetic scars that resulted from immature feeding. When flower samples or yellow trap caches were used to determine the density of thrips, which were collected on a previous sampling date, thrips densities were determined to be related to the percentage of fruits that were damaged, and a significant relationship was found for the flower samples (y = 0.3219x + 1.0792, r² = 0.8640 and for trap catches (y = 11.9209 log(x) −2.158, r² = 0.8306). The economically-tolerable ratio of damaged fruits based on control cost and market values under current greenhouse cultivation was estimated as 3.4 to 8.0%. Economic thresholds of western flower thrips ranged from 0.7 to 2.1 adults or nymphs per flower, and 2.3 to 5.7 adults per four-day sticky card count.     

Pathogenicity of three entomopathogenic nematodes against the onion thrips,Thrips tabaci Lind. (Thys.; Thripidae)

Pathogenicity of a native isolate of Steinernema feltiae (H1) and two exotic strains, Heterorhabditis bacteriophora and Steinernema carpocapsae was assessed under laboratory conditions using different concentrations i.e. 4000, 6000, 8000 and 10,000 infective juveniles/ml against second instar larvae, prepupa and pupa of Thrips tabaci Lindeman. The mortality data were recorded 24 and 48?h post-inoculation. The highest mortality rate was recorded for prepupa (62%) than second instar (12.5%) by H. bacteriophora and S. carpocapsae, respectively, 24?h after treatment. No significant differences were found in mortality between prepupa and pupa with increasing the nematodes concentrations (from 4000 to 10,000 nematode/ml) but increasing nematode concentrations increased the mortality of second instar. At the end of the experiment (48?h.), S. feltiae H1 caused the highest mortality on second instar larvae (74%), whereas all other species caused 80–83% mortalities on pupa. This study suggests that native isolate of S. feltiae (H1) had high potential to infect soil-dwelling stages of T. tabaci.