Population dynamics of western flower thrips (Thysanoptera: Thripidae) in nectarine orchards in British Columbia

Development of a control strategy for thrips attacking nectarine trees depends on an understanding of their phenology, distribution, and life history as related to characteristics of nectarine orchards. To this end, we compared the overwintering behavior, distribution, and abundance of western flower thrips, Frankliniella occidentalis (Pergande), among 11 nectarine orchards located in the dry central interior of British Columbia, Canada, during 1993 and 1994. Western flower thrips emerged from areas not previously used for agriculture (wild areas) and from within orchards before trees were out of dormancy. Flight of thrips within and around orchards peaked during early bud development, with a second major peak several weeks later after husk fall as the next generation emerged. Orchards protected from wild areas by other orchards had the lowest densities of thrips in buds. Density estimates of western flower thrips on trees were not affected by location of trees within orchards or buds within trees, but most thrips were found in the most developed buds on a tree at any one time. Thrips were not found within buds until petal was first visible on the buds. Larval feeding on buds at early petal fall resulted in serious surface russetting of fruit.     

Spatial and temporal patterns of dispersal of western flower thrips (Thysanoptera: Thripidae) in nectarine orchards in British Columbia

Thrips were sampled from six nectarine orchards in the Dry Central Interior, British Columbia, Canada, between April and June 1993 using yellow sticky cards on posts spaced around the perimeter of each orchard. Although 12 identified species of thrips were captured, >90% of individuals were the western flower thrips, Frankliniella occidentalis (Pergande). The flight patterns and abundances of western flower thrips were compared between orchards located in two differently oriented valleys (N-S and E-W) and between orchards located close to or far from areas of wild land. Results indicate that densities of western flower thrips entering orchards, and their direction of movement, were related more to the external vegetation than either location within the two different valleys or general wind flow patterns. Western flower thrips tended to move into orchards close to ground level in early spring (late April and early May) but flew higher as ground cover grew taller and temperatures increased. Densities of western flower thrips at ground level were highest in an orchard with the densest dandelion ground cover. We conclude that the location of nectarine orchards in relation to wild areas is a major determinate of western flower thrips densities.     

Sequential sampling plans for western flower thrips (Thysanoptera: Thripidae) on greenhouse cucumbers

The development of cost-effective and reliable sampling programs for the management of western flower thrips, Frankliniella occidentalis (Pergande), on greenhouse cucumbers is important for getting growers to adopt economic injury levels and economic thresholds. The objectives of this study were to develop two sequential sampling plans. A fixed-precision sequential sampling plan was designed for estimating F. occidentalis adult density at a fixed-precision level on cucumber flowers. Also, a sequential sampling plan for classifying thrips population levels as below or above economic thresholds was developed to assist in decision making for the timing of pesticide applications. Both sequential sampling plans were validated using a resampling simulation technique on nine independent data sets ranging in density from 1.25 to 12.95 adults per flower. With the fixed-precision sampling plan, average means obtained in 100 repeated simulation runs were within the 95% CI of the estimated mean for all data sets. Appropriate levels of precision for the different population densities were recommended based on the simulation results. With sequential sampling for classifying the population levels of thrips in terms of an economic threshold, it has the advantage of requiring smaller sample sizes to determine the population status when the population densities differ greatly from the critical density (i.e., economic threshold). However, this plan needs a great number of samples when population density is close to the critical density. In this case, use of a combination of both sampling plans is recommended.     

Evaluation of sampling methodology for determining the population dynamics of onion thrips (Thysanoptera: Thripidae) in Ontario onion fields

Onion thrips, Thrips tabaci Lindeman, are an economic pest of alliums worldwide. In Ontario onion-growing regions, seasonal abundance and population trends of onion thrips are not well known. The objectives of this research were to investigate onion thrips population dynamics by using both white sticky traps and plant counts, to gain insight into flight height, and to determine the genus and sex of thrips fauna present in monitored fields. Adult thrips were captured on white sticky traps placed in two commercial onion fields in the Thedford-Grand Bend Marsh region as early as mid-May in 2001, 2002, and 2003. Thrips were not recorded on onion plants in these fields until late June and early July. A comparison of sticky trap captures to plant counts revealed a strong, positive correlation, indicating that sticky traps, which consistently detected thrips earlier than plant counts, could be used instead of plant counts early in the season to monitor onion thrips populations. Pole traps placed in onion and an adjacent soybean, Glycine max (L.) Merr., field revealed that regardless of crop type, most thrips were captured 0.7-0.95 m above the soil surface. During this study, 70% of 137,000 thrips captured on sticky traps and 89% of 1,482 thrips captured in pan traps were female onion thrips. No male onion thrips were identified in this study: most of the remaining thrips were Frankliniella spp.     

Temperature-mediated relationship between western flower thrips (Thysanoptera: Thripidae) and chrysanthemum

Modeling the effect of temperature on the sustainability of insect-plant interactions requires assessment of both insect and plant performance. We examined the effect of temperature on western flower thrips, Frankliniella occidentalis (Pergande), a generalist herbivore with a high reproductive rate, and chrysanthemum inflorescences, a high quality but relatively fixed, ephemeral resource for thrips population growth. We hypothesized that different thrips versus plant responses to temperature result in significant statistical interaction of temperature with thrips abundance and flower damage attributes over time. Experiments were conducted at five temperatures between 20.7 and 35.3 degrees C, with thrips infestation and time after infestation as main effects. Only minor, uncontrolled variations in relative humidity and light intensity may otherwise have influenced the results. High temperatures lead to an initially rapid increase in density of thrips followed by abrupt declines in abundance. The rate of floral senescence increased with temperature and thrips infestation, as indicated by a reduced fresh biomass and greater leaching of yellow pigments. Multiple regression indicated that indices of plant damage responded more directly to thrips density at low than high temperature, supporting the conclusion that temperature affected the outcome beyond what was predictable simply from differential plant and insect optima. The relative intensity of damage caused by individual thrips decreased with increasing temperature, likely caused by thrips competition and reduced survival, growth, and fecundity on depleted inflorescences. Reduced per capita damage at high temperature may be common in insects exploiting fixed plant resources that exhibit an accelerated rate of deterioration at high temperatures.     

Daily flight activity of the western flower thrips (Thysan., Thripidae) in nectarine orchards in British Columbia, Canada

Abstract:   Hourly flight patterns of western flower thrips, Frankliniella occidentalis (Pergande) (Thysan., Thripidae) were assessed over a 24 h period in two nectarine orchards located in the Similkameen Valley, British Columbia, on 13 separate occasions between March and October 1994. The numbers of western flower thrips (male or female, dark or pale morphs) trapped per hour at between four to eight heights (0.25–2.00 m) were compared with temperature, wind speed, time of year and location of trap placement. Traps were placed within orchards, at the border of orchards and adjacent wild land, and within wildlands. Temperature and wind speed thresholds are defined, although the effect of wind speed varied with height of flight. Height of flight was affected by both the height of prevalent vegetation and temperature, with mean height of flight increasing linearly with temperature. Numbers of thrips in flight decreased with height of sticky trap from the ground. The pale morph was generally more common than the dark morph of western flower thrips. Males were not present until after emergence of the first generation in May, and were found at an approximate ratio of 4 : 1 (F : M) for the rest of the year. There were no differences between sexes or morphs of western flower thrips for location, height or daily patterns of flight.     

Life-stage variation in insecticide resistance of the western flower thrips (Thysanoptera: Thripidae)

The life-stage variations in insecticide resistance of western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), to selective insecticides (acrinathrin, formetanate, and methiocarb) were studied using resistant laboratory strains. In each strain, the second-instar larva was less susceptible to the insecticides tested than the adults. The lower the resistance level of the adults, the higher the difference between larva and adult susceptibility: 32-fold to methiocarb, 15.4-fold to formetanate, and 180-fold to acrinathrin in the reference strain. In laboratory-selected resistant strains, these differences were much lower: 5.8-fold to methiocarb, 4.8-fold to formetanate, and 2.0-fold to acrinathrin. In selected strains, higher resistance levels for each insecticide were found, both for larvae and adults, compared with the reference strain. These results show that after insecticide resistance selection in adults, the resistance is carried over to the larvae, but at lower levels.     

Economic injury levels for western flower thrips (Thysanoptera: Thripidae) on greenhouse cucumber

Low, medium and high densities of western flower thrips, Frankliniella occidentalis (Pergande), were established in three greenhouses at the Greenhouse and Processing Crops Research Centre, Ontario, Canada, in 1996 and 1998 to develop economic injury levels for thrips on greenhouse cucumber. Thrips densities were monitored weekly using yellow sticky traps and flower counts. Fruit was harvested twice a week, graded for size, weighed, and rated for thrips damage using three damage categories. Significant yield reduction was detected 4 wk after severe fruit damage was observed in the high and medium thrips density treatments in 1996 and 7 wk in 1998. Percentage of severe damaged fruit (P(F3)) has significant linear relationships with the adult thrips density (x) that was sampled by sticky traps 1 wk before harvest (P(F3) = -0.2533 + 0.0828x) and that was sampled by flower counts 2 wk before harvest (P(F3) = -0.2025 + 0.5490x). Based on the regression equations, economic injury levels, expressed as adult thrips per sticky trap per day or adult thrips per flower, were calculated for various combinations of control costs, yield potential and fruit prices. The economic injury levels for F. occidentalis ranged from 20 to 50 adults per sticky trap per day or 3 to 7.5 per flower as determined under average greenhouse production conditions in Ontario, Canada.     

Systemic effects of neem on western flower thrips,Frankliniella occidentalis (Thysanoptera: Thripidae)

The systemic effects of neem on the western flower thrips, Frankliniella occidentalis (Pergande), were investigated in laboratory trials using green bean, Phaseolus vulgaris L., in arena and microcosm experiments. In arena experiments, systemic effects of neem against western flower thrips larvae on primary bean leaves were observed with maximum corrected mortality of 50.6%. In microcosm experiments using bean seedlings, higher efficacy in the control of western flower thrips were observed with soil applications of neem on a substrate mixture (i.e., Fruhstorfer Erde, Type P, and sand) in a 1:1 ratio (93% corrected mortality) compared with application on the commercial substrate only (76% corrected mortality). However, longer persistence of neem was observed with soil application on the commercial substrate, which showed effects against thrips for up to 6 d after application. In addition to systemic effects observed on all foliage-feeding stages of western flower thrips, mortality on contact and repellent effects were observed on soil-inhabiting stages after soil applications of neem. Finally, bean seedlings grown from seeds pregerminated for 3 d in neem emulsion were also toxic to western flower thrips.     

Sampling methods, dispersion patterns, and fixed precision sequential sampling plans for western flower thrips (Thysanoptera: Thripidae) and cotton fleahoppers (Hemiptera: Miridae) in cotton

A 2-yr field study was conducted to examine the effectiveness of two sampling methods (visual and plant washing techniques) for western flower thrips, Frankliniella occidentalis (Pergande), and five sampling methods (visual, beat bucket, drop cloth, sweep net, and vacuum) for cotton fleahopper, Pseudatomoscelis seriatus (Reuter), in Texas cotton, Gossypium hirsutum (L.), and to develop sequential sampling plans for each pest. The plant washing technique gave similar results to the visual method in detecting adult thrips, but the washing technique detected significantly higher number of thrips larvae compared with the visual sampling. Visual sampling detected the highest number of fleahoppers followed by beat bucket, drop cloth, vacuum, and sweep net sampling, with no significant difference in catch efficiency between vacuum and sweep net methods. However, based on fixed precision cost reliability, the sweep net sampling was the most cost-effective method followed by vacuum, beat bucket, drop cloth, and visual sampling. Taylor's Power Law analysis revealed that the field dispersion patterns of both thrips and fleahoppers were aggregated throughout the crop growing season. For thrips management decision based on visual sampling (0.25 precision), 15 plants were estimated to be the minimum sample size when the estimated population density was one thrips per plant, whereas the minimum sample size was nine plants when thrips density approached 10 thrips per plant. The minimum visual sample size for cotton fleahoppers was 16 plants when the density was one fleahopper per plant, but the sample size decreased rapidly with an increase in fleahopper density, requiring only four plants to be sampled when the density was 10 fleahoppers per plant. Sequential sampling plans were developed and validated with independent data for both thrips and cotton fleahoppers.     

Lack of fitness costs of insecticide resistance in the western flower thrips (Thysanoptera: Thripidae)

The fitness costs of spinosad and acrinathrin resistance was investigated in the western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae). Fitness studies were conducted on susceptible and resistant strains of F. occidentalis. Resistant females were significantly more fecund (number of eggs per female) than susceptible females. The hatching rate (fertility) for both susceptible and acrinathrin-resistant strains was significantly lower than in the spinosad-resistant strain. Mean developmental time from egg to adult did not differ between thrips populations. Similarly, female longevity did not differ between populations. These data suggest that lack of fitness costs related to insecticide resistance may accelerate the development of insecticide resistance in populations of F. occidentalis from southeastern Spain.     

西花蓟马的SCAR分子检测技术

西花蓟马Frankliniella occidentalis （Pergande）是一种世界性入侵害虫,寄主范围广,危害严重,2003年首次在我国发生危害,并有进一步扩散蔓延的趋势。针对蓟马类害虫虫体微小、形态相似,难以准确快速区分的问题,采用特征序列扩增区域（SCAR）标记技术,以西花蓟马及与之同域发生的其他种类蓟马为对象,筛选出1对西花蓟马特异性引物（FOMF/FOMR）,其扩增片段大小为320 bp。种特异性检测结果显示,该对引物只对西花蓟马的基因组DNA具有扩增能力,对同域发生的花蓟马F. intonsa （Trybom）、禾花蓟马F. tenuicornis （Uzel）、烟蓟马Thrips tabaci L. 等41种蓟马不具有扩增效果。该对引物不仅对不同虫态的西花蓟马具有扩增能力,而且在西花蓟马发生地的寄主植物组织内亦检测到了其卵的存在。同时,该检测技术灵敏度高,对成虫的最低检出阈值为1/160头。本检测技术在口岸检疫以及花卉、蔬菜和种苗调运中的害虫检测和监测中具有重要意义。     

入侵害虫西花蓟马及其他8种常见蓟马的分子鉴定

用PCR产物直接测序法对入侵害虫西花蓟马和其他8种蓟马的线粒体 COⅠ基因433 bp片段测序，获得62个个体的序列。分子数据分析显示： 种内个体间平均遗传距离在0～0.005之间，2003年在北京发现的西花蓟马与欧洲等地区报导的西花蓟马不存在明显的遗传差异； 9种蓟马种间平均遗传距离为0.213。构建的NJ树可以很好的显示蓟马的聚类，物种各单元型最初分支自展值均达到100%。结果表明，基于PCR及直接测序技术的分子鉴定可以达到准确鉴定蓟马物种之目的。     

西花蓟马在黄瓜和架豆上的空间分布型及理论抽样数

西花蓟马Frankliniella occidentalis（Pergande）已在北京部分蔬菜园区成功定居,对蔬菜生产造成危害。为了解该虫在蔬菜田的种群空间分布型,指导田间取样,本文应用几种聚集度指标的计算公式以及Taylor、Iwao的回归方程式,分析和测定了西花蓟马的种群空间分布格局。结果表明：在黄瓜和架豆上西花蓟马的空间分布型一致,均为聚集分布型;该虫的空间分布型不受种群密度的影响,并且也不随取样时间的变化而变化。种群数量动态研究显示西花蓟马成虫和若虫在黄瓜植株的中部叶片分布较多（从顶部向下数第4叶至第17叶片上成、若虫的分布数量多于其他叶片上的数量）,未展开的叶片、嫩叶和下部老叶危害较轻。用Iwao回归法中α、β参数计算出在允许误差范围内的理论抽样数。     

Damage to nectarines by the western flower thrips (Thysanoptera: Thripidae) in the interior of British Columbia, Canada

The phenology of damage by the western flower thrips, Frankliniella occidentalis (Pergande), on nectarines was investigated using sticky cards and direct sampling of buds between 1993 and 1995 in the interior of British Columbia, the most susceptible period for damage by western flower thrips to nectarines. The life stage responsible for damage and variation in susceptibility to damage of 11 different nectarine varieties were determined. To evaluate the predictive ability of 2 sampling methods, thrips were counted from both buds and sticky cards before petal fall and correlated to damage levels at husk drop. Damage to nectarines was caused almost entirely by larval feeding at petal fall. No predictive relationships between adult or larval densities of western flower thrips and subsequent damage to fruit were apparent. Varieties did not differ in terms of larval densities at petal fall or the subsequent damage to fruit. Female western flower thrips oviposit in nectarine buds from dormant through bloom stages primarily in sepal tissues in the early buds, and in filaments and petals as these become available.     

Biotic resistance limits the invasiveness of the western flower thrips,Frankliniella occidentalis (Thysanoptera: Thripidae), in Florida

The spread of the western flower thrips, Frankliniella occidentalis (Pergande), has resulted in the world‐wide destabilization of established integrated pest management programs for many crops. It is hypothesized that frequent exposure to insecticides in intensive agriculture selected for resistant populations, which allowed invasive populations in the eastern USA to overcome biotic resistance from the native community of species. Research conducted in Florida to understand the role of biotic factors in limiting the abundance of the western flower thrips is reviewed. Orius spp. (Hemiptera: Anthocoridae) are effective predators that suppress populations of thrips on crop and non‐crop hosts in southern and northern Florida. Orius are more effective predators of the western flower thrips than the native flower thrips, F. tritici (Fitch) and F. bispinosa (Morgan). The native species are competitors of the western flower thrips. Excessive fertilization and the use of broad‐spectrum insecticides in crop fields further enhances populations of the western flower thrips. Interactions with native species clearly limit the abundance of western flower thrips in Florida, but populations are abundant in fertilized crop fields where application of insecticides excludes predators and competitor species.     

Life history characteristics of the western flower thrips,Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), under fluctuating conditions of temperature or relative humidity

Frankliniella occidentalis (Pergande) is a major insect pest of greenhouse crops such as leaf vegetables, flowers and vegetable fruits worldwide. The life history characteristics of F. occidentalis were investigated at control temperature and humidity (27.3 ± 0.54 °C, 79.9 ± 2.79% RH) (mean ± SD), a 10 °C-range fluctuation in temperature (27.1 ± 5.28 °C, 81.5 ± 4.03% RH), a 20 °C-range fluctuation in temperature (26.5 ± 10.09 °C, 80.4 ± 5.76% RH), a 20%-range fluctuation in humidity (26.8 ± 0.37 °C, 80.7 ± 9.55% RH) and a 30%-range fluctuation in humidity (27.3 ± 0.41 °C, 76.3 ± 15.28% RH). Overall, the life history traits of F. occidentalis were more negatively affected by fluctuating environmental conditions. The impact of temperature fluctuation was more severe than that of humidity fluctuation. Additionally, the degree of impact increased as the fluctuation range of the temperature increased, while the reverse trend was observed with humidity fluctuations. With the 20 °C-range fluctuation in temperature, F. occidentalis died at the 1st instar larval stage. The offspring’s sex ratio was significantly higher at the 20%- and 30%-range fluctuations in humidity (0.47 and 0.49, respectively) compared to the control (0.35) and at the 10 °C-range fluctuation in temperature (0.33). From the fertility life table analysis, the intrinsic rate of increase (r) was higher at the 30%-range fluctuation in humidity and control conditions as 0.218 and 0.205, respectively. At the 10 °C-range fluctuation in temperature conditions, r was significantly lower as 0.169. High fluctuations in temperature and low fluctuations in humidity appear to be the best conditions for controlling F. occidentalis populations in greenhouses.     

UV‐absorbing films and nets affect the dispersal of western flower thrips,Frankliniella occidentalis (Thysanoptera: Thripidae)

UV‐absorbing films and nets are frequently used as covering materials for netted greenhouses and film tunnels in protected cultivation systems. This study explored the effects of such materials on the dispersal behaviour of western flower thrips (WFT), Frankliniella occidentalis, in flight cages under greenhouse conditions with additional artificial UV‐A light sources. The study involved release–recapture experiments in choice and no‐choice layouts. Different trapping methods were compared (blue sticky cards, plants and transparent cards) for recapture of thrips. In choice experiments, insects were released from a black box compartment between two tunnels covered with either UV‐transmitting or UV‐absorbing materials. A significantly higher proportion of (82–98%) WFT was recaptured in UV‐transmitting tunnels compared with UV‐absorbing tunnels. In no‐choice experiments, WFT were found to infiltrate the tunnels at different rates depending on the trap type used and experimental layout. In small‐scale dispersal experiments using blue sticky cards and plants as traps, infiltration was not significantly different between UV‐absorbing and UV‐transmitting tunnels, whereas when using transparent cards, WFT penetrated further into the UV‐transmitting plastic film tunnels. In larger‐scale dispersal experiments, plants or blue sticky cards were arranged in concentric circles around a source plant at the release point. Dispersal was found to differ depending on the method of release, but WFT tended to exhibit reduced dispersal from source plants under UV‐deficient conditions. In conclusion, our data support the hypothesis that manipulation of spectral light properties using UV‐absorbing cladding materials for protected crop stands interferes with the orientation and host finding of WFT, resulting in reduced dispersal into and within plant stands in UV‐deficient environments.     

温度对西花蓟马生长发育、 繁殖和种群增长的影响

西花蓟马Frankliniella occidentalis (Pergande)是一种入侵我国的重要害虫, 温度是决定蓟马能否建立稳定种群的最基本因素。为明确温度对西花蓟马种群增长的影响, 本研究在室内观察了西花蓟马在15℃, 20℃, 25℃, 30℃和35℃温度条件下的生长发育、 存活与繁殖能力, 并计算各温度条件下的种群增长参数。结果表明： 在35℃条件下, 西花蓟马不能完成发育, 其他温度条件下西花蓟马从卵孵化至蛹羽化成成虫, 以20℃条件下的存活率最高, 为62.8%。西花蓟马发育速率随温度升高明显加快, 在15℃下, 完成发育需要近30 d; 而在30℃下, 西花蓟马完成发育仅需10 d 左右。西花蓟马成虫寿命随温度的升高而明显缩短, 在15℃下, 平均寿命为36 d, 最长寿命达60 d; 在30℃下, 西花蓟马的平均寿命为10 d。西花蓟马在 15℃, 20℃和25℃条件下的平均繁殖力差异不显著, 分别为37.70, 32.56, 37.80头1龄若虫/雌, 但显著高于30℃条件下的平均繁殖力（9.36头1龄若虫/雌）。西花蓟马的种群增长参数净生殖率(R0)、 内禀增长率(rm), 在25℃时达最高值, 分别为20.10和0.178 d-1, 而在15℃下分别仅为18.67和0.096 d-1。据此得出, 20~25℃是最适宜西花蓟马生长发育和繁殖温度范围, 温度过高或过低都不利于西花蓟马种群增长。西花蓟马的发育起点温度为7.4℃, 充分完成发育所需的有效积温为208.0日·度。不考虑其他阻碍生长发育因素的情况下, 华南、 华中、 华北和东北地区的年发生代数分别为24~26, 16~18, 13~14和1~4代, 西南地区昆明与丽江分别为13~15和8~10代。     

High nucleotide diversity in the para-like voltage-sensitive sodium channel gene sequence in the western flower thrips (Thysanoptera: Thripidae)

In a search for a pyrethroid resistance diagnostic marker, a partial sequence of the para-like sodium channel gene was obtained from 78 diploid females of the arrhenotokous insect pest species Frankliniella occidentalis (Pergande), the western flower thrips. Although all the insects analyzed came from a single laboratory population, nine different haplotypes were obtained. Two haplotypes did have the well-known L to F kdr mutation, but only one of these could be statistically linked to pyrethroid resistance in our population. This haplotype did not have the superkdr mutation, but did have a unique mutation a few amino acids downstream, at a position already linked to resistance in Plutella. Although this para-like locus seemed to have a role in pyrethroid resistance in our population, other resistance mechanisms were also probably involved. The fact that our laboratory population, open to migration, contained ahigh genetic diversity forthis selected gene shows that "pest tourism" is a major factor for resistance dynamics in this greenhouse pest. This, with the possible occurrence of an original resistance mutation, might preclude the use of very specific approaches for resistance monitoring in the field in this species.