The impact of irrigation frequency on population density of thrips, Thrips tabaci Rom (Thripidae, Thysanoptera) and yield of onion in E1 Rahad, Sudan

Vegetable farmers of the El Rahad Scheme (a newly developed scheme situated between latitude 13°31′–14°25′ north and longitude 33°31–34°32′ east) used to extend irrigation frequency for onion production as they believed it would hamper and suppress thrips incidence. Thrips, T. tabaci, is the only major insect pest of onion in the El Rahad Scheme and the influence of irrigation intervals on the population density of the pest and on onion yield was not quantified. Irrigation is a factor in the development of crop pests and the levels of the pest population are related to the commencement of irrigation. The effect of irrigation frequency on the development of onion thrips and yield was investigated and the response was found to be a significant increase in the population density of the pests from February to March with shorter irrigation frequency. A steady increase of thrips population was noted from February and March and a sharp decline was recorded in April during both the 1992/93 and 1993/94 seasons. At wider irrigation intervals, levels of the pest population were significantly less from February to March during both seasons. Total bulb yield and average bulb weight were significantly higher at shorter irrigation frequencies when compared with extended frequencies. The same pattern of results existed throughout the course of the experiment.     

Winter and spring abundance of Frankliniella spp. and Thrips tabaci Lindeman (Thysan., Thripidae) on weed host plants in Turkey

Abstract:  The species composition of thrips and abundance of three pestiferous thrips species, Frankliniella occidentalis (Pergande), Thrips tabaci Lindeman and Frankliniella intonsa (Trybom) (Thysan., Thripidae) on weed species during winter and spring time were studied in vegetable production and polycultured areas in the eastern Mediterranean region of Turkey in years 2002 and 2003. Of a total of 61 543 thrips extracted from 8933 samples collected, 77% were adults. F. occidentalis was collected from all of 49 weed species sampled and F. intonsa was detected on 18 plant species between May and June. T. tabaci was collected from 42 of a total of 49 weed species. Immature thrips were extracted from 42 of a total of 49 weed species. Four weed species of a total 35 weed species sampled between April and May in the sampling areas hosted 63% of larvae and approximately 50% of F. occidentalis collected. A total of 23 thrips species was recorded from weeds sampled in this study. F. occidentalis accounted for 83% of a total 47 640 adult thrips collected and followed by low rates of T. tabaci and F. intonsa (9 and 1%, respectively). Melanthrips spp. accounted for 3.54% of total numbers of adult thrips. Flower-inhabiting thrips species and T. tabaci were most abundant on weeds between April and May when the number of weed species in bloom was greatest, then decreased to low levels after May. Most pestiferous thrips on weeds species were collected from vegetable production areas. F. occidentalis was the predominant thrips comprising over 80% in most months in the vegetable grown area. In polycultured area, the composition of adult thrips shifted monthly. While Melanthrips was the more prevailing thrips by rates 53% in February and 81% in March, F. occidentalis was the predominant thrips with 60 to 62% in spring, respectively.     

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.     

Determination of soil depth inhabited by Frankliniella occidentalis (Pergande) and Thrips tabaci Lindeman (Thysan., Thripidae) under greenhouse cultivation

The objectives of this study were to determine the depth of penetration into the soil by Frankliniella occidentalis (Pergande) and Thrips tabaci Lindeman (Thysan., Thripidae) in cucumber and tomato crops in greenhouses. A metal sampling apparatus sampled for the two species of thrips at five levels of soil depth (0–10 cm), over seven dates of sampling, each in eight replications. In general, thrips were found to be in greater numbers in the first 2 cm of soil, with the number of insects decreasing with depth increasing. In the depth of 8–10 cm, no insects were found. Number of thrips found in the soil under cucumber plants was greater in comparison with the tomato samplings. Frankliniella occidentalis was found in greater numbers in comparison with T. tabaci. The date of sampling showed a significant interaction with soil depth, with the number of insects found increasing from the first to the last date of sampling.     

Colour preference and potato cultivar oviposition choice by onion thrips,Thrips tabaci (Thysanoptera: Thripidae)

Thrips tabaci (Lindeman) is an important polyphagous pest, and vector of Tomato spotted wilt virus responsible for sporadic, but devastating epidemics in potato. T. tabaci shows significant preferential differences between potato cultivars that may be important for field resistance. To investigate the role of visual cues in host choice we tested colour preference using two-choice assays. Thrips tabaci showed a significant preference for mid-green over red, blue, and white coloured cards, a preference for both light-green and mid-green over dark-green, and light-green over yellow, but no preferential difference between mid-green and yellow, and between light-green and mid-green. Analysis of the spectral reflectance of potato cultivars differing in thrips preference, revealed significant differences within the 400–700 nm wavelengths. In most cases cultivars preferred by T. tabaci had lighter green foliage and higher reflectance at 552 nm, and thus colour preference may be important for host selection. Oviposition choice of T. tabaci for potato cultivars was determined from counts of larvae and unhatched eggs from leaf disks under choice and no-choice conditions. In contrast to the colour choice experiments, onion thrips showed oviposition preference for cultivars with darker green foliage and lower reflectance at 552 nm.     

Regional and temporal variation in susceptibility to lambda-cyhalothrin in onion thrips, Thrips tabaci (Thysanoptera: Thripidae), in onion fields in New York

Populations of onion thrips, Thrips tabaci Lindeman, from commercial onion fields in New York were evaluated for their susceptibility to the commonly used pyrethroid, lambda-cyhalothrin (Warrior T), using a novel system called the Thrips Insecticide Bioassay System (TIBS). To use TIBS, thrips are collected directly from the plant into an insecticide-treated 0.5-ml microcentrifuge tube that has a flexible plastic cap with a small well into which 0.08 ml of a 10% sugar-water solution with food colorant is deposited. The solution is sealed into the well with a small piece of stretched parafilm through which the thrips can feed on the solution. Thrips mortality is assessed after 24 h with the help of a dissecting stereoscope. In 2001, onion thrips populations were collected from 16 different sites and resistance ratios were >1,000 in five populations. Percent mortality at 100 ppm, a recommended field rate, varied from 9 to 100%, indicating high levels of variation in susceptibility. Particular instances of resistance appeared to be the result of practices within an individual field rather than a regional phenomenon. In 2002, we also observed large differences in onion thrips susceptibility, not only between individual fields but also between thrips collected in a single field at mid season and late season, again suggesting that insecticide-use practices within an individual field caused differences in susceptibility. Additional tests indicated no differences in susceptibility between adult and larval onion thrips populations and only relatively minor differences between populations collected from different parts of the same field. Using TIBS, several populations of onion thrips with different susceptibilities to lambda-cyhalothrin were identified and then subjected to lambda-cyhalothrin-treated onion plants. There was a highly significant positive relationship between percent mortality of thrips from TIBS and percent mortality from the treated onion plants, indicating that results from TIBS could be used to predict spray performance. These data suggest that use of TIBS for evaluating susceptibility to particular insecticides could be instrumental for developing a resistance management strategy for onion thrips.     

Investigation on the optimal term of one-fold insecticide application for decreasing onion thrips ( Thrips tabaci Lindeman, Thysanoptera, Thripidae) damage on early white cabbage

An impact of the term of one-fold insecticide application to reduce the damage of the onion thrips in early white cabbage was established. During the growing season plants were treated with insecticide (abamectin) in three different terms (treatment 1: 9 June, treatment 2: 16 June, treatment 3: 23 June, and treatment 4: untreated). On the exterior leaves of cabbage heads, treated with the insecticide, statistically significantly lower mean index of damage was determined as compared to the untreated plants. No significant differences were found between three different terms of application. The majority of the economically important damages in all of the treatments was found between the 3rd and 6th exterior leaf in the head. The highest mean weight of heads (1517.3 g) and mean net weight of heads I = mean weight of heads - weight of damaged and removed leavesl (1166.3 g) were established in plants which were treated the last. These parameters were the lowest in untreated plants (1083.3 g / 805.6 g). The yield loss due to damaged leaves removal amounted from 22.9% (treatment 2) to 25.6% (treatment 4). Based on the results obtained in this research we concluded that yield loss due to onion thrips attack in plants with one-fold insecticide application is not substantially lower as in untreated plants (though statistically significant differences between them were established), because insecticide cannot reach the interior leaves in the head. Substantial differences in the total and net weight of heads between the treatments and especially between the treated and untreated plants are explained by the fact that feeding of numerous thrips populations in the heads and on the exterior cabbage leaves negatively affects plant physiology and yield. The highest average yield in plants which were treated the last indicates a possibility that insecticide also inhibits plant growth and development to a certain extent.     

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.     

Laboratory and greenhouse evaluation of a new entomopathogenic strain of Beauveria bassiana for control of the onion thrips Thrips tabaci

Abstract

The onion thrips Thrips tabaci is one of the most important pests of greenhouse and open-field broccoli, onion and other crops. However, the current strategy of using synthetic pesticides for its control is inadequate and unsustainable, leading to a growing interest in novel and effective biological control alternatives such as entomopathogenic fungi. Among 20 isolates of Beauveria bassiana tested for virulence against T. tabaci in laboratory bioassays, we found strain SZ-26 as the most potent, causing 83–100% mortality in adults at 1×10⁷ mL^?1conidia after 4–7 days. Further experiments in greenhouses showed the strain SZ-26 significantly lowered the numbers of adult and larval stages.     

Thrips see red – flower colour and the host relationships of a polyphagous anthophilic thrips

Abstract 1. The common blossom thrips, Frankliniella schultzei, is a polyphagous anthophilic species that colonises a wide range of host‐plant species across different plant taxa. The environmental cues used by these polyphagous insects to recognise and locate host plants are not known. We therefore determined if colour is an important environmental signal used by F. schultzei to recognise flowers of eight of its more significant host‐plant species. 2. The effect of flower colour on the colonisation of different host plant species by F. schultzei was investigated by collecting and analysing the following: (a) numbers of thrips from different heights and aspects of the primary host plant Malvaviscus arboreus, (b) thrips distribution within flowers of Hibiscus rosasinensis, (c) colour reflectance from flowers of eight different host‐plant species, and (d) reflectance from different coloured sticky traps and the number of thrips trapped on them at different times of the day and on different dates. 3. The results indicate that: (a) the thrips (both sexes) concentrate towards the top of the primary host plant M. arboreus and are not distributed differentially according to sunny or shady aspect of the plant, (b) the number of female thrips on H. rosasinensis was higher in anthers compared to petals (corolla) and the basal parts of the flower, and males were as numerous on the petals as were females, and (c) there is a common floral reflectance pattern (but with different intensities) across the eight host plant species, mainly in the red part of the spectrum (600–700 nm). 4. Results of colour sticky trapping show that red attracts more female thrips compared to any other colour and that most were caught between 09.00 and 11.00 hours. By contrast, more male thrips were trapped between 07.00 and 09.00 hours. Males were more evenly distributed across the different colours but the highest numbers were associated with the yellow traps. 5. The higher densities of thrips at the top of their host plant may be related to the early morning (07.00–11.00 hours) activity of the thrips, when the top portions of the plant are more exposed to sunlight. The sex‐related distributions of F. schultzei thrips across time, coloured sticky traps, and various parts of the flowers seem to be related to mating swarm formation by the males, on the one hand, and the relative frequency and intensity of the use of M. arboreus by the females, on the other, as a feeding and oviposition site. Frankliniella schultzei females respond more strongly to red than to any other colours, so it is predicted that the spectral properties of colour recognition by this species will correlate with the predominant red reflectance of its primary host, M. arboreus, and that there may well be a sex‐related difference in colour recognition within this species.     

Host plant effects on the functional response of Neoseiulus cucumeris to onion thrips larvae

Abstract:  The performance of herbivores, natural enemies and their interactions may be affected directly or indirectly by host plant traits, e.g. the physical plant characteristics may influence the search pattern and the functional response of predators. We studied the functional response of adult females of the predatory mite Neoseiulus cucumeris to first instar larvae of Thrips tabaci on three host plants (sweet pepper, eggplant and cucumber). The 24-h leaf disc experiments conducted at 25 ± 1°C, 60 ± 10% relative humidity and 16 : 8 h (light : dark) showed that N. cucumeris exhibited a type II functional response on all host plants. The following search rates and handling times were estimated from fitting the data to the disc equation 0.043/h and 1.798 h (cucumber); 0.048/h and 1.030 h (sweet pepper) and 0.0441/h and 2.294 h (eggplant) giving an estimated maximum predation of 13.35, 23.31 and 10.46 larvae per day respectively. The data from sweet pepper could also be described by the random predator equation ( a ': 0.051/h; T _h: 0.472 h). The host plant species interacted significantly with prey density on the functional response of N. cucumeris with the relative differences in the number of thrips eaten on each host plant increasing with density. It is suggested that it is mainly the difference in trichome density between the three host plants that is responsible for the observed differences in the functional response of N. cucumeris . These results emphasize the importance of the host plant characteristics on the performance of natural enemies and for optimizing their use in biological control of pests.     

Effect of temperature on the sex ratio and life table parameters of the leek-(L1) and tobacco-associated (T) Thrips tabaci lineages (Thysanoptera: Thripidae)

Temperature strongly affects insect biological response. Effects of temperature on the sex ratio, preoviposition, fecundity, egg hatchability rate, and adult longevity of the L1 and T Thrips tabaci lineages were studied under laboratory conditions at three constant temperature levels (15, 23, and 30°C) on cabbage and tobacco leaf discs, respectively. Results showed that temperature affects all the tested life table parameters of L1 and T lineages. The proportion of female's log-linearly increased as temperature increased. Preoviposition period, fecundity, adult longevity, and egg hatchability rate were reduced significantly with increasing temperature. In both lineages, the lowest egg hatchability rates were recorded at 30°C and the highest rates were recorded at 15°C. Mean fecundity of the L1 and T lineages were 107.63 and 84.83 at 23°C, respectively, and mean fecundity of the L1 and T lineages were 19.88 and 70.82 at 15°C, respectively. The lowest mean fecundity was recorded at 15°C and 30°C for the L1 and T lineages, respectively. Mean longevity were 34.22 and 81.82 days at 15°C for L1 and T lineages, respectively. Whereas, the shortest mean longevity (13.63 and 13.91 days) was observed at 30°C for the L1 and T lineages, respectively. Our results demonstrated that 23°C was the optimum temperature for fecundity among the studied temperature levels for the studied population of the L1 and T T. tabaci lineages.     

The Thrips orientalis group from South-east Asia and Australia: some species identities and relationships (Thysanoptera, Thripidae)

Abstract  A thrips associated in Thailand with the flowers of Paederia foetida , a rubiaceous weed in Florida that is a threat to Australia, is here recognised as Thrips morindae Priesner, a species previously considered a synonym of T. javanicus Priesner. Systematic relationships of these species with others in South-east Asia and the Australasian region are discussed. Two new species-groups are designated: the T. obscuratus group of six species from New Zealand and New Caledonia, and the T. orientalis group of 12 South-east Asian species that are associated with scented white flowers such as Gardenia . Four species of this second group are recorded from tropical Australia.     

Species composition of thrips (Thysanoptera: Thripidae) and spider mites (Acari: Tetranychidae) on cultivated chrysanthemum (Asteraceae) in Okinawa,southwestern Japan

We investigated the species composition of phytophagous thrips and spider mites on cultivated chrysanthemum in Okinawa, southwestern Japan. Eight thrips species belonging to the genera Frankliniella, Microcephalothrips, Scirtothrips and Thrips were found on chrysanthemum leaves. Among them, Thrips nigropilosus was the predominant species irrespective of season, island or cultivation environment (meshed greenhouse or open field), with its infestation frequency being 89% of the fields in which thrips occurred. This high frequency of occurrence suggests that T. nigropilosus is a major pest of chrysanthemum in Okinawa, even though this species has rarely been regarded as an important pest of chrysanthemum or other crops in any other areas. Thrips palmi was the second most dominant (infestation frequency 36%) and other species were scarce (<14%). Tetranychus urticae (green form) was the only tetranychid species on chrysanthemum in our survey. This lack of diversity among spider mites on chrysanthemum is peculiar considering that eight Tetranychus species have been found on vegetables in the same area. Since T. urticae (green form) has been shown to be resistant to a number of pesticides, severe pesticide applications might have simplified the spider mite fauna on chrysanthemum in Okinawa.     

Pollen-feeding and the host specificity and fecundity of flower thrips (Thysanoptera)

Abstract. 1. Flower thrips could feed on pollen of a wide range of sizes and were unaffected by exine spines. One species of thrips was unable to feed on very adhesive grains.
2. The proportion of time spent feeding on each species of pollen (in seconds per minute of exposure) reflected the known host specificities. Host-specific thrips could distinguish their host pollen, apparently without probing.
3. Kakothrips pisivorus (Westwood) appeared to have a higher rate of ingestion of a host pollen than of four non-host pollens.
4. In cages in the laboratory, Thrips fuscipennis Haliday laid significantly more eggs over 48h when given pollen than when given other floral tissues or no food. K. pisivonis and Ceratothrips ericae (Haliday) also laid significantly more eggs over 48h in the presence of pollen than in its absence.
5. In comparisons between pollen diets, T. fuscipennis, a generalist, laid similar numbers of eggs with each species of pollen, but K. pisivorus and C. ericae laid significantly more eggs with their respective host pollens. The differences in oviposition rate between pollen diets reflected the differences in the proportion of time spent feeding.     

The Jurassic thrips Liassothrips crassipes (Martynov, 1927) and its taxonomic position in the order Thysanoptera (Insecta)

The Late Jurassic thrips Liassothrips crassipes (Martynov) is redescribed, and its taxonomic position discussed. It is shown that the male genitalia of Liassothrips are similar to the terebrantian type, while the female genitalia resemble the tubuliferan type. Therefore, the Liassothripidae shows the possible evolutionary pathway of tubuliferan thrips. Liassothrips is treated as the oldest known member of the suborder Tubulifera (previously known beginning from the Eocene), the ancestors of which belonged to the family Aeolothripidae (Terebrantia) rather than Thripidae.     

Molecular differences in the mitochondrial cytochrome oxidase I (mtCOI) gene and development of a species-specific marker for onion thrips, Thrips tabaci Lindeman, and melon thrips, T. palmi Karny (Thysanoptera: Thripidae), vectors of tospoviruses (Bunyaviridae)

A quick and developmental-stage non-limiting method of the identification of vectors of tospoviruses, such as Thrips tabaci and T. palmi, is important in the study of vector transmission, insecticide resistance, biological control, etc. Morphological identification of these thrips vectors is often a stumbling block in the absence of a specialist and limited by polymorphism, sex, stage of development, etc. Molecular identification, on the other hand, is not hampered by the above factors and can easily be followed by a non-specialist with a little training. The mitochondrial cytochrome oxidase I (mtCOI) exhibits reliable inter-species variations as compared to the other markers. In this communication, we present the differences in the mtCOI partial sequence of morphologically identified specimens of T. tabaci and T. palmi collected from onion and watermelon, respectively. Species-specific markers, identified in this study, could successfully determine T. tabaci and T. palmi, which corroborated the morphological identification. Phylogenetic analyses showed that both T. tabaci and T. palmi formed different clades as compared to the other NCBI accessions. The implication of these variations in vector efficiency has to be investigated further. The result of this investigation is useful in the quick identification of T. tabaci and T. palmi, a critical factor in understanding the epidemiology of the tospoviruses, their management and also in quarantine.     

Temperature and precipitation affect seasonal patterns of dispersing tobacco thrips, Frankliniella fusca, and onion thrips, Thrips tabaci (Thysanoptera: Thripidae) caught on sticky traps

Effects of temperature and precipitation on the temporal patterns of dispersing tobacco thrips, Frankliniella fusca, and onion thrips, Thrips tabaci, caught on yellow sticky traps were estimated in central and eastern North Carolina and eastern Virginia from 1997 through 2001. The impact that these environmental factors had on numbers of F. fusca and T. tabaci caught on sticky traps during April and May was determined using stepwise regression analysis of 43 and 38 site-years of aerial trapping data from 21 and 18 different field locations, respectively. The independent variables used in the regression models included degree-days, total precipitation, and the number of days in which precipitation occurred during January through May. Each variable was significant in explaining variation for both thrips species and, in all models, degree-days was the single best explanatory variable. Precipitation had a comparatively greater effect on T. tabaci than F. fusca. The numbers of F. fusca and T. tabaci captured in flight were positively related to degree-days and the number of days with precipitation but negatively related to total precipitation. Combined in a single model, degree-days, total precipitation, and the number of days with precipitation explained 70 and 55% of the total variation in the number of F. fusca captured from 1 April through 10 May and from 1 April through 31 May, respectively. Regarding T. tabaci flights, degree-days, total precipitation, and the number of days with precipitation collectively explained 57 and 63% of the total variation in the number captured from 1 April through 10 May and from 1 April through 31 May, respectively.