Within-plant distribution of onion thrips (Thysanoptera: Thripidae) in onions

Two aspects of the within-plant distribution of Thrips tabaci Lindeman (Thysanoptera: Thripidae) on onion, Allium cepa L., plants were investigated: 1) diurnal variations in the distribution of adults and larvae between basal and upper sections of onion leaves, and 2) between-leaf and within-leaf distribution of the eggs. The diurnal investigations showed that higher proportions of larvae than of adults congregated at the basal sections of plants, particularly when plants were young and thrips density was low. As plants matured and thrips density increased, the larvae became more dispersed. Regardless of plant size, there were always more adults in the upper than basal plant sections. There were no clear time-windows during the 24-h diurnal cycle when more thrips were in the upper plant parts. T. tabaci eggs were laid everywhere in the plant. Leaves of intermediate ages had more eggs than older or younger leaves. Within leaves, the white leaf sheath received the least eggs and leaf tips received slightly more eggs than leaf sheaths. The highest egg density was found between the green leaf base and the leaf tips. Regardless of plant size, more than half of all eggs were laid above the basal sections. The percentage increased to >95% in mature plants. Except when plants were small the outer leaves were preferred over inner leaves and upper leaf sections preferred over lower leaf sections as egg-laying sites by adults. Implications of the results in the management of T. tabaci are discussed.     

Patterns of insecticide resistance in onion thrips (Thysanoptera: Thripidae) in onion fields in New York

To develop an insecticide resistance management program for onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripidae), on onions (Allium spp.), we surveyed populations in commercial onion fields in New York and evaluated their susceptibility to the two most widely used classes of insecticides plus two new insecticides during 2003-2005. All insecticide evaluations were conducted using the Thrips Insecticide Bioassay System (TIBS). As in our surveys conducted during 2002-2003, there were large temporal and spatial variations in susceptibility to the pyrethroid lambda-cyhalothrin (Warrior) across onion-growing regions in 2003. New data indicate that the field rate of methomyl (Lannate LV) still provides control but that the genes for resistance to methomyl are present in some populations. Tests with the two new insecticides, acetamiprid (Assail 70 WP) and spinosad (SpinTor 2CS), indicated they provided > 85% mortality at the field rate. To determine the spatial variation in insecticide susceptibility within a region, a series of systematic assays were conducted with lambda-cyhalothrin and methomyl. In 2004 and 2005, our data indicated that the within-region spatial variation in susceptibility to lambda-cyhalothrin was not large at the field rate or for the 100 ppm rate of methomyl. In 2005, a year in which T. tabaci densities in most fields were much higher than in 2004, growers were unable to control T. tabaci in particular fields and attributed this lack of control to resistance. Yet, we found similar levels of high susceptibility in all fields when using TIBS. This finding suggests that resistance had not developed and that variation in control may have been due to other factors, such as localized higher populations, poor spray coverage, too much time between spray applications, or different onion varieties.     

Impact of straw mulch on populations of onion thrips (Thysanoptera: Thripidae) in onion

Development of insecticide resistance in onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripidae), populations in onion (Allium spp.) fields and the incidence of the T. tabaci transmitted Iris yellow spot virus have stimulated interest in evaluating alternative management tactics. Effects of straw mulch applied in commercial onion fields in muck areas of western New York were assessed in 2006 and 2007 as a possible onion thrips management strategy. In trials in which no insecticides were applied for thrips control, straw mulch-treated plots supported significantly lower T. tabaci populations compared with control plots. In both years, the action thresholds of one or three larvae per leaf were reached in straw mulch treatments between 7 and 14 d later than in the control. Ground predatory fauna, as evaluated by pitfall trapping, was not increased by straw mulch in 2006; however, populations of the common predatory thrips Aeolothrips fasciatus (L.) (Thysanoptera: Aeolothripidae) were significantly lower in straw mulch plots in both years. Interference of straw mulch in the pupation and emergence of T. tabaci was investigated in the lab and their emergence was reduced by 54% compared with bare soil. In the field the overall yield of onions was not affected by the straw mulch treatment; however, the presence of jumbo grade onions (>77 mm) was increased in 2006, but not in 2007. These results indicate that populations of T. tabaci adults and larvae can be significantly reduced by the use of straw mulch without compromising overall onion yield. The use of this cultural practice in an onion integrated pest management program seems promising.     

Advances in control of onion thrips (Thysanoptera: Thripidae) in cabbage

Onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripidae), feeding injury results in discoloration and a rough texture on cabbage, Brassica oleracea capitata (L.), leaves, and damage may occur deep inside the head. It has become a key pest of cabbage in the United States and many other countries. Previous studies have indicated poor control using insecticides. The present study identified imidacloprid drenches and sprays of acetamiprid, dimethoate, spinosad, and imidacloprid as insecticides that performed better than the industry standard, lambda-cyhalothrin. However, additional tests with foliar sprays of dimethoate and acetamiprid indicated there was not an ideal crop stage (precupping, cupping, or postcupping) at which either insecticide could be applied for reliable control of T. tabaci, possibly because of multiple flights of thrips into the crop or the asynchrony of flights and susceptible crop stages. In tests in a commercial field, a soil drench of imidacloprid 4 wk after transplanting reduced the number of damaged leaves in the head by 32%, whereas five sprays of acetamiprid reduced damage by 51%. Combining both insecticide regimes reduced damage by 85%, but resulted in a very costly management program. Cabbage varieties varied considerably in susceptibility with some having negligible thrips injury, regardless of being treated with an insecticide. Planting date affected susceptibility of cabbage to some degree, but not as much as other tactics. Overall, these studies indicate that increased emphasis should be placed on breeding cabbages to be resistant to T. tabaci as the foundation for its management.     

Response of omnivorous leafroller (Lepidoptera: Tortricidae) and onion thrips (Thysanoptera: Thripidae) to low-temperature storage

Eggs and first-fifth instars of omnivorous leafroller, Platynota stultana Walshingham, had a mean percentage survival to the adult stage of 60.7-95.2% for nonexposed immatures and 14.5-54.3% for immatures exposed to 1 wk at 0-1 degree C. Exposures of 2-5 wk resulted in 0-6.7% survival, and a 6-wk exposure resulted in < 1% survival of all stages tested. A significant reduction in survival of all larval stages occurred between exposures of 0 and 1 wk and between 1 wk and 2-6 wk. Survival of eggs after exposures of 0 and 1 wk was significantly different than survival after exposures of 2-6 wk. The second instar was the stage least susceptible to low-temperature storage. Adults that were exposed to low temperature for 1 wk in the third through fifth instars laid a mean of 120-289 eggs per female, and the mean percentage viability of the eggs ranged from 56.2 to 71.4%. Mean percentage survival of adults and nymphs of onion thrips, Thrips tabaci Lindeman, was inversely related to the duration of exposure from 1 through 3-6 wk at 0-1 and 5 degrees C and was lower at 0-1 (0.2-52.5%) than at 5 degrees C (17.6-66.6%). Exposure to 0-1 degree C for 4 wk attained 91.2% control, which increased to 99.8% after 6 wk. Low-temperature storage has potential to control omnivorous leafroller in table grapes, Vitis vinifera L., and onion thrips in onions, Allium cepa L.     

Onion thrips (Thysanoptera: Thripidae): a global pest of increasing concern in onion

During the past two decades, onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripidae), has become a global pest of increasing concern in commercial onion (Allium cepa L.), because of its development of resistance to insecticides, ability to transmit plant pathogens, and frequency of producing more generations at high temperatures. T. tabaci feeds directly on leaves, causing blotches and premature senescence as well as distorted and undersized bulbs. T. tabaci can cause yield loss > 50% but can be even more problematic when it transmits Iris yellow spot virus (family Bunyaviridae, genus Tospovirus, IYSV). IYSV was identified in 1981 in Brazil and has spread to many important onion-producing regions of the world, including several U.S. states. IYSV symptoms include straw-colored, dry, tan, spindle- or diamond-shaped lesions on the leaves and scapes of onion plants and can cause yield loss up to 100%. Here, we review the biology and ecology of T. tabaci and discuss current management strategies based on chemical, biological, and cultural control as well as host resistance. Future directions for research in integrated pest management are examined and discussed.     

Overwintering locations and hosts for onion thrips (Thysanoptera: Thripidae) in the onion cropping ecosystem in New York

Identifying locations where onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripidae), overwinter and subsequently disperse is important for designing control strategies. In upstate New York from 2003 through 2006, potential overwintering sites in the commercial onion, Allium cepa L., cropping system were investigated early in the spring before onion seedling emergence and again late in the season after onions were harvested. Onion thrips adults were sampled directly from the soil and indirectly from the soil by using emergence cages. Sampling locations included onion field interiors and edges and areas outside of these fields, including woods. Host material sampled included onion culls; volunteer onions, which sprout from cull onions left behind after harvest; and weeds. Onion thrips adults were found in all sections of onion fields and in locations outside of onion fields, with the fewest emerging from woods. Emergence began in early May and extended into June. Peak emergence occurred during the last half of May, at which time 50-75% of the population had emerged. Adults colonized volunteer onions as early as late March and as late as mid-November. No adults were found overwintering in onion cull piles. Adults also colonized several weed species, especially pigweed, Amaranthus hybridis L., and lambsquarters, Chenopodium album L., late in the fall. Our results indicate that onion thrips adults overwinter in the soil within and near onion fields and that they probably colonize volunteer onion plants before subsequent generations infest the onion crop in the spring. Volunteer onions and weeds also provide onion thrips with a host after onions are harvested. Consequently, onion thrips management strategies should include tactics that reduce volunteer onion and weed abundance.     

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.     

不同施钾水平对苜蓿营养物质及抗蓟马性的影响

【目的】明确施钾是否能有效提高苜蓿对蓟马的抗性以及钾-营养物质-抗虫性之间的关系。【方法】以感蓟马紫花苜蓿Medicago sativa品种甘农3号(Gannong No.3)和抗蓟马紫花苜蓿品种甘农9号(Gannong No.9)为材料,设0,6,9,12和15 g/m2(K2O)5个钾水平,在大田蓟马为害高峰期,评价和测定了不同钾水平处理下苜蓿的受害指数、虫口数量、叶片钾含量、碳氮比和游离氨基酸变化。【结果】在第2茬中,随着钾水平的升高,甘农3号和甘农9号老叶和心叶的钾含量上升,可溶性糖和碳氮比升高,游离氨基酸含量下降,虫口数量无显著性差异,受害指数显著降低(P0.05),且在9 g/m2(K2O)水平下最低。在第3茬中,甘农3号和甘农9号的受害指数、叶片钾含量、碳氮比及游离氨基酸的变化规律和第2茬相同,但虫口数量上升。第2茬和第3茬中,不同施钾水平下,苜蓿叶片钾含量与受害指数负相关但不显著(P0.05)。施钾后甘农3号的受害指数均低于未施钾甘农9号的受害指数。【结论】钾元素可通过提高苜蓿碳氮比,降低游离氨基酸的含量来提高苜蓿对蓟马的耐害性。在大田条件下,通过施钾管理来提高苜蓿品种对蓟马的耐害性是一种有效的措施。     

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.     

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.     

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.     

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.     

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.     

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.     

The occurrence and species composition of thrips (Thysanoptera) on onion

The aim of the present research work was to investigate the population density and species composition of thrips infesting crops of onion in South Poland. The flight activity of thrips was monitored using blue sticky traps and plant samples were taken to record the number of adult and Larvae of thrips on onion. In 2004 the thrips were caught into blue sticky traps from the start of June to the end of the first decade of September. The peak flight activity was noticed in the middle of July. In 2004 the significant growth of the numerousness of adult thrips on onion grown from seeds was recorded in the middle and in the third decade of August, whereas on onion grown from sets in the first decade of August. Thrips larvae were not observed during the whole vegetation season. In 2005, the thrips were caught into blue sticky traps form the half of June to the first decade of September. The peak flight activity was noticed in the first decade of August. In 2005 the rapid growth of the numerousness of adult thrips on onion grown from seeds was recorded in the third decade of July. The highest number of adult thrips on onion grown from sets was noticed at the beginning of August. Thrips larvae were observed on onion grown from seeds in the third decade of July and at the end of the second decade of August. In both years of observations the most numerous species was Frankliniella intonsa Tryb. The second most numerous species was Thrips tabaci Lindeman. In 2004, the most numerous species was predacious Aeolothrips intermedius Bagnall.     

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.     

Soil application of Beauveria bassiana JEF-350 granules to control melon thrips,thrips palmi Karny (Thysanoptera: Thripidae)

The melon thrip, Thrips palmi Karny, is a worldwide pest that causes severe damage to a wide range of host plants. Current management mainly relies on sprayable synthetic chemicals that target adult, egg, and larval stages. However, these chemicals have adverse effects on the environment and induce resistance in thrips. An alternative environmentally sound management method with a different mode of action, targeting the soil-dwelling stage of thrips, was considered in this work. Bioassay of seven Beauveria bassiana isolates under laboratory conditions revealed that two isolates, JEF-350 and JEF-341, were highly virulent against melon thrips. Conidial production and thermotolerance of JEF-341 and JEF-350 were compared with those of the commercial isolate B. bassiana ERL836 (Chongchaesak GR). B. bassiana JEF-350 showed similar conidial production to ERL836 but had significantly higher thermotolerance than JEF-341 and ERL836. Dose-dependent virulence of JEF-350 was tested in pot and field conditions, and JEF-350 millet-based fungal granules were applied to the soil surface to target the pupal stage of melon thrips. In the pot and field trials, JEF-350 granules significantly reduced thrip population with greater efficacy than JEF-341 and ERL836 granules. In application, JEF-350 needs to be carefully applied when in combination with chemical fungicides to ensure survival. Our results suggest that B. bassiana JEF-350 could be effectively used to control the pupal stage of melon thrips in soil.