Mass Trapping for Onion Thrips

Comparison of Sampling Methods and Application of Mass Trapping for Onion Thrips, Thrips tabaci Lindeman, on Onion Crop


Results showed that significant (P < 0.05) difference was found in the density of adults between sampling methods, i.e., blue sticky trap (BST), yellow sticky trap (YST), plant sampling (PS), and field sampling (FS) in 2011/12 and 2012/13. Sampling methods revealed that T. tabaci populations migrated to onion field early in December and established in the mid-February. Blue sticky trap continually captured and detected adult thrips earlier than other sampling methods with total average of 2.18-9.21 adult/plant in onion field. Yellow sticky trap could not be recommended for monitoring onion thrips in onion field, because it has captured and attracted numbers of non-targeted insects that causes more effort in counting of thrips. Further investigations are required for application of sticky traps for estimation of ETL in relation to the density of adult on onion varieties/cultivars under field condition, which will be helpful in the management of thrips. A non-significant (P > 0.05) reduction was found in either stage or absolute estimate of thrips in both mass trapping (Vaseline petroleum jelly and motor oil) on two cultivars (OW and OK). In addition, a non-significant (P > 0.05) effect was found in the yield neither between cultivars nor traps and their interaction treatments in both years. Therefore, mass trapping could not be recommended in the management of thrips under field condition, but it can be used for monitoring thrips on onion crop.


Onion crop (Allium cepa L.) is a major vegetable crop in India. India is the second largest producer of onion next to China (FAO 2015). Onion thrips (OT), Thrips tabaci Lindeman (Thysanoptera: Thripidae), a serious and global insect pest of onion. It causes damage to onion crops either directly and/or indirectly. The reduction in onion crop yield was reported up to 90% India (Pandey et al. 2011) and >50% in USA (Boateng, 2012). It is also predominant vector of Iris Yellow Spot Virus (IYSV) that can cause yield loss up to 100% (Birithia et al. 2013).

Monitoring and detection of insect pests and their relative abundance are the foundation of an authentic IPM approach (Sarwar 2012). Population of T. tabaci in relation to abiotic and biotic factors was studied by numbers of methods; number of thrips over plant counts (MacIntyre-Allen et al. 2005b; El-Sherif and Mahmoud 2008a; uric and Hrncic 2011; Moraiet and Ansari 2014), water trap (Mustafa and Turaikhim 2001; Kadri and Goud 2005b; Ibrahim and Adesiyun 2010a; uric and Hrncic 2011), and sticky trap; almost blue color (Mustafa and Turaikhim 2001; Bergant et al. 2003; Ben-Yakir and Chen 2008),white (Macintyre-Allen et al. 2005b) or yellow color (Morsello et al. 2008).

These sticky traps of different colors, materials, and shapes have been used for sampling and monitoring, estimating populations, and controlling various species of thrips, including T. tabaci, T. palmi, and F. occidentalis under green house and field conditions (Lu 1990; Cho et al, 1995; Tsuchiya et al. 1995; Roditakis et al. 2001, Szenasi et al. 2001). Efficacy of sticky trap may depend on where traps are placed in relation to crop phenology. It can be used as decision making tool to reduce the load of insecticides as well as to intervene for control of thrips by application of insecticides and when biological and cultural practices fail (Binns and Nyrop 1992). However, few works were carried out to evaluate different sampling methods for T. tabaci in last decade resulted in good recommendations and suggestion (Liu and Chu 2004; Macintyre-Allen et al. 2005b; Natwick et al. 2007) but related to more parameters and extensive work is necessary.

Management of thrips is also required either by increasing the yield or application of IPM technology. Krauthausen et al. (2001) showed that more than 50% of the pesticides could be saved without any concession to yield or quality by employing the threshold strategy. Therefore, economic threshold level (ETL), action threshold [AT] or economic injury level [EIL] may adequately utilized as decision-making for management of thrips if density of the thrips reaches or exceeds the threshold level. Nault and Shelton (2010) stated that an AT is one of the most important decision-making tools in IPM. ATs are not usually derived from an EIL model, but ATs could be identified from research-based strategies and pest-crop relationships, but they may also be determined on the basis of experience. In addition, understanding of ATs for thrips could help farmers to minimize the use of insecticide sprays, saving money and time, and potentially mitigating the development of insecticides-resistance (Gill et al. 2015). Different levels of action threshold of T. tabaci were investigated in relation to different effect factors, e.g., cultivars, age of plant, insecticides resulted in different ATs. Variation in AT could be due to climate change, regions, cultivars, market prospects, and methodology of research, e.g., insecticides, design, type of parameters, technical of measurements, etc., (Rueda et al. 2007; Nault and Shelton 2010; Gill et al. 2015 ). It was suggested that influential factors should be considered before selecting an AT to be used in making decisions for thrips control (Gill et al. 2015). Therefore, extensive investigations are desired to comparison of sampling methods with different parameters on onion crop, the relationships between sampling methods and ETL, and application of sticky traps for control of T. tabaci have been investigated in this study. The outcomes will be helpful in the management of thrips on onion crop.

Material and methods:

Two field experiments, (A) comparison of sampling methods (B) and Application of mass trapping in the management of T. tabaci, were conducted at the Department of Plant Protection, Faculty of Agricultural Sciences, Aligarh Muslim University, Aligarh, India for two consecutive years (2011/12 and 2012/13). Two cultivars were considered in these experiments, i.e., Onion White and Onion Kessar, which sown in the nursery beds (1×3 m) on October 10, and standard agronomic practices were adopted for commercial cultivation of onion. The seedlings were transplanted in the main fields of A and B experiments on 45 (November 23) and 39 (November 17) days after sowing (DAS), respectively. Plant spacing was 30×15 cm (within rows x within plants) and accommodated four rows of 134 and 21plants/row for each plot in the A and B experiments, respectively. Size of the fields with ridges and channels were about 990 and 132 m2 in the A and B experiments, respectively. One dose of phosphate (P2O5) @45 kg/ha was applied at the time of transplanting, and three doses of nitrogen (N) @90 kg/h were given, where first dose (50%) was at the time of transplanting, second (25%) and third (25%) doses were on 35 and 45 days after transplanting (DAT), respectively for each experiment. The experiments were performed in a randomized complete block design (RCBD) with three replications. Insecticidal treatments had not been applied on onion cultivars during the seasons of experiments. However, at the time of transplanting, the seedlings were treated with thiophorate 70 WP @2 gm/L for 20-30 min against root rot.

(A) Comparison of sampling methods

Extensive investigations were during this experiment: (1) survey of insect pests and natural enemies associated with onion crop, (2) population dynamics of onion thrips in relation to abiotic and biotic factors, (3) and economic threshold level of onion thrips in relation to sticky traps. Therefore, four sampling methods were used for monitoring and estimation in field of onion-ecosystem:

I. Blue sticky trap (BST) II. Yellow sticky trap (YST): both sticky traps; blue and yellow plastic sheets (7.60×12.70 cm, locally manufactured plastic sheets) were planned to make squares (2.40×3.40 cm) at the printing press in Aligarh. They were coated with Vaseline® petroleum jelly (Anglo-Dutch Company Unilever, India) on one side only. The sheets were hung on a wooden stake with a binder clip 2-3 cm above the plant canopy and placed in centre of plots. The height of the traps was adjusted with the growth of the plants. The traps were fixed in the field one week after transplanting (WAT) and changed weekly. Routine examination of traps was done by using binocular stereo microscope. Average number of thrips (adult) and other insects including natural enemies were determined from each trap.

III. Plant sampling (PS): five plants/plot were carefully cut at base of canopies (neck of plant) from the middle lines in ‘W’ form with avoiding two plants at the end of middle lines. These plants were carefully handled to avoid disturbing the thrips on the plants, kept in polythene bags (70×50 cm) tied by rubber band, and then brought to laboratory. These bags were filled with 500 ml water with alcohol 70% (9:1) to wash off insects and sieved the water through two layers of wet tissue paper. In the way, all sieved thrips and other associated insects on a plant could be counted using binocular stereo microscope on the wet tissue paper. In addition, all leaves of the plant were gently separated and inspected for thrips and other insects. Several parameters were measured: average number and length of leaves and damage area to calculate percentage (%) of leaf damage (Equation 1), average number of thrips (adult and immature stages), average number of eggs and other insects (pests and natural enemies) per plant.

The percentage (%) of damage leaves = (Total average damage area of leaves )/(Total average area of leaves) x 100 (1)

To estimate the number of eggs/leaf (EL), three leaves from each plant/plot (total of 15 leaves/plot) were examined. Method for preservation of eggs was described by Martin and Workman (2006). The population dynamics of T. tabaci and the effect of abiotic and biotic factors on the population were ascertained through this method. Data of weather parameters (abiotic factors): temperature (maximum, minimum, and average °C), relative humidity (%RH), rainfall (mm), wind velocity (Km/hr), and dew point (%) were collected weekly during experimental periods from the Meteorological station, Department of Physics, AMU, Aligarh, India. The distance from experimental field to the Meteorological station is about 100 m. Correlations and linear regression among parameters in PS were analyzed by Statistical Package for the Social Sciences (SPSS) version 16.0.0 (SPSS 2007) in relation to abiotic and biotic factors.

IV. Field sampling (FS): sampling of thrips was done by visual count using hand lens (10-15x). Five plants were selected randomly from each of the centre and middle lines of the plot with ‘W’ form with avoiding re-selection of the same in the next observation. Average number of thrips (adult and immature stages) and other insects per plant were considered in the method.

Sampling of thrips and other insects was started 2 WAT. The survey of the insect fauna and natural enemies associated with onion crops was carried out and collected through different sampling methods (plants and sticky traps). Captured insects were carefully examined for identification. For proper identification of T. tabaci and other species of thrips, standard techniques were adopted as described by Hoddle et al. (2012) for rotation identification, while permanent slides were prepared as mentioned by Bhatti (1999) and Hoddle et al. (2012) for taxonomy. The collected specimens from sticky traps were treated with turpentine oil for dissolving the Vaseline oil. The permanent slides of T. tabaci and other thrips species were prepared and photographed using microscope (Olympus CX-21i and Nikon Eclipse Ci-L) digital camera (Olympus PEN E-PL3, LMI MCA-1000, and Nikon DS-Fi1c). Coccinellid beetles and spiders were also preserved and photographed for identification. The identification was done by taxonomists.

The sampling methods were compared based on number of adult/sticky trap (AS), adult/plant (AP) and other insects per plant or trap. These experimental data were analyzed by using two-way ANOVA. The number of insect per experimental plot was divided by the number of scouted plants to calculate average of insect per plant (means±SE), while number of scouted trap to estimate average number of insect per trap (means±SE). Tukey-HSD (P = 0.05) test was used for multi-comparison treatment and interaction means using Statistical Analysis System (SAS) program version 9.1.3 (SAS 2012). In addition, correlations and linear regression between AS and average number of TP in the PS and FS were analyzed (SPSS 2007). Economic threshold level was also studied in relation to the number of trapped adults on sticky traps through linear regression analysis (SPSS 2007).

(B) Application of mass trapping in the management of T. tabaci:

Two adhesives and one control plot (free of traps), i.e., Vaseline petroleum jelly and motor oil were coated on blue and yellow colors of plastic sheets. Detailed information regarding sticky traps was mentioned earlier. Both colors of sticky plastic traps were hung separately on a wooden stake with a binder clip 2-3 cm above the plant canopy. The height of the traps was adjusted with the growth of the plants. In each plot, both of plastic sheet traps (blue and yellow) were placed in the middle line of the plot 1 WAT, and changed weekly up to harvesting. The observations of T. tabaci population were estimated weekly on onion cultivars through field visual counting as FS, which was described earlier.

The densities of thrips were observed on plant from 2 WAT till harvesting time when 50-60% of canopy fell down 141 DAT (April 6). They were taken on three plants selected randomly from each plot’s middle lines in ‘V’ shape with avoiding re-selection of the same plant. Two plants were also avoided at the end of middle lines from both sides. Counting of thrips was made 3:00-5:00 pm with the help of a magnifying hand lens (10x to 15x) to avoid the hottest hours of the day. Yield was estimated by weighing the onion produced in one line (middle) per plot using meter balance immediately (before curing). Harvested onions were kept in the hanger, Faculty of Agricultural Sciences, AMU, for three weeks for curing, and they were weighted again (after curing). Then it was converted to t/ha before and after curing and expressed as means±SE. A Tukey-HSD test was used for comparing multiple treatments (SAS 2012).

3. Results:

A. Comparison of sampling methods:

1. Survey of the insect fauna and natural enemies associated with onion crop

Through the comparison of sampling methods, survey of the insect fauna and natural enemies associated with onion crop were carried out. the survey revealed that there were 36 insect and spider species belonging to 17 families and 10 orders associated with onion crop (Table-12). Nineteen of them were insect pests belonging to 7 families and 4 orders; however, 14 species belonging to 10 families and 6 orders were predators. Immature and adults of T. tabaci were predominant insect pest, which were found abundantly on onion crop in both years at Aligarh. Three species from Thysanoptera were also collected as adults namely: Aeolothrips intermedius (Aeolothripidae), Megalurothrips distalis (Thripidae), and Haplothrips gowdeyi (Phlaeothripidae) on leaves of onion plants. However, other species of thrips were caught by sticky traps.

Adults of aphids: Lipaphis erysimi, Myzus persicae, and Brevicoryne brassicae (Hemiptera: Aphididae) were mostly found as winged form either on plants or sticky traps. Sticky traps either yellow or blue have attracted large number of aphids during the season in both years but rarely found on plants. Leafminer, Liriomyza spp. (Diptera: Agromyzidae) and onion maggot fly, Hylemya antiqua [Delia antique] (Diptera: Anthomyiidae) were observed in larval stage feeding upon leaves and bulbs of onion, respectively, but their damage was not significant that did not adversely effect on the yield of onion. Cutworm, Agrotis ipsilon and tomato fruit borer, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) were rarely found feeding upon onion plants.

Generally, natural enemies and predators were also monitored, which belong to Coleoptera; coccinellidae: Coccinella septempunctata, C. transversalis, C. sexmaculata, and Scymnus sp.; Anthicidae: Anthicus erinitus and Anthelephila multiformis; Lathridiidae: Corticaria fulva; Araneae (spiders): Neoscona sp., Araneus spp., Gnaphosid spp., and Lycosa sp. The adults and larvae of beetles and spiders were found feeding upon adults and larvae of T. tabaci even into inner base of onion leaves. Green lacewings, Chrysoperla carnea (Chrysopidae); green bug, Nezara spp. (Pentatomidae); Indian earwig, Labidura riparia (Labiduridae); syrphid flies, 3 unknown species (Syrphidae). Generally, they were observed at adult stage except green lacewings, which were found at egg and adult stages on onion plants.

Table 1 List of the insect fauna associated with onion crop (2011/12 and 2012/13):

Order Family Genus

Insect pest Thysanoptera Thripidae Anaphothrips sudanensis Trybom

Bregmatothrips sp. Hood

Chirothrips sp. Haliday

Craspedothrips sp. zur Strassen

Frankliniella schultzei (Trybom)

Megalurothrips distalis (Karny)

Microcephalothrips abdominalis (Crawford)

Thrips palmi Karny

Thrips tabaci Lindeman

Phlaeothripidae Haplothrips gowdeyi (Franklin)

Aeolothripidae Aeolothrips intermedius Bagnall

Aeolothrips sp. Haliday

Diptera Agromyzidae Liriomyza spp. Mik

Anthomyiidae Hylemya antiqua [Delia antique] (Meigen)

Lepidoptera Noctuidae Agrotis ipsilon (Hufnagel)

Helicoverpa armigera (Hübner)

Homoptera Aphididae Brevicoryne brassicae (Linnaeus)

Lipaphis erysimi Kaltenbach

Myzus persicae (Sulzer)

Natural enemy Coleoptera Coccinellidae Coccinella septempunctata Linnaeus

C. transversalis Fabricius

Cheilomenes sexmaculata (Fabricius)

Scymnus spp. Kugelann

Anthicidae Anthicus erinitus

Anthelephila multiformis Kejval

Lathridiidae Corticaria fulva (Comolli)

Neuroptera Chrysopidae Chrysoperla carnea (Stephens)

Hemiptera Pentatomidae Nezara spp. Amyot & Serville

Dermaptera Labiduridae Labidura riparia (Pallas)

Diptera Syrphidae Syrphid flies 3 species (unknown)

Araneae Araneidae Araneus spp. Clerck

Neoscona sp. E. Simon

Gnaphosidae Gnaphosa spp. Latreille

Lycosidae Lycosa sp. Latreille

Identification Thrips tabaci:

Thrips tabaci Lindeman 1889

(Fig. 3, 74-75)

Thrips solanaceorum Widgalm in Portschinsky, 1883: 44.

Thrips tabaci Lindeman, 1889: 61.

Thrips indigenus Girault, 1929: 29. Synonymised by Mound & Houston, 1987: 9

Ramaswamiahiella kallarensis Ananthakrishnan, 1960:564-565. Synonymised by Bhatti, 1980: 157.

Diagnosis: Abdominal tergum II with 3 lateral marginal setae, Posterior margin of abdominal tergum VIII with a comb of microtrichia; upper vein of forewing with 5 distal setae. Lateral tergites II-VII with rows of microtrichia.

Description: Body color variable (pale yellow, brown, or bicolored); Legs paler than thorax; antennal segments III-IV brown with basal half pale; fore wing pale yellow to light gray or light brownish gray; body setae brown or yellowish brown. Head broader than long; with two pairs of ocellar setae; pair III, inserted inside ocellar triangle; postocular setae I-III subequal to ocellar setae III in length. Eyes without pigmented facets. Antennae 7-segmented; with forked sense cone on segments III and IV; microtrichia present on segments III-VI; segment I with 6-5 setae (none at middorasal apex); II with median dorsal seta bases of campaniform sensillum. Maxillary palp 3-segmented. Pronotum with 2 long setae at each posterior angle; with 4 pairs of setae inner to the major angulars. Setae on pronotal disc usually not especially developed. Basantra membranous, without setae; but with a somewhat elongate, triangular microtrichia-bearing patch at middle in front of ferna. Ferna undivided. Mesonotum without anterior pair of campaniform sensilla. Mesoanepimeron reticulate and with a pubescence of microtrichia; katepimeron with 2 setae. Mesothoracic sternopleural sutures present. Metanotum with longitudinally striate to reticulate sculpture at middle; median pair of setae inserted far back of anterior margin; campaniform sensilla absent. Metaepimeron with 2 setae; metaepisternum with several setae. Spinula present on mesosternum only; absent on metasternum. Tarsi 2-segmented; fore tarsus unarmed. Hind tibia without long setae on outer margin. Forewing with 5 setae on anal vein; radius (upper vein) with 7 to 10 setae and 4 or 5 distal setae; cubitus (lower vein) numerous setae. Posterior fringes undulated. Abdominal tergum II with 3 lateral marginal setae. Abdominal terga III-V with setae S3-S5 arranged in a triangle. Abdominal terga V-VIII each with lateral ctenidia; on tergum VIII these are postspiracular in position. Posterior margin of abdominal tergum VIII with complete comb of long and closely set microtrichia. Setae on intermediate abdominal terga: usually 2+1m+2+1m or sometimes 2+1m+3 to 11+1m. Female abdominal tergum IX with 1 pair of campaniform sensilla. Tergum X split longitudinally. Sternum I without microsetae. Abdominal sterna and laterotergites without accessory setae. Postmarginal flange absent on terga and sterna. Sternum II with 2 pairs of primary setae; sterna III-VII each with 3 pairs of setae Median pair of primary setae on sterna II-VI inserted at posterior margin, on VII in female usually inserted far ahead of posterior margin. Abdominal segment XI with a median epiproct (tergum) bearing one pair of setae, and a ventrolateral paraproct (sternum) on each side bearing 4 setae.

Material studied. Uttar Pradesh, Aligarh, AMU, Field of Plant Protection Department, 210 ♀, 2011/12, 210 ♀, 2012/13; all leg. Maher, taken as follows:

7-XII to 26-IV, 10 ♀ per week from onion crop, A. cepa (Amaryllidaceae)

Distribution. Worldwide.

Habitat. Polyphagous.

2. Population dynamic of T. tabaci in relation to abiotic and biotic factors:

Population dynamics:

In order to compare between different sampling methods, population dynamic of T. tabaci in relation to abiotic and biotic factors were under consideration in this study. Gradually, thrips population was gradually increased from December with fluctuation peaks till harvesting time on onion plants.

Adults of T. tabaci

A non-significant (P > 0.05) difference was determined between cultivars in number of adults in 2011/12 and 2012/13 (Table-18). Similarly, the density of thrips was identical to the results of sampling methods. Results (Table-13) showed that a significant (P < 0.05) difference was found between cultivars (7.05 and 1.86 AP on OK and OW, respectively) in 2011/12, but non-significant (P > 0.05) difference was (1.61 and 1.29 AP on OK and OW, respectively) in 2012/13.

By using the sampling methods, results showed that adult of thrips was initially caught on December 7 (2 WAT) that ranged from 0.83 to 3.00 AP or AS in 2012, while 0.04-0.50 AP or AS by most of the sampling methods at 17.37 °C with 72.79% RH on December 14 (3 WAT), 2011 (Fig. 6). However, 0.50 AP were detected late on March 8 (15 WAT), 2012 and 0.28 AP by FS method on February 15, 2013. Generally, adult density was increased gradually from the mid-December to mid-April with fluctuation peaks and then gradually declined in the mid-April as determined by different sampling methods. The highest peak (20.33 TS) detected by YST on April 19, 2012 and 61 AP by PS on April 12 (20 WAT), 2013. The results (Table-18) indicated that a significant (P < 0.05) difference was found in the density of adults among sampling methods in both years. The maximum mean of adult was 9.21 AS caught by BST in 2011/12 and 2.98 TS by YST in 2012/13. However, a minimum density (0.87 and 0.42 AP in 2011/12 and 2012/13, respectively) was observed by FS. On other hand, result of interaction revealed that adults were first detected on December 14 by BST (0.33 AS) and YST (0.67 AS) on OW compared to 0.33 AS by YST on OK in 2011 (Fig. 7). In 2012, adults were caught on December 7 (2 WAT) by sticky traps (BST and YST) on both cultivars and PS on OK only. A total density was ranged between 0.33 and 1.67 TS or AP. However, 0.67 AP were recorded on OW on March 1 (14 WAT) and 0.33 AP on OK, while 0.33 AP on OW by FS on March 15 (16 WAT) in 2012. Population of adults was not stable, but fluctuation peaks were occurred from January 4 (6 WAT) to April 19, 2012 and December 14, 2012 to April 19, 2013. The highest number of adult was 111.20 AP estimated by PS at 29.94 °C with 38.07% RH on April 12, 2012 and 22.33 AS by YST on April 19, 2013. Multi-comparison of Tukey-test (Table-18) revealed that a significantly (P < 0.05) different result was obtained in total average of adults among interaction treatments in both years. The maximum mean of adults were caught by using BST (9.21 TS) in 2011/12 compared to 3.01 TS by YST on OW in 2012/13. A minimum mean of adults (0.85 and 0.40 AP in 2011/12 and 2012/13, respectively) was caught by FS on OK. Immature of T. tabaci In the present study, there were no significant (P > 0.05) differences in immature of thrips when they observed on cultivars, PS and FS, and their interactions in both years except between cultivars in 2012/13. Result (Table-13) showed that total average of immature/plant (IP) was 13.32 IP on OK and 10.39 IP on OW in 2011/12; however, 7.57 and 5.22 IP were observed on OW and OK, respectively in 2012/13.

Immature of T. tabaci (larva, pre-pupa, and pupa) was detected early by PS (0.1 IP) at 12.03 °C with 82.50% RH on December 21 (4 WAT), 2011, while 0.6 IP at 14.55 °C with 74.07% RH on February 1 (10 WAT), 2013. Then they were observed by using PS (0.1 IP) and FS (0.17 IP) at 18.54 °C with 57.43% RH on March 1 (14 WAT), 2012, and 1.03 and 0.22 IP were counted by using PS and FS on at 18.71 °C with 70.21% RH on March 1, 2013. The highest peak was 85.30 IP counted by using PS on April 12, 2012 and 53.83 IP at 25.26 °C with 53.36% RH on March 29, 2013 (Fig. 4 & 5).Result (Table-13) showed that mean of 11.86 IP was higher on PS than FS (9.13 IP) in 2011/12; however, 6.40 IP were higher on FS than PS (5.29 IP) in 2012/13.

In interaction analysis, total of 0.2 TP was detected early on OW by using PS at on December 21, 2011, but it was 1.2 TP late at on February 1, 2013. The highest peaks were 126.60 and 83.07 TP observed by PS on OK at on April 12, 2012 and OW on March 29, 2013, respectively. Higher mean of immature was calculated by PS on OK (13.32 IP) in 2011/12 and OW (7.57 IP) in 2012/13.

Absolute estimation

Comparison of absolute estimation was limited in PS and FS, because the sticky traps could not trap or catch the insect at immature stage. The results (Table-19) of absolute estimate of thrips (TP) indicated that a significant (P < 0.05) difference was found between FS and PS in both years as well as cultivars, and their interactions in 2011/12. Infestation on OK (20.38 TP) was higher than OW (12.25 TP) in 2011/12, but it was higher on OW (8.87 TP) than OK (6.83 TP) in 2012/13 (Fig. 4 & 5). Initial infestation of 3.0 TP was observed early by using PS on December 7, 2012, but 0.10 TP on December 14, 2011. However, it was delayed when it was counted by using FS (0.67 and 0.28 TP) to March 1, 2012 and at 16.94 °C with 71.57% RH on February 15, 2013, respectively. A number of peaks was recorded in different dates of months; January, March, and April on both years. The highest peak of population was observed by PS (146.40 TP) on April 12, 2012 and 61.33 TP March 29, 2013. The highest mean of thrips was 16.32 and 7.85 TP obtained by PS in 2011/12 and 2012/13, respectively as compared to FS (10.00 and 5.15 TP in 2011/12 and 2012/13, respectively). Interaction analyses (Table-13) indicated that early infestation (6.00 and 0.20 TP) was detected on OK by PS on December 7, 2012 and December 14, 2011, respectively. The maximum peaks of population were recorded by PS on OK (240.80 TP) on April 12, 2012 and OW (94.07 TP) on March 29, 2013. The mean of infestation on OK (20.38 TP) was significantly (P <0.05) the highest when counted by PS than other interaction treatments in 2011/12, but it was non-significantly (P >0.05) higher on OW (8.87 TP) counted by FS than other interaction treatments in 2012/13 (Fig. 4 & 5).

Percentage (%) of leaf damage

By using PS only, because it is difficult to measure the damage of plant by FS without damaging plant during the investigation. Initial percentage (%) of leaf damage was 1.87 and 7.61% observed on OW at 19.58 °C with 74.00% RH on December 7 (2 WAT), 2011 and 15.90 °C with 67.86% RH on December 7, 2012, respectively and it was 10.74% on OK on December 7, 2012. Gradually, it was increased up to harvesting with fluctuation peaks on both cultivars. The damage did not occur on OW at 12.39 °C with 81.29% RH on January 11 (7 WAT), 2012; however, it was also observed at 9.77 °C with 81.71% RH on January 11 and 12.21 °C with 77.71% RH on January 25 (9 WAT) in 2013.

Peaks of damage were considerably varied in both cultivars and years. They were appeared on both cultivars in December, January, February, March, and April 2011/12. In 2012/13, they were observed in February, March, and April on OW, whereas December and April on OK. The maximum damage was 78.25% estimated on OW at 29.66 °C with 51.50% RH on April 12 (20 WAT) and OK (76.50%) in 2012 (Fig. 4 & 5). In 2013, the highest damage was 80.35% monitored on OW at 30.26 °C with 49.64% RH on April 26 (22 WAT) and. 82.01% on OK at 30.09 °C with 42.5% RH on April 19 (21 WAT). ANOVA analysis (Table-13) revealed that a non-significant (P > 0.05) difference was determined in total average %leaf damage between onion cultivars in 2011/12 and 2012/13. The highest %leaf damage was measured on OW (25.37%) in 2011/12, while 23.28% on OK in 2012/13.

Eggs of T. tabaci

Thrips tabaci lays the eggs inside the tissue of leaf by the pointed ovipositor. Therefore, eggs were sampled by using PS as it is impossible by FS method. Results revealed that a total of 0.07 egg/leaf (EL) was found on OW at 12.80 °C with 78.07% RH on December 28 (5 WAT), 2011, while 0.8 EL on OK on January 11, 2012. In 2013, 11.05 EL were observed on OW at 16.08 °C with 82.14% RH on February 8 (11 WAT), and 0.14 EL was observed on OK on February 15 (12 WAT). The highest peak (44.33 EL) was recorded on OK and 40.20 EL on OW on April 12, 2012. A similar trend of eggs (21.40 EL) was on OK and 19.22 EL on OW at 26.44 °C with 46.93% RH on April 5 (19 WAT) in 2013 (Fig. 4 & 5). A non-significant (P > 0.05) difference was found in total average number of EL among cultivars in both years (Table-13). A total number of 6.88 and 4.38 EL in 2011/12 and 2012/13, respectively recorded on OK, which was higher than OW (5.76 and 3.62 EL in 2011/12 and 2012/13, respectively).

Correlation analysis

The number of trapped adults was significantly (P < 0.05) positively correlated between BST and YST in both years. A significant (P < 0.05) correlation was determined between PS and FS in relation to estimation of adult in 2011/12 but non-significant (P > 0.05) in 2012/13. However, average number of adults caught on YST and BST were significantly (P < 0.05) positively correlated with those on PS and FS, but a non-significant (P > 0.05) between YST and PS in 2011/12 as well as both sticky traps and FS in 2012/13 (Table-20).

A non-significant (P > 0.05) correlations was determined between numbers of immature counted by PS and FS as well as on onion plants and the adults caught on BST and YST in both years (Table ).

Results (Table-21) revealed that a significantly (P < 0.05) positive relationship existed between absolute estimate (TP) on PS and FS and adult on sticky traps, but non-significantly (P > 0.05) positive between density of adults on sticky traps and absolute estimate on PS for OW in both years.

Immature stage had a non-significant (P > 0.05) positive correlation by using PS and FS on both cultivars and years, except on OK in 2011/12 (Table ).

Results of PS correlations (Table-14) showed a significantly (P < 0.05) positive relationship between %leaf damage and adult, immature, absolute estimate, and egg of thrips on both cultivars. Average number of adults was also significantly (P < 0.05) positively correlated with immature, absolute estimate, and eggs in both cultivars and years. In addition, immature stage showed a significantly (P < 0.05) positive correlation with absolute estimate and eggs in both cultivars and years except with eggs on OK, which was non-significant (P > 0.05) in 2012/13. Moreover, absolute estimate was significantly (P < 0.05) positively correlated with the number of eggs. Thrips tabaci activities Immature and adult of thrips were feeding upon the inner bases of onion leaves in early season, but later on, they moved everywhere on the leaves and also noticed on top of leaves particularly in cloudy weather. Generally, the larvae were more aggregated at the bases of leaves than adults. The damage and eggs were more concentrated at inner bases of leaves and then dispersed everywhere on leaves from bottom to top of leaves. Other insect pests A significant (P < 0.05) difference was found in total average of other thrips species either between cultivars or sampling methods as well as their interaction treatments. However, a non-significant (P > 0.05) difference was calculated among cultivars in 2012/13 (Table ). Numbers of thysanopterans were very miniscule compared to T. tabaci on onion. The highest number of thrips was caught by BST (2.75 and 4.29 in 2011/12 and 2012/13, respectively) as compare to the other sampling methods. Most of the insects were recorded from mid-March when temperature > 20 °C and relative humidity < 50% RH to the end of April, but they concentered in the beginning of April. Leafminers were infesting the leaves of onion during the experimental periods at larval stage only. Therefore, non-significant (P < 0.05) differences were found in total average of leafminer insects either between cultivars or sampling methods as well as their interaction treatments. However, significant (P > 0.05) differences were found in 2012/13 (Table ). Plant Sampling (0.01 and 0.10 larva/plant (LP) in 2011/12 and 2012/13, respectively) were higher average of leafminer larvae as compared to other sampling methods. Their occurrence was started in the mid-February (temperature ≥ 15 °C and relative humidity > 60% RH) to the end of April particularly in second season.

Huge number of winged aphids had been caught by traps more than onion thrips and other insects. However, rarely aphids were counted on plant by using PS or FS. Thus, there was significant (P < 0.05) different in average number of aphids either between cultivars, sampling methods, or their interaction in both years except between cultivars in 2011/12 (Table ). The means of 53.11 and 47.96 aphids/sticky trap were the highest number of aphids caught by YST as compared to BST and other sampling methods in 2011/12 and 2012/13, respectively. The aphids presented in field through the seasons from the date of transplanting to April 19 when temperature was over 28 °C and relative humidity was ranged 40-56% RH. The high peaks of aphids were 361.83 and 331.83 aphid/sticky trap recorded by using YST on at 19.86 °C with 52.57% and 24.29 °C with 56.43% RH on March 15 (16 WAT) in 2011/12 and 2012/13, respectively. Natural enemies A significant (P < 0.05) difference was observed in relation to beetles and spiders by using sampling methods and interaction treatments, but there was no significant (P > 0.05) difference between cultivars in both years (Table-13). Higher means of spiders were counted from PS (0.04 and 0.34 spider/plant in 2011/12 and 2012/13, respectively) as compared to other sampling methods. Spiders were recorded in February, March, and April and ranged 0.00-1.6 spider/plant or sticky trap, but they were concerted in March and April. The highest peak of spiders (0.40 and 1.60 spiders/plant) was observed by using PS on April 12, 2012 and March 15, 2013, respectively.

Coccinellid beetles were either adults or larvae found predating upon T. tabaci on onion plant with low average ranged between 0.01 and 0.05/plant. They were mostly observed in March and April. Total of 0.03 insect/plant was higher mean of PS and FS in 2011/12 and 2012/13, respectively.

Abiotic and biotic factors

Abiotic and biotic factors were studied in relation to the population parameters by using PS, because it has most of the data and are more reliable and accurate than other sampling methods.

Effect of abiotic factors

Temperature showed a sequential increase from the end of December up to the end of April. However, relative humidity had in a decreasing trend after December up to the mid-April, but variation occurred in February then increased in the end of April. Wind velocity varied during the cropping seasons up to the end of April. Dew point gradually increased with fluctuation within the cropping season. However, rainfall was scanty in winter season in both years (Table-15).

Incidence of T. tabaci: its damage, eggs, stages, and absolute estimate were carried out in relation to weather parameters on both cultivars (Table-16). Results showed that a significantly (P < 0.05) positive correlation was estimated between temperature and thrips parameters (%leaf damage, eggs, different stages, and absolute estimate of thrips) on all cultivars in 2011/12 and 2012/13. A non-significant (P > 0.05) relationship was found between immature stage and temperature on OW in 2012/13. However, average relative humidity was significantly (P < 0.05) negative with thrips parameters in both cultivars and years, but it was negatively non-significant (P > 0.05) in relation to adults and absolute estimate of thrips on OK in 2011/12. Average wind velocity showed a non-significant (P > 0.05) positive correlation with parameters of thrips population on both cultivars in 2011/12. In 2012/13, it was non-significantly (P > 0.05) positive in relation to most of thrips parameters but correlated significantly (P < 0.05) positive with %leaf damage and eggs for both cultivars as well as adult stage on OW. Average dew point was significantly (P < 0.05) positively correlated with thrips parameters on both cultivars in 2011/12 and 2012/13; however, it was non-significantly (P > 0.05) positive with adult on both cultivars and with immature and absolute estimate on OW in 2012/13. Average rainfall affected negatively non-significant (P > 0.05) in relation to adults, immature, and absolute estimate of thrips on OW in 2011/12, while non-significantly (P > 0.05) positive in relation to %leaf damage on both cultivars and adult and absolute estimate of thrips on OK. In 2012/13, it was non-significantly (P > 0.05) negatively correlated with thrips parameters on both cultivars, but positive with %leaf damage on OK.

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