The country has a good potential in cotton yield from areas varying in altitude from sea level to about 1000 m.a.s.l. (EIAR, 2012, Abebe et al., 2021). Both abiotic and biotic stresses affect cotton production. Major abiotic stresses affecting cotton production include drought, salinity and heat stress; while biotic stresses include insect pests, diseases and weeds (Maiti et al., 2020). Cotton is considered as a museum of insect pests due to the diverse insect herbivore species feeding on it. A total of 70 species of insect and mite pests are known to attack cotton in Ethiopia (Ermias et al., 2009; Bayeh and Meisso, 2013). Insect pest infestation causes substantial losses in cotton production which could reach up to 70% in the absence of pest control measures (Rehman et al., 2016).

Sap feeders have been reported to cause a loss in yield to the extent of 8.45 q/ha in hirsutum cotton (Radhika et al., 2006). A complex of thrips species may infest the seedling stage cotton. These species include tobacco thrips, Frankliniella fusca (Hinds), flower thrips, Frankliniella tritici (Fitch), onion thrips, Thrips tabaci (Lindeman), western flower thrips, Frankliniella occidentalis (Pergande), and soybean thrips, Neohydatothrips variabilis (Beach) (Stewart et al., 2013). The onion thrips and western flower thrips are more invasive and damaging than tobacco thrips throughout the world (Faircloth et al., 2001; Greenberg et al., 2009; Stuart et al., 2011; Gao et al., 2012).

The two more invasive species are also present in cotton in Ethiopia. When the species co-occur, one species tends to eventually predominate over the other. Both T. tabaci and F. occidentalis were mostly collected from weeds flowering in spring and summer when these plants were most abundant (Silva et al., 2018). The seasonal composition of thrips populations on cotton changed from predominately T. tabaci on seedling cotton to F. schultzei and F. occidentalis on mature flowering cotton later in the season (Silva et al., 2018). High T. tabaci abundance on early-season cotton was attributed to the abundance of T. tabaci on the surrounding weed species. In contrast, the abundance of F. occidentalis and F. schultzei on cotton was not connected to their abundances (Silva et al., 2018).

Thrips are found on the underside of the leaves damaging them by piercing the epidermis  of  the  tissues and sucking the sap oozing out of wounds (Sanjta and Chauhan, 2015). As a result, leaves become slivery due to the formation of white patches or streaks which finally cause scarring and distortion of leaves (Patel and Patel, 2014). The detection and estimation of damage caused by cotton thrips can be done using hyperspectral radiometry (Ranjitha et al., 2014). These insects have piercing-sucking and rasping mouthparts and feed on almost all portions of the cotton plant and stage, with the most significant injury occurring on seedlings at plant emergence to five true leaves (Terefe and Shonga, 2006; Cook et al., 2011; Reay-Jones et al., 2017). Excessive feeding injury can produce severely stunted plants, often resulting in loss of yield or at least a delay in crop maturity.

Preventive control tactics are commonly recommended to manage early-season infestations of thrips in cotton (Lopez et al., 2008; Toews et al., 2010). The most commonly applied insecticide classes in Ethiopia include Carbamate, organophosphate, organochlorine and pyrethroid, which are often applied repeatedly and indiscriminately in the Central Rift Valley of Ethiopia (Belay et al., 2017).  However, the efficacy of these insecticides used on thrips has not been assessed in cotton-producing areas of Ethiopia. Even though, most of the insecticides used were registered in onion and cotton, their comparative efficacy decreases or increases with the plant canopy nature and degree of pesticide exposure in the production. This lack of information is the cause of concern which needs to be addressed to provide accurate information to cotton producers. Hence, this research was designed to evaluate the efficacy and efficiency of the different insecticides used in the management of cotton thrips in middle Awash, Ethiopia.

The experiment was conducted at Werer Agricultural Research Center (WARC) in irrigated cotton production period in the main seasons for two consecutive years (2017-2018).  The cotton variety Deltapine 90 was used. The efficacies of 10 different insecticides were compared with the controls for the experiment (Table 1).

The eleven treatments (including one untreated check) were laid out in a Randomized Complete Block Design (RCBD) with three replications per treatment. Each total plot size had a plot size (6.3 m * 5 m) of 31.5 m². The distance between the row to row and plant to plant was 90cm and 20 cm, respectively.  Agronomic practices such as thinning and weeding were done manually as recommended.

Thrips pest assessment random sampling was done on 10 plants in each plot, and 5 leaves were checked for thrips pest; the number of nymphs and adults were counted on weekly basis.

Spraying frequency of the insecticides in the field

For  foliar  treatment,  the  insecticides  were diluted with water (200 l/Ha). Each was sprayed using a knapsack sprayer with one nozzle. Each year, the treatments were applied two times after reaching economic threshold level of 5 thrips per plant during the growth period at two weeks intervals in the field starting 60 days and 105 days after planting in 2017 and 2018, respectively. Late application of the spray in 2018 was due to late ETHL as a result of high rainfall.

Data collected

Data were collected on pre and post-spray insect count, and cotton plant population stand was counted. The number of opened and unopened bolls per plant as well as plant height measurements was taken from randomly selected plant. 10 plants were tagged from each of the treatment plots. The 65% boll opening of the experimental material of the variety DP-90 was known, so we took that time and recorded if there was a difference between treatments.  The height of ten randomly selected plants from the central five rows at maturity time (one or two days before first peaking) was measured in cm from each plot using wooden ruler from the ground level to the tip of the plants and was averaged.

For the assessment of the numbers of thrips, a random sampling was done on 10 plants in each plot, and 5 leaves were checked. The number of nymphs and adult thrips was counted on a weekly basis. The pre-assessment of thrips was done weekly starting from seedling to maturity of the cotton plants. The post spray assessment of the number of thrips was conducted on the 3^rd, 5^th, 7^th, and 10^th days after insecticide application. Cotton seed weight was measured by weighing the total harvested cotton in each plot and then converted to quintals per hectare.

Economic analysis (cost-benefit ratio)

The relative economic returns of the treatments were calculated by subtracting the cost of insecticides and their application cost from the gross return. The price of cotton was estimated based on the farm gate price in Ethiopia 21 Birr^-1kg in 2017. The application cost in the first experiment was estimated at ETB^-1 ha 100 birrs for the two-round 200 ETB. In the second season, the price of cotton was estimated based on the farm gate price in Ethiopia as 23 Birr^-1 kg in 2018 and the insecticide application cost in the first round was estimated at ETB  per ha 100 birrs for the two-round 200 ETB. The prices   of     sulfoxaflor,    deltamethrin,    ethiopyrifos,   ethiothoate, chlorfenapyr, lufenuron + profenofos, Ethiozinon, Imidaclorpride, Imidaclorpride + lambda-cyhalothrin, and Acetamiprid per liter were ETB 3400, 800, 480, 600, 3000,1000, 700, 1400, 1000  and 800, respectively. During the study period the exchange rate of the ETB to US dollar was 45 ETB=1$.  The partial budget analysis was used for comparing the impact of a technological change on-farm costs and returns.

Partial Marginal Benefit = (TB₁ - TB₀) / (Q₁ - Q₀)

When TB₀ = Initial total benefit at quantity Q₀, TB₁ = Final total benefit at quantity Q_1, Q₀ = Initial quantity and Q₁ = Final quantity.

Benefit-Cost Ratio = ∑ Present Value of Future Benefits / ∑ Present Value of Future Costs.

Data analysis

Thrips mortality analysis

The mean number of insects per plant for the overall effect of sprays was determined for each treatment. Percent mortality for the cotton thrips as well as the population was calculated using Abbott’s formula for corrected mortality (Abbott, 1925):

Where n in T = population in the treated plot after treatment; n in Co = population in control after treatment. The data collected including the efficacy derived pre and post spray mealybug count data were subjected to efficacy calculation using the formula of Fleming and Retnakaran (1985). Then ANOVA analysis was done using R software. The mean data least significant difference (LSD at 5%) level was used for treatment mean comparison.

First year (2017)

The result showed that the thrips population decreased significantly  after  the application of insecticides (DF= 10, 20; F= 1.0^ns, 4.19^**, 6.12^***, 16.56^***, 2.71^* and 8.05^***; P>0.01) at pre, three, five, seven, ten days post spray count, respectively. The efficacy result showed that the population decreased significantly (DF= 10, 20; F= 6.61^***, 12.89^***, 30.36^***, 0.64^ns, and 14.95^*** at, three, five, seven, ten days post spray efficacy, respectively (Table 2). All the insecticides were found to reduce the thrips up to ten days after application; however, treatment Closer 240% SC, Dimethoate 40% EC, Rectro 20%SC and Imidacloprid were most effective in reducing the number of thrips (Table 2). The number of thrips was lower at the early growth stage and during the boll formation stage and the population increased in the cool dry period of September and October. During the rainy hot periods, this trend was not observed as the populations were generally variable with the amount of rainfall. Among the insecticides applied in 2017 treatment Closer 240% SC, Dimethoate 40% EC, Rectro 20% SC and Confidor SL 200 resulted in fewer thrips population numbers per plant (Tables 2 and 3). The numbers of thrips were significantly higher on the untreated checks (Tables 2 and 3). Sulfoxaflor reduced thrips by 60 to 70% compared to the reduction caused by spinetoram.

The results revealed that the thrips population decreased significantly after the application of insecticides (DF= 10, 20; F= 3.33^*, 5.70^***, 21.58^***, 13.43^***, 7.48^***, and 43.4^***; P>0.01) at pre, three, five, seven, ten days post spray count, respectively. The efficacy result showed that all the treatments decreased the thrips population significantly (DF= 10, 20; F= 1.15^ns, 6.56^***, 2.36^*, 1.04^ns, and 3.41^**) at three, five, seven, ten days post spray efficacy, respectively. All the insecticides reduced thrips population up to ten days after application. However, Treatment 5 Deltahock 0.6% ULV,  Dimethoate 40% EC, Confidor SL 200 and Closer 240 SC are effective in reducing thrips with good efficacy at the mean of post spray count application (Table 3).

Second year (2018) population density of cotton thrips

The result showed that the thrips population decreased significantly after the application of insecticides (DF= 10, 20; F= 0.70^ns, 20.02^***, 16.45^***, 16.57^***, 13.55^*** and 37.26^***; P>0.01) at pre, three, five, seven, ten days post spray count, respectively. The efficacy result showed that the population decreased significantly (DF= 10, 20; F= 19.23^***, 24.36^***, 11.08^***, 11.28^***, and 31.86^***) at three, five, seven, ten days post spray efficacy, respectively (Table 4). All the insecticides  reduced the thrips up to ten days after application; however ttreatments Tutan 36 SC, Curador 55 EC, Confidor SL 200, Closer 240 SC and Pritact 10% EC were effective in reducing thrips population (Table 4). Some level of resistance or reduction in efficacy on the 10^th day was still observed with the chemical Confidor SL 200; Closer  240 SC and Dimithoate40% EC were better in reducing the thrips population as shown in Table 4.

The result also shows that the cotton thrips chemical management shows great efficacy even in field condition at three to ten days interval. Even though there is no significant difference at ten-day, the insecticides  Closer 240 SC(86.7%), Pritact 10% EC (85.6%), Dimethoate 40% EC(85%) and Confidor SL 200 (83.4%), insecticides have good efficacy (Table 4 and 5).

The result showed that the thrips population decreased significantly  after  the application of insecticides (DF= 10,  20; F= 1.99^ns, 12.26^***, 81.62^***, 74.72^***, 66.44^*** and 101.80^*; P>0.01) at pre, three, five, seven, ten days post spray count, respectively (Table 5). The efficacy results showed that the population decreased significantly (DF= 10, 20; F= 2.92^*, 30.08^***, 24.27^***, 15.54^***, and 30.08^***) at three, five, seven, ten days post spray efficacy, respectively. All the insecticides reduced the thrips population up to ten days after application. However, treatment Dimethoate 40% EC, Curador 55 EC, Confidor SL 200 and Closer 240 SC were most effective in reducing thrips population (Table 5).

Effect of insecticides on yield and yield components of cotton in 2017 and 2018

The results showed that the yield and boll number parameters significantly increased after the application of insecticides (DF= 10, 20; F= PHT (0.70 ^ns, 1.44 ^ns), stand count at emergency (1.03^ns, 1.04^ns) and at harvest (1.45^ns, 1.07^ns), boll number (0.94^ns, 1.04^ns), yield (0.85, 12.06^ns) P>0.01) during 2017 and 2018, respectively (Table 6). The yield and yield components of cotton was affected  with  the  insect  population  and   application  of insecticdes. The agronomic parameter did not affect the plant height and boll number of the cotton; however, it affected the yield of cotton greatly. The treatments showed asignificant differences (Table 6).  

Thrips population effect on cotton yield and yield component in 2017

In  the   first   year,  the  thrips  population  was  observed increasing from time to time possibly due to the re-infestations from neighboring onion fields. Over the crop growth period in the first year, the thrips population in the control plots, where only water was sprayed during each spraying time (37, 51, 67, and 86 days after planting), was relatively higher than the rest of the treatments.

The yield and yield component result of each insecticide or the sum gain of a product for a farmer can be approved with their cost benefit ratio. The yield and yield components  of cotton affected the insect population and the application of insecticdes. The agronomic parameter did not affect the plant height and boll number of the cotton; however, it affected the yield of cotton greatly and the treatments showed asignificant differences (Table 7). 

Ten (10) insecticides products were assessed for the control of thrips during flowering and boll setting periods, when they reached ETHL. The products, Imdacloropride 20 % SC, Rectro 20% SC, and Curador 45% EC had 1.33, 1.24, and 1.24 birr return economic advantage through their good efficacy. The result confirmed the application of more effective insecticides when the thrips population was high resulting in better control and higher economic returns. Therefore, the sequential applications of insecticides Closer 240 % SC, Dimethoate 40% EC, Rectro 20%SC, and Imidacloprid were recommended to manage the cotton thrips in the middle awash areas of Ethiopia. Treated plots had the lowest thrips population across all assessment dates (Figure 1). On the third spraying time (67 and 110 days after transplanting), Closer 240 % SC, Dimethoate 40% EC, Rectro 20%SC, and Imidacloprid had a significantly lower populations. In line with our findings, preliminary data from Queensland indicated that there are very few effective insecticides that will control thrips. Repeat applications will undoubtedly increase any residue levels within the crops and increase resistance on the thrips such as F. occidentalis. Marquini et al. (2002) found that imidacloprid sprays to the foliage gave up to 8 days control of T. tabaci. The literature has shown various thrips species to be susceptible to a wide range of insecticides (Marquini et al., 2002; Thoeming et al., 2006; Mo 2007; Nderitu et al., 2007); it is just a matter of getting the chemicals to where the thrips are hiding.

This is clearly a waste of time and money. Jianhua (2004) showed that Dimethoate was effective for adult thrips management. Thoeming et al. (2006) also investigated the systemic effects of neem against western flower thrips larvae on primary bean leaves and observed maximum corrected mortality of 50.6%. Pesticides neonicotinoid (Imidacloprid) interfere with nicotine acetylcholine receptors in the nervous system of insects (Yamamoto, 1996). The results agreed with Aslam et al. (2004), who discovered Confidor profoundly compelling against thrips. Further, the management of T. tabaci was also evaluated through agronomic practices in onion field by Khaliq et al. (2016), and Faircloth et al. (2002) also reported that cotton seedlings were more susceptible to thrips attack and observed the effect of insecticide treatment and environmental factors on thrips population, plant growth and yield of cotton. Besides, the eco-friendly management practices were necessary to keep pest population below economic damages by assuring safe mode to beneficial reported by (Khaliq et al., 2014).

The efficacy of the insecticide chemicals listed above and below resulted in good cost-benefit ratio for Cotton thrips management. Rectro20% SC (2.55), Closer240 % SC (1.93), Dimithioate40% EC (1.49) showed the insecticides  have   good   comparative   advantage.  Still, each birr investment to the protection of Cotton thrips using the insecticide Imdacloropride twntysc, Chlorfenapyr, and  Curador  45%  EC  would  return 1.33, 1.24, and 1.24 birr. The management of T. tabaci was also evaluated through agronomic practices in onion field by Khaliq  et  al. (2016). Faircloth et al. (2002) also stated  that cotton seedlings were more susceptible to thrips attack and observed the effect of insecticide treatment and environmental factors on thrips population, plant growth and yield of cotton. Besides, the eco-friendly management practices were necessary to keep pest population below economic damages by assuring safe mode that is beneficial (Khaliq et al., 2014). Sahito et al. (2017) observed the same kind of the research studied on comparative efficacy of novel pesticides against sucking complex as jassid on cotton crop under field conditions and found significant results (p<0.05). More work needs to be undertaken to look at what damage if any the different species of thrips do to bean pods. Growers are known to spray their crops when they find thrips in the flowers and if they know that F. occidentalis is one of these thrips then there is a clear need to apply a suitable insecticide, which is generally a spinosad spray.

The results of the study on field efficacy showed that Rectro20% sc (2.55), Closer 240% SC (1.93), Dimithioate40% EC (1.49) showed good partial economic return advantage against Cotton thrips management. Closer 240 SC at 0.36 g a.i.L^-1 was highly effective in controlling thrips in pomegranate, did not show any phytotoxic effects, relatively safer to natural enemies and realizing higher yield. Hence, it may be recommended for the management of thrips in pomegranate. This saves the farmer's yield and its value appreciably. The combined two years' data showed a significant yield increase in treatments, insecticides and botanicals, particularly the tree tobacco. Thus, it provides better and wide control options, locally available, ecologically sound and cost-effective solutions. The present study confirmed that the application of Imdacloropride, 20%SC, Dimethoate 40% EC, Closer 240% EC and Rectro 20% EC resulted in good yield and economic returns with the lowest population density of thrips populations. Therefore, to alleviate the loss of yield due to thrips population these insecticides are recommended to farmers for the management of this pest in middle Awash. Future studies are needed to monitor the level of insecticide resistance and test botanical pesticides that are ecologically sound and cost-effective solutions.

The two years of research result indicates positive findings which reflect significant differences between the parameters that are of interest to the producers. The population dynamics of the pest in the controlled plots and overall pest population for first and second spray mean was higher. Thrips are consistent and predictable insect pests of seedling cotton with the dry spiel in the Awash valley during June and sometimes in September. Although a complex of species infests seedling cotton in the region, onion thrips are the predominant species requiring management in the crop. Control strategies  rely heavily on chemical control used at planting. Cultural control strategies, such as the use of cover crops, reduced tillage operations, delayed planting date, targeted irrigation, and starter fertilizer, can complement chemical control likely help slow the population of thrips development. Producers and managers of cotton in Awash Valley should consider using a multi-tactic approach that uses several best management practices in an overall IPM approach for managing thrips. But insecticide management issues with insecticide resistance will still emerge, so it is better to consider candidates of insecticides that reduce resistance.

The author does not have any conflict of interests.