Varietal resistance of Bt cotton against sucking complex under field conditions at upper Sindh-Pakistan

The research work was carried out during Kharif season, 2018 to investigate the varietal resistance of six Bt cotton varieties i.e., FH-Lalazar, SS-32, Bt-CRIS-508, CRIS-510, FH-142 and CRIS-533 against sucking pests, jassid, whitefly, thrips and mealybug under field conditions. The maximum resistivity against jassids was observed in SS-32 (1.98) per leaf followed by Bt-CRIS-508 (2.01), CRIS-533 (2.10), CRIS-510 (2.11), FH-142 (4.02) and FH-Lalazar (4.06).Against whitefly SS-32 gave maximum resistance (5.99) per leaf followed by CRIS-533 (6.00), CRIS-510 (8.55), Bt-CRIS-508 (8.93), FH-142 (9.12) and FH-Lalazar (11.04). Against thripsSS-32 variety showed maximum resistance (8.57) per leaf followed by CRIS-510 (8.81), CRIS-533 (8.87), Bt-CRIS-508 (9.50), FH-142 (10.27) and FH-Lalazar (15.31), while against mealybug Bt-CRIS-508 found resistive (0.42) followed byCRIS-510 (0.43), FH142 (1.49), SS-32 (1.88), CRIS-533 (1.98) and FH-Lalazar (2.02).The overall results showed that cotton variety SS32 gave better resistance against overall sucking pests (4.60) per leaf followed by CRIS-533 (4.74), and CRIS-510 (4.97), while varieties Bt-CRIS-508 and FH-142 showed moderate resistance against sucking pests (5.21) and (6.23), respectively however; variety FH-Lalazar had not shown enough resistance (8.11) against all the four mentioned sucking pests, so cotton variety FH-Lalazar was considered as most susceptible Bt variety and SS-32 variety was found to be most resistive variety against all the four sucking pests except mealybug. So it is suggested that the variety SS-32 should be cultivated as it is less attacked by sucking pests.


Introduction
Cotton (Gossypium hirsutum L.) is a very important crop containing fiber and maintaining cash. In 2017 the growing area of cotton crop in Pakistan was 2.489million hectares and its production was 10.671million bales [1]. It is commercial, historical, industrial and cultural crop, its silver fiber attracts world's interest. In Pakistan it earns a substantial foreign exchange of 68% by exporting raw cotton and 7.1%in agriculture value addition and about 1.6% to GDP [2]. It is attacked by various sucking and chewing insect pests but major sucking complex among them are jassid, Amrasca biguttula biguttula (Ishida) whitefly, Bemisia tabaci (Gennadius), thrips, Thrips tabaci (Linderman), mealybug, Phenacoccus solenopsis (Tinsley) and aphid, Aphis gossypii (Glover) which are cause of the most damage to the cotton crop. Cotton jassid is the notorious sucking insect pest of cotton [3,4]. This pest mainly found on lower side of plant leaves, sucks sap from lower side portion of the leaves and injects the poisonous substance into the plant tissues. So, plant leaves become wrinkled and this is the symptom of jassid infestation [5]. Cotton whitefly also lives underside of cotton leaves, sucks the cell sap, causes 50% reduction of the production of the bolls [6], also it is vector of famous disease of cotton crop namely cotton leaf curl virus (CLCuV) [7]. There are about 600 host plants of whitefly, which is menacing to agriculturebased countries [8]. Thrips are tiny insect pests which live lower side of cotton leaves and gave harm to plant by sucking sap of the cell, due to that, the leaves of the cotton plant become cupshaped and of silvery appearance [3].
Mealybug is a small soft-bodied pest, which found mostly on the stem and leaves of the plant and sucks the cell sap [9]. Its adult female is covered over by waxy powder due to this; its common name is mealybug [10]. Its hosts are different like harvests, vegetables, decorative plants and weeds [11], it gave 14% loss to cotton yield, recently it become serious pest of cotton [12]. These sucking pests are playing major role in reducing cotton production throughout the Pakistan [13,14]. Due to massive usage of pesticides, these pests got high resistance and the environment becomes polluted along with different health risks [15,16]. From 1996 world-renowned that Bt cotton is the definitive solution to overcome the resistivity problems against chewing pests. An enormous resistivity has been adopted by chewing pests such as Helicoverpa armigera, Pectinophora gossypiella, Earias vittella and Earias insulana against Bt cotton under field and laboratory conditions [17]. Therefore, without using insecticides, varietal resistance tactic is of huge importance [18,19,20] for developing the pest management strategy, it is main requirement to understand the effect of different morphological characters of plants for the host selection behavior. In Pakistan, the breeders have evolved a number of varieties by focusing their concentration to enhance the yield capacity. Plant pests and natural enemies can be positively or negatively affected by many plant characters [21,22]. In Pakistan the preceding efforts were taken to investigate the tactics to increase host plant resistance to insect pests in cotton; like those of [15, 20, 23, 24], etc, but still a lot remains yet to be completed to reach some specific outcomes. Keeping in view; the importance of resistant varieties the present studies were conducted to screen out 6 cotton genotypes. The natural resistances against these sucking complexes were also determined. Thus, the main object was to observe the population fluctuation for most resistant and susceptible Bt cotton varieties against sucking insect pests.

Materials and methods
The experimental trial was conducted during Kharif season, 2018 to investigate the resistivity of Bt cotton varieties against sucking complex i-e., jassid, whitefly, thrips and mealybug under field conditions. For this purpose the seeds of six different Bt cotton varieties i.e., T1= FH-Lalazar, T2=SS-32, T3= Bt-CRIS-508, T4=CRIS-510, T5= FH-142 and T6= CRIS-533 were sown on ridges in North to South direction in a Randomized Complete Block Design (RCBD) having a treatment size of 100 sq feet, which were replicated 4 times and kept without using of insecticides. The space of 2 feet was maintained between treatments and replications by separating each plot, while 18'' distance was maintained between the ridge to ridge and 9'' to 12'' distance between plant to plant. Thinning, weeding and other agronomical practices were manually done. Before sowing the cotton crop, the pre-weedicide Stomp (FMC Pvt. Ltd. Pakistan) was applied for controlling the weeds. When the sucking insect pests had made their initial appearance sporadically, the crop was checked for the presence of the sucking insect pests on a weekly basis. Twenty plants of each cotton variety were chosen, randomly per treatment/plot for recording pest population. Three leaves, one each from the bottom, middle and top portions of cotton plants were kept under observation for insect pest population. The information was obtained by the average counts of the pest per leaf. The varietal resistance was observed during the season of the most insect pest activity.

Statistical analysis
The information on the individual population of each sucking complex species was displayed through a different examination of the mean values. The methods were isolated by observing the Least Significant Difference (LSD) test at (P<0.05) through Statistics (8.1) computer software, student package, USA. The relationship framework between the overall and individual population dynamics of the sucking complex on various cotton cultivated varieties was analyzed.

Results
As the infestation of the pest is to be reciprocal of the insect pest susceptibility, so with an increase in pest population per leaf, the relative resistance of the variety was considered to decrease as reported by Aslam et al. [25].

Resistance of Bt cotton varieties to jassid
The population of jassid started from the month of July when plant bore 5-6 leaves after one month of cultivation of the cotton crop under field conditions which remained active up to the end of September or harvesting of the crop. All the six cotton varieties were cultivated on the same date and at the same location which was infested by the sucking complex vigorously. From which cotton jassid pest was observed with the overall maximum mean population (4.06) per leaf on FH-Lalazar under field conditions (Table 1). This variety had maximum number of jassids followed byFH-142 (4.02), CRIS-510 (2.11), CRIS-533 (2.10) and Bt-CRIS-508 (2.01) per leaf throughout the cotton crop season but the least number of jassids were found on SS-32 (1.98) per leaf that found comparatively resistant when compared to the other tested varieties. Further, the ANOVA showed a significant difference in two groups (DF= 11, 5; F= 75.93; P= 0.001) against jassid population, one of them included FH-Lalazar and FH-142, while the second group included CRIS-533, CRIS-510, Bt-CRIS-508, and SS-32, respectively.

Resistance of Bt cotton varieties to whitefly
The population of whitefly on all the six Bt varieties of cotton was observed and It was found thatSS-32 variety was relatively more resistant to whiteflies among the tested varieties, as it is shown in (Table 2) per leaf least number of whiteflies (5.99) followed by CRIS-533 (6.00) whiteflies. The FH-Lalazar was found to be the most vulnerable as it showed maximum whiteflies per leaf (11.04), followed by FH-142, Bt-CRIS-508 and CRIS-510 having (9.12), (8.93) and (8.55) whiteflies respectively. Overall results showed that FH-Lalazar was significantly different (DF= 11, 5; F= 13.24; P= 0.001) from all other varieties containing the maximum number of whiteflies while Bt-CRIS-508, CRIS-510, FH-142 were statistically similar and CRIS-533, SS-32 were also similar to each other for whiteflies per leaf.

Resistance of Bt cotton varieties to thrips
The population of thrips was also observed on all the six tested varieties of cotton, the results revealed that SS-32 showed maximum resistance to thrips comparatively to other tested varieties (Table 3). It showed least number of thrips (8.57) per leaf followed by CRIS-510 (8.81), CRIS-533 (8.87). Similarly; the maximum number of thrips was found on FH-Lalazar (15.31) per leaf followed by FH-142 (10.27) and Bt-CRIS-508 (9.50). All the varieties were significantly different (DF= 11, 5; F= 76.97; P= 0.001) from one another except CRIS-510 and CRIS-533 which were nonsignificant to each other.

Resistance of Bt cotton varieties to mealybug
The population of mealybug appeared late in the 2 nd week of August on all the six tested varieties of the cotton crop as shown in (

Resistance of Bt cotton varieties to overall sucking pests
The resistance level of tested Bt cotton varieties against different sucking complex is shown in (Fig. 1). It was observed that cotton variety SS-32 showed better resistance to overall sucking pests (4.60) mean population per leaf followed by CRIS-533 (4.74) and CRIS-510 (4.97), while varieties Bt-CRIS-508 and FH-142 showed moderate resistance to all sucking pests (5.21) and (6.23) mean population per leaf respectively; however, variety FH-Lalazar had not shown enough resistance (8.11) against all the four mentioned sucking pests, so it was considered as most susceptible among the tested transgenic varieties. Further, the ANOVA showed that against sucking pests all the varieties were significantly different (DF= 11, 5; F= 65.74; P= 0.001) from one to another except SS-32 and CRIS-533 which were non-significant to each other. Means followed by common letters in the respective category are not significantly different from each other by LSD at α = 0.05.

Figure 1. Overall mean population of sucking pests on different Bt varieties of cotton crop under field conditions Weekly population fluctuation of sucking complex
The results of the population fluctuation of all the four sucking complex on various transgenic cotton varieties are shown in (Fig. 2) which illustrates that infestation of the jassid remained above economic threshold level (ETL) almost throughout experimental period except 1 st week of data collection and its peak population was found during 3 rd week of August which is (3.94) per leaf. While the attack of whitefly remained below ETL in starting; during the first three weeks of July then gradually increased and crossed the ETL during the month of August and its population even more increased in September reached at peak (13.46) per leaf during 2 nd week of September. Likewise mean population of thrips on various Bt cotton varieties remained below ETL in July then its population crossed the ETL Further, the ANOVA showed the overall significant difference (DF= 5, 11; F= 128.68; P= 0.001) against sucking pests among all the twelve weeks (throughout the season) from one to another except 9 th and 10 th week, which were non-significant to each other.

Figure 2. Weekly infestation ratio of sucking complex on Bt cotton varieties throughout the season Discussion
In current research trial, six Bt cotton varieties were cultivated which were infested by the sucking complex vigorously, from which cotton jassid pest was observed with the overall per leaf maximum mean population (4.06) on FH-Lalazar under field conditions followed by FH-142 (4.02), CRIS-510 (2.11), CRIS-533 (2.10) and Bt-CRIS-508 (2.01) per leaf throughout the cotton crop season but the least number of jassids were found on SS-32 (