Prevalence and Impact of Sucking Insect Pests on Cotton Crop

Cotton is a staple textile with versatile usage and its fiber has excellent thermal conductivity and anti-static properties (Majumdar et al., 2010). The Cotton Development Board of Bangladesh has released some varieties of Gossypium hirsutum recently, which are cultivated in Bangladesh. The yield of cotton in Bangladesh is low (2.4 Mt/ha) due to climatic conditions, unavailability of suitable variety and insect pest infestation (Azad et al., 2011).
       
The sucking insects, aphid Aphis gossypii Glover (Hemiptera: Aphididae) and jassid Amrasca devastans Distant (Hemiptera: Cicadellidae) prevail in the cotton fields of Bangladesh throughout the season and cause severe damage (Amin et al., 2009; Azad et al., 2011). The nymph and adult of the sucking insects suck sap from cotton leaves, squares, flowers and developing bolls and secrete honeydew, which facilitates the development of sooty mold fungus. These insects act as a vector of viruses and toxin injected by jassid into the cotton plants cause hopper burn symptoms. Severely infested plants produce cripple bolls and low-grade lint and create a plucking and ginning problem (Hossain et al., 2012).
       
Inherent crop characteristics such as hair and trichomes, cuticle thickness and phytochemicals, such as reducing sugars, proteins, proline and presence of toxic allelo-chemicals influence the foraging, feeding, oviposition, growth and development of the herbivore insects and affect their abundance (Zalucki et al., 2002; Amin et al., 2020b). Singh and Agarwal (1988) observed that cotton genotypes containing lower amounts of reducing sugars, proteins and free amino acids exhibited resistance to leafhoppers.
       
To maximize the quality and quantity of cotton yield, it is crucial to overcome pest incidence and keep healthy ecosystem conditions. Cultivation of resistant varieties is the prime consideration of ecological pest management and this purpose is increasing daily (Al-Chaabi et al., 2000). With this viewpoint, this study was done to determine the incidence and impact of sucking insects, specifically aphid and jassid, on five cotton varieties, namely, CB12, CB13, CB14, CB15 and HC1.

Cotton plants
 
The cotton varieties CB12, CB13, CB14, CB15 and HC1 were sown on August 2, 2018 in the experimental field of Bangabandhu Sheikh Mujibur Rahman Agricultural University, Gazipur, Bangladesh (25°25' N and 89°5' E). Seeds of the cotton varieties were sown in 3.0 m × 3.0 m plots, replicated five times and arranged in a randomized complete block design. The spacing between blocks and plots was 1.5 m and 1.0 m, respectively. Each plot contained five rows and each row had five plants. Fertilizing and intercultural operations were done on a necessity basis and the plants were grown without insecticide spray to allow insect infestation.
 
Sucking pests
 
Data of the aphid and jassid’s abundances were collected at the cotton plant boll formation stage. Ten plants for each variety were randomly selected to collect data and the numbers of nymphs and adult aphids on the apical leaf and the numbers of nymphs and adult jassids on the fifth leaf from the top of the plants were recorded. Observations were done using a magnifying glass (FD 75, Ballon Brand, China).
 
Numerical and phytochemical characteristics of cotton plants
 
At the plants’ boll maturation stage, ten plants of each variety were randomly selected to collect the data of the number of branches, sympodial branches, leaves and bolls plant^-1 and trichomes leaf^-1. The fifth leaf from the top of a branch of the selected plants was collected to count the trichome density. The number of trichome per leaf was observed under a stereo microscope (BOE3200, BOECO, Germany). Plant heights (length from top to base) were measured using an mm-scale tape measure. Ten bolls were randomly collected for each variety and the boll diameters were measured using a slide caliper and the weights of the bolls were weighed using a digital balance (CANRY, China). The bolls were exposed and the numbers of locules per boll were counted.
       
Reducing and total sugar content of the leaf was estimated following Burtrand’s method (Chandraju et al., 2016). Then, non-reducing sugar content was calculated by subtracting reducing sugar content from total sugar content. Micro Kjeldahl method was used to estimate the nitrogen content of leaves and results obtained were multiplied by 6.25 to get the protein percentage (Maehre et al., 2018). A digital pH (Lutron PH-207) meter was used to measure the pH levels in the leaves.
 
Yield and quality attributes
 
After bursting the bolls, the bolls were plucked manually and kept separately in polythene bags and then sun-dried. The cotton seeds were separated after proper drying and the weights were measured using a digital balance. The seed cotton yield of each plot was converted into t/ha.
       
Yield-quality characters such as ginning out-turn (GOT%), seed index, lint index and micronaire value were estimated with ten replications of each variety. The bolls were ginned with a single roller electric gin in the Regional Cotton Research and Extension Center laboratory, Gazipur, Bangladesh. Boll sample data included: percentage GOT (% GOT: percentage of lint obtained from 100 g of seed cotton), seed index (weight of 100 seeds), lint index (amount of fiber retained on 100 seeds) and micronaire (fiber fineness and maturity). Micronaire values were tested using a cotton micronaire testing machine (Zhenjiang KDL Machinery Co. Ltd., Jiangsu, China).
 
Data analysis
 
One-way analysis of variance (ANOVA) followed by Tukey HSD posthoc test was used to analyze data for abundance of aphid and jassid and morphological traits, yield and quality characteristics. Correlation matrix following principal component analysis was performed to observe the relationship between the abundance of insects with the morphological characteristics and biochemical contents of the cotton plants. All the analyses were performed using IBM SPSS v.21.0 software (IBM SPSS statistics v.21.0, Georgia, USA).

Variations in insect pest abundance
 
The mean abundance of aphid and jassid on the cotton varieties ranged from 4.6±0.9 to 31.3±6.2 leaf^-1 and 2.4±0.7 to 5.6±1.1 leaf^-1, respectively and the results differed significantly (Fig 1). The varieties HC1 and CB14 revealed significantly the highest and the lowest number of aphid populations, respectively. Jassid population was the highest on CB15 and the lowest on HC1 cotton variety. The results are in close conformity with Amjad et al., (2009) who found significant variations of the abundance of aphid and jassid on different cotton varieties.
 

 
Plant responses to pests
 
Differences in plant characteristics such as leaf bronzing, leaf curl, leaf dropping, boll browning, hopper burn symptom and plant dwarfism due to aphid and jassid incidence are shown in Table 1. Results showed that CB15 had extreme leaf and boll bronzing, severe leaf curling and dwarf growth and high leaf dropping with excessive hopper burn symptom. CB12, CB13 and HC1 showed moderate to high response and CB14 depicted minute response to the above symptoms.
 

 
Numerical plant characteristics
 
The cotton varieties showed significant variations in plant agronomic characteristics such as the plant height, branch, leaf and sympodia and boll plant^-1, locules boll^-1, boll diameter and weight (Table 2). Results showed that plant height, number of branch, sympodial branch and boll plant^-1 and boll weight were the highest in CB14 with a mean value of 144.4±2.0 cm, 17.2±0.5, 10.1±0.5 and 33.8±0.9 and 14.4±0.8 g, respectively. Statistically, CB12 had the smallest height (118.6±1.8 cm) and CB13 had the lowest number of branches (14±0.8 plant^-1). HC1 and CB15 had the highest and the lowest number of leaves (122.8±2.8 and 59.9±1.4 plant^-1, respectively). Statistically, HC1 had the lowest number of locules boll^-1, boll diameter, boll weight, number of boll and sympodial branch plant-1 (3.5±0.2, 2.81±0.1 cm, 9.4±0.3 g, 20.2±0.8 and 5.7±0.5, respectively). Amin et al., (2017) reported significant variations among different varieties of cotton.
 

       
Aphid and jassid’s populations had a significant negative correlation with ventral trichomes on CB12, CB15 and HC1 varieties and dorsal trichomes on CB14 and HC1 varieties, respectively (Table 3). Leaves plant^-1 had a significant positive correlation with the aphid population on CB12 and CB14 varieties and with jassid population on CB12 and CB15 varieties. The abundance of aphid showed a significant positive correlation with the abundance of jassid on CB12, CB14 and CB15 varieties. The result conformed with that of Amin et al., (2017) who reported the number of trichomes on the midrib of the cotton varieties showed significant negative correlations with the populations of aphid and jassid.
 

 
Phytochemical characteristics
 
The phytochemicals of leaves among the varieties varied significantly (Table 4). The moisture content was the highest and lowest in CB12 and HC1 with a mean value of 64.12±1.0% and 56.29±0.6%, respectively. Statistically, the highest amount of reducing sugar, total sugar content and protein was recorded for CB15 (2.50±0.05, 2.78±0.11 and 29.5±0.3, respectively). Statistically the lowest amount of reducing, total sugar and protein was found for CB14 variety (0.08±0.04, 0.11±0.03 and 25.4±0.2, respectively). pH was the maximum in CB13 with a mean value of 6.34±0.1. The lowest pH was recorded in HC1 (5.68±0.0). According to Singh and Agarwal (1988), reducing sugar ranged from 0.04 to 2.55 and moisture content ranged from 71.5% to 74.33% in different cotton varieties. This finding reflects Wei et al., (2010) results who noted reductions in soluble sugar content in various host plants due to infestations of the sucking insect. Aphids take nutrient from host plants and reduce soluble sugar, amino acid and protein contents in the plants (Douglas 2003; Zhou et al., 2013). 
 

       
Aphid and jassid abundance had a significant negative correlation with protein content on CB12 and CB13 varieties, respectively (Table 5). Both insects had a significant negative correlation with pH on the CB15 variety and jassid had a significant negative correlation with pH on the CB12 variety. Aphid and jassid populations had a significant positive correlation on CB12, CB14 and CB15 varieties.
 

 
Yield and quality attributes
 
There were significant differences in seed cotton yield, GOT%, seed index, lint index and micronaire among the varieties (Table 6). Among the tested varieties, HC1 had the lowest yield and quality characteristics. CB14 had statistically the highest yield and quality characteristics. Sukhija et al., (1987) found a yield loss of 9.74 q/ha due to the infestation of sucking insect pests in cotton. Amin et al., (2016) observed significant differences in yield and quality attributes among the cotton varieties.
 

The findings of the study showed that the tested cotton varieties had variable phytochemical and morphological characteristics which influenced the abundance of aphid and jassid. Among the varieties, CB14 revealed statistically the highest yield and quality performances than other varieties in the state of aphid and jassid infestation.

Authors are grateful to the Ministry of Science and Technology, Government of the People’s Republic of Bangladesh for funding the study.