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Field evaluation of water plant extracts on sucking insect pests and their associated predators in transgenic Bt cotton
Egyptian Journal of Biological Pest Controlvolume 29, Article number: 39 (2019)
Several plant species and their compounds are well-known to have some pesticidal properties against a wide range of insect pests. Potential of two water plant extracts, Azadirachta indica A. Juss and Melia azedarach L., in comparison with the synthetic pyrethroid bifenthrin against sucking insect pests and their associated predators, as well on the yield in Bt cotton, was evaluated. The population of the sucking insect pests was found lowest in the positive control (only bifenthrin application). The A. indica extract reduced the pest population equivalent to the positive control, but the highest populations were observed in both negative controls (only water and 0.1% soap with water application). The M. azedarach did not show any harmful effect on the insect population. The predators’ abundance was higher in the plots where botanicals were applied, while the lowest population was observed in the bifenthrin treatment. In comparison to the negative control (only water application), (63.4%) cotton yield was increased by the application of A. indica and (58.8%) by the application of the synthetic insecticide. Using plant extracts of A. indica to control sucking insect pests of cotton can be as effective as synthetic insecticides in terms of crop yield beside they are safer for natural enemies in the field.
The genetically modified crops have been cultivated and adopted by the farmers worldwide including Pakistan. The cotton containing Bacillus thuringiensis genes known as Bt cotton is being cultivated in more than half of the cotton-growing countries (Ali et al. 2010). In Pakistan, it was introduced in 2005 to mitigate the resistant strains of some lepidopterous insect pests (Sabir et al. 2011) with minimizing the insecticidal applications. Reducing the synthetic insecticides use against lepidopterans led to the increasing populations of sucking insect pests (Naranjo 2011). In Pakistan, the economic sucking insect pest complex in cotton include the whitefly, Bemisia tabaci Gennadius; the aphid, Aphis gossypii Glover; the jassid, Amrasca devastans Distant; the thrips, Thrips tabaci Lindeman; the red cotton bug, Dysdercus koenigii Walk; and the dusky cotton bug, Oxycarenus hyalinipennis Costa (Majeed et al. 2016).
The cotton growers mostly rely on synthetic insecticides to manage the sucking insect pests. The insecticide bifenthrin (Talstar) is a synthetic pyrethroid having contact and stomach toxicity (Mukherjee et al. 2010). Previously, it has been reported that this insecticide is very effective against different insect pests in cotton, vegetables, fruits, and in public health for the control of mosquitoes (Reddy and Rao 2002; Gupta et al. 2009). However, the presence of residue in fruits, vegetables, and green leaves above the maximum limit is a major concern about human health (Codex, Alimentarius Commission MRL of Pesticides 2009). Alternatively, insecticidal materials extracted from the plants are more readily biodegradable, may retard the resistance development in insects, and are safer for humans and the environment (Isman 2006). Among these, neem Azadirachta indica A. Juss (Family Meliaceae) has been emerged and served as a potent biopesticide in the last decade in the subcontinent (Ahmed et al. 2001) and still considered a highly effective compound against soft-bodied insects (Chaudhary et al. 2017). Azadirachtin, the major active ingredient of neem (Alzohairy 2016), responsible for insecticidal activity has gotten much attention in recent years (Chaudhary et al. 2017). Pest control using different plant extracts may help in preventing the resistance development in insects due to the presence of various bioactive compounds. These pesticidal plants have low persistence in the environment and are cheaper mainly for small-scale farmers (Isman 2008). Similarly, M. azedarach, having the active compounds triterpenoids, is well known to inhibit the feeding of herbivorous insects (Abdelghany et al. 2012).
Contrarily, natural enemies have immense importance in managing the population of insect pests in global agriculture (Chidawanyika et al. 2012). Unfortunately, the population of natural enemies is being affected negatively through excessive use of synthetic insecticides, but use of the botanical biopesticides have showed less effect (Potts et al. 2016).
The objective of this study was evaluating the effectiveness of two plant extracts in comparison to a synthetic chemical insecticide against sucking insect pests, their predators, and the yield in Bt cotton field.
Materials and methods
A transgenic Bt cotton (Bt FH-113) field, planted on 15 May 2018 under semi-arid climatic conditions in the farmland (29° 35′ 25.7″ N 71° 10′ 35.0″ E), located at Jalalpur city of southern Punjab, Pakistan, was selected for the study. All the standard cultural practices were followed as recommended for the growing of cotton. The experiment was conducted under a randomized complete block design (RCBD), and the treatments were replicated in 3 blocks.
Collection and processing of plant extracts
The seeds of neem, Azadirachta indica A. Juss, and the white cedar, Melia azedarach L. (Family: Meliaceae), were collected from different locations around the study area. The seeds were washed and sun-dried for 10 days. The seeds were crushed and powdered by using an electrical blender. To make 10% w/v extracts, a total of 1 kg of powder was mixed in 10-L water and kept at 25 ± 5 °C for 48 h. Further, 0.1% soap as a detergent was also added to the solution (Belmain et al. 2012). The extracts were stored in a 10-L buckets and kept in the shade. Before application, the extracts were filtered twice through a fine cloth to remove the extra plant materials for the safety of sprayer.
The two botanicals and the synthetic pyrethroid pesticide bifenthrin (Talstar® 10EC, Jaffer Group of Companies, Pakistan) were tested in this study. The bifenthrin was applied as per the manufacturers’ instructions (250 ml/acre). The positive control was bifenthrin insecticide (control—B), and 2 negative controls were made: water + 0.1% soap (control—ws) and water only (control—w). The treatments applications were performed when the population of pests reached at the economic threshold level (ETL). The ETL for sucking insect pests is 5 adults or nymphs/leaf for whitefly, a single adult or nymph/leaf for jassid, and 10 adults or nymph/leaf for aphid (Asif et al. 2016). The application was carried out, using a 15-L knapsack sprayer. The sprayer was cleaned after application of each treatment.
Sampling for the presence of sucking insect pests and their associated predators
From each block, 3 inner rows were selected for sampling. To record the data of a number of insect pests and predators, total 5 plants from the selected rows were visually examined after 3, 7, and 10 days of treatment applications. The main target insect pests were A. gossypii, B. tabaci, and A. devastans. The target predators were the lady beetles (adults) (Coleoptera: Coccinellidae) and green lacewing, Chrysoperla carnea (Stephens) (Neuroptera: Chrysopidae). In the case of insect pests and C. carnea, both immature and adult populations were considered, while only adults were recorded in case of coccinellids.
Data of the population of insect pests and predators, their abundance, and cotton yield were assessed, using analysis of variance (ANOVA) to check the significance among treatments. Means were separated by Tukey’s post hoc Honestly Significant Difference (HSD) test at the 95% confidence interval. All the analyses were performed through Minitab 17.0 software.
Results and discussion
The results showed that the botanicals and bifenthrin insecticide had significant effect on the population of whitefly (F = 206.6), jassid (F = 30.2), and aphid (F = 15.06) at (P < 0.05). Only bifenthrin had greater effects, and low populations of all insect pests—whitefly (2.48 individuals/plant), jassid (1.45 individuals/plant), and aphid (1.87 individuals/ plant)—were recorded. The A. indica extract also reduced the pests’ population, showing similar effects as the positive control of the chemical pesticide. The highest population of all insect pests was observed in both negative controls (only water and 0.1% soap and water application). The M. azedarach was not efficient in suppressing the insect population (Fig. 1).
The population of all three insect pests was found lower by the application of synthetic insecticide; however, the results were comparable in case of A. indica treatment. Overall, there was a little difference in the abundance of pest population in A. indica and bifenthrin treatment. Bifenthrin was more effective against sucking insect pests of cotton in the present study but had a negative effect on the predators. The treatments had significant (F = 2.65, P < 0.05 for coccinellids, and F = 2.47, P < 0.05 for C. carnea) effect on the predator’s populations. Interestingly, the population of coccinellids was found higher (1.07 individuals/plant) in the plot where A. indica was applied, followed by M. azedarach (0.93 individuals/plant). The population of coccinellids was lower (0.53 individuals/plant) and had insignificant difference (P > 0.05) in positive and negative controls. C. carnea was found higher (1.13 individuals/plant) in the plots, where M. azedarach was applied, followed by A. indica (0.87 individuals/plant) and negative controls (0.87 individuals/plant). In case of bifenthrin, the population of C. carnea was very low (0.40 individuals/plant) (Fig. 2). Overall, pest abundance was low in bifenthrin (1.93 individuals/plant) and A. indica treatment (2.46 individuals/plant). Pest abundance was higher in both negative controls and M. azedarach treatment. The predators’ abundance was higher in plots, where botanicals were applied, while the lowest abundance was observed in positive control treatment where bifenthrin insecticide was applied (Table 1).
Previously, many researchers have reported the effectiveness of neem plant extracts against different insect pests (Kunbhar et al. 2018). Furthermore, A. indica has various excellent attributes including systemic action in plants, rapid degradation in the environment, and minimal effects on biological control agents (Mordue and Nisbet 2000). A. indica was acutely safe for the predators because the predator abundance was higher in both botanicals treatment than in the chemical pesticide. M. azedarach did not show significant toxicity against insect pests. Obtained results are in accordance with Kraiss and Cullen (2008) who reported that azadirachtin was much safer for the adults of ladybird beetle Harmonia axyridis. Patel et al. (2009) reported a higher activity of coccinellid beetles in mustard crop, using neem oil-based formulation. Similarly, Smitha and Giraddi (2006) stated that botanical pesticides are quite safer to the predatory coccinellids. The results have also been supported by Ma et al. (2000) who tested some biorational pesticides and synthetic insecticides against Helicoverpa armigera and H. punctigera (Lepidoptera: Noctuidae) and reported bifenthrin and azadirachtin were effective against insect pests and safer for coccinellids and C. carnea. In contrast, chemicals were found highly toxic to both predators. Similarly, Qi et al. (2001) reported that azadirachtin at 50–200 ppm concentration did not affect the predation rates of coccinellids beetle. Previously, researchers have studied the effect of botanical insecticides on non-target natural enemies under laboratory and field conditions (Chaudhary et al. 2017; Kunbhar et al. 2018). The harmful effect of azadirachtin compound on the immature stage of predators may be expected because it acts as a growth regulator and suppresses the ecdysteroid hormone that regulates the process of molting and development of immature insects (Gontijo et al. 2015).
The cotton yield was higher (2800.0 kg/ha) in case of A. indica treatment and (2058.5 kg/ha) with the use of M. azedarach. The yield was also comparable with the positive control (2487.6 kg/ha). In both negative control treatments, the yield of cotton was low (1024.6–1111.5 kg/ha) (Fig. 3). In comparison to the negative control (only water application), 63.4% yield was increased by the application of A. indica, 50.2% by M. azedarach application, and 58.8% by the application of synthetic insecticide (Table 1). Despite the lower number of pests’ populations in bifenthrin treatment, the yield of cotton was comparable in the pesticidal plant treatment, A. indica. It could be due to the reduction of pests through their associated predators. Two species of coccinellids were recorded in the cotton field: Coccinella septempunctata L. and Menochilus sexmaculatus Fab. However, the population of C. septempunctata was higher compared to the second one. The visual presence of C. septempunctata matches with other studies which documented the role of this coccinellid species as major predator responsible for the reduction of cotton insect pests (Arshad et al. 2017). The other possible reason of higher yield could be due to the ability of plants to tolerate the damage and to compensate physiologically to maintain overall yield (Brown 2005). Usually, the compensation requires the good health of plants with sufficient nutrients and water (Tardieu and Tuberosa 2010).
The use of plant extracts has no greater level of risk to human health as does the use of synthetic insecticides. The neem seed extract offered the cost-effective plant protection alternatives to pyrethroid bifenthrin. Smallholder farmers in Pakistan have accessibility to this plant material and have the labor availability as well. Generally, the botanicals are considered safer to users, animals, and the environment because of their non-persistent nature (Buss and Park-Brown 2002).
The present study showed that the water extract of the tested plant seeds A. indica locally available could be a less expensive and favorable alternative to mitigate the sucking insect pests and to reduce the negative impacts of insecticides. Further studies should be conducted at a larger scale and on the efficacy against different insect pests at different climatic conditions.
Availability of data and materials
Data will not be shared.
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The authors are thankful to the Department of Pest Warning and Quality Control of Pesticide, Multan, for providing facilities for the experiment.
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