Abstract: An experiment was conducted to find out the most economical number of sprays for the control of insect pest complex of cotton (Gossypium hirsutum L.). Different applications of curacron 500 EC and polytrin-C 440 EC were applied on variety gomal-93. The boll number and seed cotton yield increased, while the boll weight decreased with increase in number of sprays. Three and four spray applications gave lowest yield (1449 and 1453 kg ha-1 respectively), while five and six sprays were at par with maximum yield (2554 and 2581 kg ha-1, respectively). Although seven and eight sprays gave maximum yield (2605 and 2677 kg ha-1, respectively), but keeping in view of the cost involved and safety of the ecosystem, five sprays based on pest's economic threshold level are most economical that gave reasonable yield (2683 kg ha-1).
Introduction
Cotton is the most important cash crop of Pakistan and contributes 60 percent of export earning to the national exchequer. In addition to the lint, cotton seed for oil and meal account for 80 percent of the national oil seed production (Anonymous, 2000). Cotton with, its green, succulent leaves, many large open flowers, nectars on every leaf and flower and abundance of fruit attract a variety of insect pests and mites, such as sucking pest complex (whitefly, jassids, thrips and mites) and bollworm pest complex (Spottedd, Pink and American).
In Pakistan about 145 species of cotton pests are recorded (Bo, 1992). Pests cause low damage to G. arboreum L. but maximum to G. hirsutum L (Myint and Pierrard, 1992). These insects cause considerable damage to the crop from seedling stage to the harvesting stage. Insect pests are responsible for reflecting heavy losses to the cotton crop by reducing yield and quality of seed cotton and are the basis cause of worry and financial loss to the grower (Wilson et al., 1980). It is estimated that the losses on an average range from 30 to 40 % and could be as high as 50 to 60 % in some areas (Ahmed, 1980).
More insecticides, both in kind and quantity are used to control the insect pests of cotton, largely due to the fact that no single chemical can control all the pest species. Number of sprays has a positive influence on the physiomorphic characters as well as on the yield of seed cotton. However, insecticide applications to control insect pest complex have been increased upto alarming stage and ranges between 6 to 8 sprays. This indiscriminate use of spray has created many problems as development of insect resistance to pesticides, resurgence to target pests, secondary pest out - break, killing of non-target organisms, disturbance of biological equilibrium, environmental pollution and health hazards. Insecticide application is a very drastic control measure and should only be used as last resort, when other method are not successful to control the insect pests before economic threshold level (Korejo et al., 2000). Pesticides are to be used judiciously in combination with proper spray technology. Economic threshold levels have been recommended to reduce pesticide loads (Bakhetia et al., 1996).
The present study was undertaken to know that how many number of sprays actually needed for the control of insect pest complex of cotton.
Materials and Methods
Cotton cultivar gomal-93 was sown in a randomized complete block design with three replications and eight treatments including check at Agricultural Research Institute, D.I.Khan during 2001. The treatment size was 3 x 9 m2 and the rows and plant spacing were maintained at 30 and 75 cm2, respectively. All other cultural practices were given uniformly to the whole field.
During the study, field area was visited every day to know the pest's economic threshold level (ETL). For sucking pests, data were recorded by counting numbers of various insects through use of magnifying lens on 9 randomly selected leaves from 9 plants, in such a way the one leaf from upper portion of one plant, 2nd leaf from middle portion of 2nd plant and 3rd leaf from lower portion of 3rd plant and so on.
For bollworm pests, data were recorded from 3 randomly selected plants per treatment, by counting total number of bolls, infested bolls, squares/flowers and infested squares/flowers to worked out % infestation.
First spray of curacron 500 EC at its recommended dose to all the treatments was given when the insect pests reached to its economic threshold level (ETL). After 15 days interval 2nd spray of curacron 500 EC at its recommended dose was given to all treatments, except T1. For the next 6 sprays polytrin-C 440EC was used at its recommended dose.
All other sprays except T1 and T2 were adjusted by dividing the period from the date of 2nd spray to the end of October by the spray numbers. T2 received its 3rd spray at ETL for bollworms. T1 received every spray at pest's reaching to ETL through out the entire growing period, to know that how many sprays, based on ETL, actually needed for the control of insect pest complex. The detail of the treatments are given in Table 1A and B.
The data were subjected to ANOVA technique and LSD test by using MSTATC, a computer software package (Bricker, 1991).
| Table 1: | Details of the treatments given to cotton cultivar gomal-93 (A) T1= Number of sprays based on pests economic threshold level (ETL) |
Results and Discussion
Results (Table 2) indicate that all the plots with different number of sprays were significant regarding number of bolls/plant, boll weight and seed cotton yield. Maximum number of bolls/plant (34.33) was recorded from plots sprayed 8 times, while minimum (9.33) from check plots. Number of bolls/plant increased with increase in number of sprays and shows directly proportional trend. Maximum boll weight (3.55) was recorded from plots sprayed 3 times, while minimum (2.94) from check plots. The boll weights were almost decreased with increase in number of sprays.
| Table 2: | Effect of number of sprays on the physiomorphic features and yield of cotton |
| Means followed by different letters differ significantly at p<0.05 | |
It is a negative influence, as the boll weight increases, the number of bolls/plant and yield decreases. Maximum seed cotton yield (2677 kg ha-1) was recorded from plots sprayed 8 times, while lowest (991.7 kg ha-1) from check plots. Seed cotton yield decreased with reduction in number of sprays and shows directly proportional trend. The results are in conformity with those of Gupta et al. (1999) and Kalroo et al. (2001) as they reported yield increase with increase in number of sprays. The results are not in conformity with Korejo et al. (2000) as they reported maximum yield with 3 sprays.
Data (Table 3) revealed that maximum pesticide consumption and cost (Rs. 7641.98) was counted for plots with eight sprays. Consumption and cost of pesticides increased with increase in number of sprays and shows directly proportional trend. These findings are in agreement with the findings of Gupta et al. (1999) and Korejo et al. (2000). The maximum net benefit (Rs. 27352.16) over control was obtained with 5 sprays (based on pests economic threshold levels), while lowest (Rs. 4503.00) with 4 sprays (Table 4). The results are not in conformity with Gupta et al. (1999) as they obtained maximum net return with 9 sprays, while Korejo et al. (2000) obtained highest net return over control with 2 sprays. Economic analysis showed that present cotton production is not economical. The number of sprays can be reduced from 8 to 5 or less without significantly affecting seed cotton yield and quality. This statement is in conformity with that of Ahmad (1986) who reported reduction of sprays upto 4 from 10-14. The recommendations are contradictory to that of Shafi (1993), who recommended 2-3 sprays at an interval of 12-15 days based on ETL. The recommendations are also not in conformity with Amer et al. (1999) who recommended 6 sprays of polytrin-C 440 EC at 14 days interval. The variations may be due to cultivars used under different climatic conditions. Five sprays at an interval of 13-15 days based on damage threshold which are cost effective, affordable for farmers and safer for the environment are recommended.
| Table 3: | Consumption and cost of pesticides on cotton cultivar gomal-93 |
| Table 4: | Cost benefit ratio of cotton cultivar gomal-93 during 2001 |