CROP PROTECTION (1985) 5 (2), 105-108

An es t imate of the loss in cotton yield in Pakistan attributable to the jassid Amrasca devastans Dist.

ZAHOOR AHMAD *, M. R. ATTIQUE * * AND ABDUL RASHID

Entomology Section, Cotton Research Institute, PO Box 572, Old Shujabad Road, Multarb Pakistan

ABSTRACT. Different levels of jassid infestation were allowed in field cage and open field studies to identify the economic threshold o f jassid in Pakistan and the most effective time to introduce control measures. In field cages even 0 .5 jassid/leaf caused significant yield losses when numbers were maintained at the target level for the period of 40 days. One jassid/leaf is the critical threshold for practical control purposes in open fields and the cost-benefit ratio was maximum (18" 67) at this level o f infestation. A season-long infestation o f jassids caused significant crop loss so correct timing of an insecticide application to reduce the damage is important.

Introduction

The jassid, Amrasca devastans Dist., an injurious pest of cotton in Pakistan, is active throughout the summer with the peak infestation occurring in July and August. Heavy infestations during vegetative growth cause leaf shedding and, later, loss of flower buds and bolls. The quality of fibre is also reduced when attack is severe during boll formation (Afzal, Rajaraman and Abbas, 1943).

Insecticides have been shown to be effective against jassid infestations (Kumar et al., 1966; Moiz and Naqvi, 1971) while Sidhu and Dhawan (1978) reported a 22.53% increase in yield with the application of two sprays against A. devastans at the appearance of second-grade injury symptoms (yellowing and curling of leaves along their margins).

Hassan (1970) reviewed the literature pertaining to jassid control in the Sudan, where cotton yields are inversely related to the level of jassid infestation. A yield loss of over 40% was reported in field cage experiments when the infestation was held at a level of 1"2 jassid per leaf throughout the season compared with 0.5 jassid per leaf. Other workers have shown the importance of correct timing of insecticide sprays for efficient control of the jassid in India (Singh et al., 1982).

This paper identifies an economic threshold of jassid infestation in Pakistan, when an insecticide should be applied.

Materials and methods

Field cage studies

Different levels of jassid infestation were maintained from 10 July to 20 August, 35 days after crop emergence in field cages in the Punjab during three cotton seasons, from 1977 to 1979. Each field cage covered five cotton plants (1" 5 m2); the experiments were carried out on varieties of Gossypium hirsutum: 149F in 1977 and B557 in 1978 and 1979. During each season, 15 field cages were used, giving three replicates of five different levels of jassid infestation (Table 1). On every second day during the course of the experiments, the total number of jassids and leaves was recorded and the required number of additional jassid nymphs needed to bring numbers up to the target level was introduced to the cage. These additional nymphs were collected from early sown cotton.

Open field studies

During 1980, 15 plots (105 m 2) of cotton variety B557 were sown so that sprays could be applied if the number of jassids exceeded a particular level with three replicates of each treatment. The plots were examined every second day and the number of jassids recorded. Leaves on each of 10 randomly selected plants within a plot were sampled examining leaves from the top, middle and bottom. Each treatment was

* Project Coordinator, FAO/UNDP, PO Box 650, Rangoon, Burma. * * Correspondence to Dr Attique.

0261-2194/86/02/0105-04 $03.00 © 1986 Butterworth & Co (Publishers) Ltd

106 Jassids on cotton in Pakistan

TABLE 1. Yield of seed cotton and percentage decrease at different levels of jassid attack

Field cage Open field studies studies (1980)

Average yield of seed Decrease in yield relative cotton (g/plant) to control (o70) Decrease in

Yield of yield Treatments seed cotton relative to (no. of jassids/leaf) 1977 1978 1979 1977 1978 1979 (kg/ha) control (%)

0"0 (Control) 91a 50a 63a -- -- -- 2487a -- 0.5 72a 38b -- 21 24 -- -- -- 1.0 74a 37b 39b 19 26 38 2341a 5.87 1.5 69a 38b -- 24 24 -- -- -- 2.0 63a 28b 37b 31 44 41 2091b 15.92 3.0 -- -- 36b -- -- 42 1788c 28"10 4.0 -- -- 32b -- -- 49 1566c 37.03

SE ± 10.29 3.15 7.702 -- -- -- 101.36 --

Numbers followed by the same letter in any column are not significantly different at P<0"05 after analysis of variance.

sprayed with dimethoate 40 e.c. at 494 ga.i./ha, diluted in water, 160-2001/ha, according to the crop height, i f the jassid populat ion increased to above the desired level. Spray application was made with a lever- operated knapsack sprayer, which was fitted with a plastic hollow-cone spray nozzle, operated at a pressure of 276 kPa.

Later, when the jassid observations were completed, each plot was blanket-sprayed with triazophos 40 e.c. at 988g a.i./ha at 10-day intervals from the first week of September to the second week o f October as a protect ion against subsequent bol lworm and other insect attack.

Seed cot ton from the plants in both cages and open field trials was weighed at the end of each season and used to compare the effects o f different levels o f jassid infestation. Yield saved was calculated from data obtained where there was an infestation level o f four jassids/leaf and the loss was calculated f rom plots kept free f rom jassids.

T h e value o f yield lost or saved was determined at the wholesale Government-guaranteed price o f rupees 200/40kg seed cotton. T h e cost o f pest control included the cost o f pesticide (rupees 148/spray/ha) and labour charges (rupees49 per spray/ha). T h e cos t - benefit ratio was calculated as the value o f net saving by pest control divided by the cost o f pest control.

Results and discuss ion

Field cage studies

In field cages (Table 1) the highest yield was obtained from the plants in cages kept free f rom jassid attack in each of the years 1977 to 1979. In 1977 this was not significantly greater than yields f rom the infested cages, but in that year yields were generally much

higher than in subsequent years, i.e. a loss o f 17-28 g o f seed cotton/plant was less significant in terms o f a highest yield o f 91 g than similar losses in 1978 and 1979 when peak yields were 50g and 63g. F rom the farmer 's point o f view, losses in all three years caused by jassid attack would be significant. In terms o f establishing an economic threshold the most interest- ing feature is that 'significant' losses occurred at all tested levels o f jassid attack, and losses were not substantially greater at the higher levels o f infestation.

Open field studies

In the field, jassid populations rose steadily through- out the season, so insecticides were applied when numbers rose to 1" 2, 2" 1 and 3" 04 in the plots con- taining one, two or three jassids per leaf, respec-

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Date

FIGURE 1. Jassid population in field studies. - - One jassid per leat~ -- -- -- two jassids per leaf~ . . . . . three jassids per leaf~ 0--0--0--o four jassids per leal~ spraying dates.

ZAHOOR AHMAD, M. R. ATTIQUE AND ABDUL RASHID

TABLE 2. Yield loss caused by A. devastans to cotton at different population densities in open field trial

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Value of net saving

Yield saved Value of by pest Treatments Yield loss by pest control yield loss control Benefit:cost (no. of jassids/lea0 (kg/ha) (kg/ha) (rupees/ha*) (rupees/ha) ratio

0 -- 921 -- 4014 6-79 1 146 775 730 3678 18"67 2 396 525 1980 2428 12"32 3 699 222 3495 913 4-60 4 921 -- 4605 -- --

* US$1. 00 -- rupees 15.

tively (Figure 1). These sprays were applied succes- sively on 29 July, 13 August and 19 August. No spray was applied to plots infested with four jassids per leaf as numbers did not rise above this target level before observations ceased. Three sprays were applied to the plots containing no jassids, from the second week of July to the third week of August. The highest yield of seed cotton was recorded in the plots with no jassid infestation, but unlike the field cage results the yield from the plots with one jassid per leaf was only slightly less than that from the plots with no jassids (Table 1). Much lower, and statistically different, yields were obtained from the plots with two or more jassids per leaf, and percentage decreases from the highest yields were similar to those in the field cage trials.

General

After the dimethoate spray had been applied to open field plots with one, two and three jassids per leaf, the population did not develop beyond 0.39, 0-32 and 0.01 jassids/leaf. The level of one jassid per leaf was, therefore, obtained briefly, unlike the field cage trials where numbers were maintained at the target level for the period of 40 days. These results indicate the importance of correct timing of insecticide applica- tions. In the open field plots with one jassid per leaf, jassid levels were higher than 0.5 jassid/leaf until the spray date and the field cage results suggested that even 0.5 jassid/leafwould cause significant yield losses with a longer exposure period.

The effect of different jassid levels on yields and, in consequence, on the financial return to the farmer (Table 2) show that the best cost-benefit ratio was achieved at a level of one jassid per leaf. To keep plots free from jassids required two more sprays, so the higher yield necessitated more expenditure.

Evidently the timely intervention of insecticides at the end of July 1980 was the reason for the minimal reduction in yield. The importance of crop loss caused by secondary pests in the plots with no jassids can be minimized because of the treatment with insecticides

TABLE 3. Levels of whitefly infestation recorded during the open field studies

Jassid plot target Whitefly population (no./lea0 (no./tea0

0 0.12-0"37 1 0.37-1"6 2 0.37-2"1 3 0-37-2"5 4 0-37-6"65

so that the top yield can be considered comparable with that in the field cages. However, losses in the plots which were not sprayed until later, in August, may be partly attributable to other pests, for example whiteflies (Table 3). Nevertheless, jassid attack alone can cause severe reductions in the cotton yield and this confirms the observations of Jackson, Schultz and Faulkner (1965) in Sudan who found that severe jassid infestations caused a yield reduction of more than one- third compared with cotton kept free from insect attack. Indeed, in years of severe jassid and thrip infestation, Jackson (1969) found that up to 56% of the potential yield can be lost.

Conclusions

A season-long infestation of jassids causes significant crop loss, but timely intervention with insecticides can reduce the damage. One to four jassids per leaf are very common in the Multan District of Punjab but the level of infestation varies greatly from year to year. The critical threshold, for practical control purposes, is one jassid per leaf. Careful monitoring of the cotton crop in the Punjab has been recommended so that an insecti- cide may be applied before jassids build up to cause significant damage to the cotton crop. Earlier indica- tions are that plant protection at a level of one jassid per leaf is successful, but likely to change with variety, sowing date, developmental stages of the crop and other such factors (Afzal and Ali, 1983).

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References

AFZAL, M. AND ALI, M. (1983). Insect pests of cotton. In: Cotton Plant in Pakistan, pp. 431-476. Lahore: Ismail Aiwan-i- Science. 449 pp.

AFZAL, M., RAJARAMAN, S. AND ABBAS, M. (1943). Studies on the cotton jassid (Amrasca (Empoasca) devastans Dist.) in the Punjab. III. Effect ofjassid infestation on the development and fibre properties of the cotton plant. Indian Journal of Agri- cultural Science 13, 191-203.

HASSAN, H. M. (1970). Progress in chemical control of pests of cotton in the Gezira. In: Cotton Growth in the Gezira Environ- ment, pp. 232-246 (ed. by M. A. Siddig and L. C. Hughes). Sudan: Agricultural Research Corporation.

JACKSON, J. E. (1969). Cotton crop environment, growth and yield in the Sudan Gezira. Cotton Growing Review 46, 315-316.

JACKSON, J. E., SCHULTZ, L. R. AND FAULKNER, R. G. (1965). Effect of jassid attack on cotton yield and quality in the Sudan Gezira. Empire Cotton Growing Review 42, 295-229.

Jassids on cotton in Pakistan

KUMAR, K., KISHAN, J., RAI, B. K. AND LAL, R. (1966). Duration of the effectiveness of insecticides on cotton in the field for controlling Amrasca devastans (Dist.). Indian Journal of Agri- cultural Science 36(5), 273-277.

Molz, S. AND NAQVI, K. M. (f971). Insecticidal trial against cotton jassid (Amrasca devastans Dist.). Agriculture Pakistan 22(1), 71-76.

SIDHU~ A. S. AND DHAWAN, A. K. (1978). Testing of various spray schedules against cotton pests. Indian Journal of Entomology 40(3), 324-327.

SINGH, J. I., GATORIA, G. S., SIDHU, A. S. AND BINDRA, O. S. (1982). Studies on the supervised control of cotton jassid (Amrasca biguttula biguttula Ishida) in the Punjab. Ento- mologist 7(3), 275-279.

Accepted 15 May 1985