The present study on the varietal resistance of okra against Earias spp were carried out at the experimental farm, Sindh Agriculture University, TandoJam, for two years (1997 and 1998). For recording pest infestation, five plants per treatment were thoroughly observed from one month after sowing and continued till harvesting. Healthy and infested fruits were noted at weekly intervals. Okra varieties tested in the present study were: Jalandri, Green polo, Parbhani karanti, Pusa sawani, Faisalabad-M-1 and Desi. The results revealed that infestation varied significantly in different varieties of okra. The okra varieties Green polo was found the least susceptible and the Desi variety was the most susceptible with the minimum and maximum percent infestation of 9.06 and 18.92, respectively.
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Okra, Abelmoschus esculentus (L.) belongs to the Malvaceae family is an important vegetable crop in many parts of the world. It is a native crop of Africa, South East Asia and North Australia to the pacific. Okra is the herbaceous annual plant of the old world tropics and widely cultivated or naturalized in the tropical and sub-tropical countries. Okra is severely attacked by the spotted bollworms, Earias spp. The severity of damage varies from place to place during different seasons. The extent of loss to okra due to infestation of Earias spp. has been reported by various workers in the range of 20 to 51%[2-4].
The spotted boll worm, Earias insulana (Boisduval) and Earias vittella (Fabricicus) (Lepidoptera: Noctuidae) are serious polyphagous insect pests on many economic crops widely distributed in North Africa, India, Pakistan and other countries of the world. The E. insulana is distributed in Europe, Asia, USSR and most of Africa, while E. vittella in Asia, Australia the pacific Islands and Seychellus in Africa.
Vegetables are mostly consumed fresh. Use of chemical insecticides for the protection of vegetable crops cause human health hazards. Therefore, alternate non-chemical methods are sought for the management of vegetable insect pests. Host plant resistance is one of the self perpetuating and cost effective methods of pest management. Varietal resistance has often been used for the management of Earias spp. on okra[7-10].
Since okra is a very common and widely consumed vegetable of Pakistan. It is grown almost throughout the year in southern Sindh due to climatic conditions favourable for cultivation of okra. Okra is heavily infested by Earias spp so much so that farmers usually apply pesticides on every alternate days in some localities where pest pressure is high on okra crop. These heavy uses of highly toxic insecticides cause human health hazards. No study on the varietal resistance of okra against Earias spp is reported in Pakistan, Therefore, present investigation was under taken to study extent of damage and varietal resistance of okra varieties available in Pakistan against Earias spp for the benefit of farmer and consumer communities.
MATERIALS AND METHODS
The present studies on varietal resistance of okra against Earias spp were carried out at Latif Experimental Farm, Sindh Agriculture University, TandoJam. The seeds of different varieties were obtained from the seed store, Hyderabad and vegetable section, Horticulture Research Institute, Mirpurkhas.
The sowing of different varieties of okra namely, Green polo, Desi/local, Parbhani karanti, Pusa sawani and Faisalabad-M1 were done on April 21, 1997 in a randomized complete block design with four replications. The plot size for each treatment was 10x20 ft. The varieties were sown in parallel lines through a hand drill. The observations on pest infestation were started in third week of June and continued till harvest of the crop in the, fourth week of July 1997. The second experiment on varietal resistance of okra was conducted in the late summer season as well. The seeds of different varieties of okra were sown on August 11, 1997 in a randomized complete block design. The observations on pest infestation were started in the fourth week of September and continued till the harvest of the crop, in the fourth week of December, 1997. During the year 1998, crop was sown on March, 11 in a Randomized Complete Block design with four replications. The observations on pest infestation were started in the fourth week of April and continued till the third week of December, 1998.
The observations were recorded by observing 5 plants selected from each treatment at random. The whole plant was observed carefully and healthy and infested fruits were recorded. The observations were taken at weekly interval in the morning hours. From each treatment, damaged fruits were picked and brought to the laboratory, Department of Entomology. All damaged fruits were opened and the larvae of Earias spp were collected and kept in plastic jars for adult emergence to ascertain the species composition. The data obtained were statistically analyzed.
RESULTS AND DISCUSSION
The results on early summer okra crop, indicated that pest infestation started in the third week of June, 1997 and continued till harvest of the crop. There was significant (P<0.05) difference of infestation in different varieties of okra. The highest Earias spp infestation of (18.77%) based on seasonal average was observed in Desi (local) variety which was followed by Faisalabad-M1 (17.37%), while the minimum infestation was observed in Green polo (10.87%) Table 1.
Infestation of Earias spp in late summer crop started in the fourth week of September and continued till fourth week of December, 1997 (Table 2). Okra variety Desi (local) was significantly the most susceptible of all other varieties in this study. It had the highest (24.95%) infestation followed by Parbhani karanti (20.96%) and FaisalabadM1 (15.8%), where as the minimum infestation (6.64%) was recorded in Green polo variety.
The results on varietal resistance of okra against Earias spp during 1998 indicated that pest remained active from the fourth week of April till the third week of December. The infestation of Earias spp on different varieties of okra was significantly (P<0.05) different from each other. On overall basis, Faisalabad-M1 was the most susceptible variety with the highest pest infestation (16.80%) followed by Parbhani karanti (16.18%), while the most resistant variety was found to be Green polo showing minimum infestation of (9.67%), Table 3.
Table 4 indicated the overall varietal resistance in okra against Earias spp based on two years data. The Green polo variety was the most resistant which showed minimum infestation (9.06%) by Earias spp, while Desi (local) variety was the most susceptible one with the highest infestation level (18.92%).
The infestation by Earias spp varied between different varieties, different seasons and years in this study. Many studies reported earlier, support the results of present study, for example, Saha and Singh reported more than 25% loss of yield okra was due to Earias spp. Similarly, Singh et al. also noted that the average length of okra fruit and seed production was reduced because of Earias spp infestation and seed staining was maximum in infested pods compared with healthy ones. Kumar and Urs studied the seasonal incidence of E. vittella on okra and found that the pest started its infestation on shoots and fruits. The infestation of fruits varied from 8.4 to 73.2% in different weeks. Singh and Brar reported that the losses in the yield due to Earias spp infestation varied from 32.06 to 40.84%. Abhishek-Shukla et al. also reported a peak infestation of 41.25% of okra fruits by E. vittella.
There are many studies reported which support the findings of present study on varietal resistance. Madav and Dumbre studied the resistance of 14 varieties of okra to E. vittella and reported that AE-76, Pusa sawani, Long green and White-velvet showed tolerance, while Indo-American hybrid and Koparwadi local were resistant to the pest[8,14,15].
|Table 1:||Percent infestation of fruits of different varieties of okra by Earias spp under field conditions during season, 1997 (±S.E)|
|Table 2:||Percent infestation of fruits of different varieties of okra by Earias spp. under field conditions during late season, 1997 (±S.E)|
|Table 3:||Percent infestation of fruits of different varieties of okra by Earias spp under field conditions during early season 1998 (±S.E)|
|Figures followed by the same letter (s) are not significantly different from each other (P<0.05)|
Kumbher et al. found okra resistance to E. vittella correlated with increased fruit hair density. While, Brar et al. conducted studies on losses in fruit yield of okra due to Earias spp as influenced by dates of sowing and varieties.
Two species of Genus Earias spp are found in Pakistan that is, Earias vittella and Earias insulana, during the year 1998, studies were undertaken to investigate the species position under field conditions. Larval collections were made from different varieties of okra and mix crops, brought to the laboratory and kept for adult emergence.
|Table 4:||Overall seasonal mean percent infestation of fruits of different varieties of okra by Earias spp. under field conditions during 1997 and 1998 (±S.E)|
|Table 5:||Species wise position of Earias vittella and Earias insulana collected from different varieties of okra grown under field conditions during 1998|
|1 = E. vittella||2 = E. insula|
It was found that no single insect of E. insulana emerged from larval collection on different okra varieties, whereas only 10 insect of E. insulana were collected from the okra (cv. Faisalabad-MI) growing in mixed crop along with cotton and other crops such as, till, mung, maize and guar (Table 5). Working on the alternate hosts in the carry over of Earias spp in Punjab, Pakistan. Arif and Attique have reported that the incidence of E. vittella was higher on cotton, okra and kenaf, while, E. insulana was more common on ornamental plants viz., hollyhock, china-rose, cotton-rose and Turks cap and weeds, abutilon and small mallow. Similar results have also been reported by Khan.