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Abstract The present study was carried out at the experimental farm of Faculty of Agriculture, Moshtohor, Zagazig University using com variety (Treble cross 310) which considered one of the major cereal crops. Field experiments were carried out during two successive maize seasons 1996 and 1997 to determine the effect of three planting dates (April, May and June), three nitrogen fertilizer levels (70, 100 and 130 N units/fed.) and two insecticides (Lannate 900/0 and Malathion 57%) on rate of infestation by the three corn borers (Sesamia cretica Led., Chilo agamemnon Bles. and Ostrinia nubilalis Hbn.), rate of infestation by the com leaf aphid Rhopalosiphum maidis, number of aphid and its aphidivorous insects, yield of com grains and number of hibernating com borers larvae inside the plants before and after harvesting. I. Effect of planting dates: 1. Data revealed that infestation of maize plants with corn borers was higher in 1996 than 1997 in the three sowing dates. The three planting dates may be arranged in descending order according to their rates of infestation for the two seasons as follows, April plantation received highest rates of infestation (74.2 and 50.6%) followed by the third (11.4 and 12%) and finally the May plantation which received the lowest infestation with corn borers (4.8 and 3.1%) for the two seasons, respectively. So to avoid the heavy infestation with com borers, maize plant must be sown during May. The analysis of varience showed highly significant differences between first and both and third plantations in 1997 season, while in 1996 season there was high significant differences between the three plantations regarding corn borers infestation. 2. The rate of infestation with corn leaf aphid and number of its main predators were studied under the effect of the three sowing dates. The seven predators were Chrysoperla carnea, Paederus aifierii, Scymnus spp, Coccinella septempunctata, C. undecimpunctata, Cydonia vicina var. nilotica and Cydonia vicina var isis. Generally, the infestation with aphids was higher in 1996 than 1997 maize season. The plants of first plantation (sown in April) were not infested with aphids until harvesting time while the plants of the second sowing date (sown in May) received the highest number of aphids than the third plantation in 1996. There were high rates of infestation on the first half of August which reached 1000/0. There was high significant differences between the three sowing dates under study in the two seasons regarding the number of aphids and subsequently the rate of infestation with aphids. The highest number of predators recorded ten plants throughout the two seasons 1996 and 1997, respectively were 15 & 18 individuals for the first sowing date, 115 and 9 individuals for the second sowing date while the highest abundance were 27 and 15 individuals on the third sowing date. But generally, it may be concluded that the overall means of predators were higher in 1996 (8.8, ~2.8 and 13.7 predators/plant) than 1997 (9.3,4.0 and 5.1) on first, second and third plantation, respectively. This is may due to the infestation of maize plant with com borers and com leaf aphid was higher in 1996 than 1997. As for the effect of planting dates on yield of com ears, results obtained in the present experiment showed that the difference in yield of ears per feddan between them was significant in both seasons. In 1997 season, May and June plantation gave the best yield (19.5 and n03 ardab/feddan, respectively), while the lowest yield was obtained from April planting (14.0 ardab/feddan) which harbourd more com borers than the other two plantations. So, the differences in yield/feddan between early and late plantations may be attributed to the levels of infestation by com borers and also to the complex of the prevailing environmental factors. II. Effect of nitrogen fertilizer levels: Ammonium nitrate was used at three levels (130, 100, 70 N units of fertilizer/feddan). Results indicated that there was significant relation between the degree of maize cultivars infestation by com borers and nitrogen fertilizer application at 70 units level, while insignificant at the two other levels and control in 1996 season, while in 1997 the three levels of nitrogen fertilization showed insiginficant effects on maize plant infestation with com borers. As for infestation with com leaf aphids and their predators data showed that infestation rates with aphids were markedly affected by adding nitrogen fertilizers in 1996, where the overall means were 92, 85, 87.5 and 60% for the levels 130, 100, 70 and zero units of nitrogen/fed., respectively. There were significant variation between the number of aphids and aphid infestation rates at the three levels of nitrogen and control. In 1997 the present study showed that rates of infestation with aphids were higher on fertilized plants than control where the overall means of percentage of infestation were 8.6, 8.3, 7.1 and 3.40/0 on maize plants applied with 130, 100, 70 and zero units N.Ifed., respectively. The relation between aphid infestation rates of maize plants and levels of nitrogen fertilization were significant between zero fertilization and other nitrogen levels while was insignificant between the three position addition of nitrogen. Number of aphids were higher on fertilized than unfertilized plants, where the numbers were 8.2, 4.6, 3.6 and 0.5 individuals/plant as seasonal overall means at the levels 130, 100, 70 and zero units of nitrogen/fed., respectively in 1997. Data showed that total numbers of predators were higher on fertilized maize plants with three levels of nitrogen. The numbers of predators were 805, 1074 and 548 individuals for 1996 and 351, 301 and 340 predators for 1997 at the levels 130, 100 and 70 units of nitrogen/fed., respectively. These numbers were higher comparing with those in the case of control showing 240 and 157 individuals in the two successive seasons, respectively. The predators, can be arranged in a descending order according to their abundance on maize plants to 3 groups. Highest abundance (ladybird), relatively high abundance (Rove beetle) and lowest abundance (Aphid lion). The present study showed that the addition of any of the different nitrogenous fertilization levels caused increase in infestation rate of com borers, corn leaf aphid and ears yield/fed. compared with untreated maize plants which showed reduction in infestation rates with com borers and aphids and also reduction in ears yield. The means of ears yield/fed. were 23.5, 19.8, 17.5 and 8.3 ardab/fed. for the two seasons with the following nitrogen levels 130, 100, 70 and zero units of nitrogen/fed. , respectively. III. The effect of certain insecticides on rate of damage: Hl.a, Effect of Lannate on corn borers: Generally, infestation rate of maize plants by com borers did not abviously increase after application of Lannate 90%. Lannate used at two concentrations 300 g/400 L water/fed. (recommended concentration) and 200 g/400 L water/fed. (lower than recommended). The infestation rates by borers on control were higher than treated plants, during the two seasons, where the overall means of infestation were 1.20/0in 1996 and 3.8% in 1997 with recommended concentratien, while on untreated plants were 7.0 and 5.7 during 1996 and, respectively. Lower concentration of Lannate was less effective on the rate of infestation by com borers showing 3.0 and 4.40/0during 1996 and 1997, respectively as overall mean of infestation. III.b. Effect of Malathion on the infestation percentage and abundance of R. maidis and its predators: Generally, infestation to maize plants by com leaf aphid was higher in 1996 than 1997. In the two seasons, rate of infestation by aphid, number of aphids and their predators were higher on untreated maize plants than plants treated with two concentrations of Malathion. The used concentrations were 1.5 & 1 L/600 L water/fed. The overall means of infestation rate by aphid were 95.5 and 14%) on untreated plants during 1996 and 1997, respectively. While, those were 88 & 94 and 8.7 & 13.30/0on maize plants treated with recommended and low concentrations of Malathion during 1996 and 1997, respectively. The overall mean of aphid number with recommended and low concentrations of Malathion and untreated plants were 391.7,512.2 and 1439.4 individuals/one plant during 1996. Also, the overall mean number of aphidivorious insects on treated plants with recommended and low concentration of Malathion and on untreated maize plants were 13.2, 13.5 and 24.6 predators/l 0 plants throughout 1996, while during 1997, were 3.6, 4.4 and 4.6 individuals/lO plants, respectively. Higher effect on number of aphids and their predators was obtained using recommended concentration of Malathion than lower concentration. I.V. Effect of different treatments on hibernating larvae after harvesting: Because of the hibernating larvae are the main source of infestation to the plants of the next season, this point was carried out to determine the effect of different treatments on number of hibernating larvae inside dry stalks of treated com. The plants were examined at the mid of winter (lOth of January). Data showed that the dry stalks of the late sowing possessed higher infestation with com borers and number of hibernating larvae and pores than the stalks of early sowing. Also, infestation with com borers and number of hibernating larvae increased by increasing the level of nitrogen fertilization up to 100 units of nitrogen/fed. and after which they decreased by increasing the level of nitrogen fertilization. Also, the present work indicated that the treatments with Malathion at 1.5 and 1 L/fed. and with Lannate at 300 and 200 glfed. gave a good protection to the treated green plants from infestation with com borers but this protection decrease by lapse of time. |