Possibility of Replacing the Conventional Insecticides with Safety Environmental Compounds for Controlling the Two Corn Borers Sesamia Cretica Led. And Ostrinia nubilalis Hun.

MATERIALS AND METHODS

Tested Insecticide

Experimental Design
Experiments were conducted at Mansoura University Experimental Station, Dakahlia Governorate. The field was cultivated with single hybrid maize (SC 224) during the early and the late summer seasons, 2005. Maize was seeded on April 16, 2005 for the early season plantation and on June 15, 2005 for the late season. Maize fields were subjected to normal agriculture practices such as planting, thinning, irrigation and hoeing. A randomized complete block design with 5 treatments and 3 replicates (42 m²/plot) was used for both experiments. A knapsack sprayer provided with one nozzle delivering (200 L water/fed.) has proved to be sufficient to give good coverage on the tested maize plants. The tested insecticides were applied with recommended concentrates at two times; the first when the plants reached 15 days to control S. cretica and the second when it becomes 45 days old to control O. nubilalis.

Procedures of Evaluating the Treatments
The stem borer S. cretica infestation was recorded in 30 randomly selected plants from the inner rows of each plot at 24 h. before first spray and post treatment interval of 3 days and 1, 2, 3, 4 weeks. Degree of the leaf eating and dead heart due to the infestation with S. cretica, were estimated as follows 0 = healthy plant ; 1 = slight leaf eating; 2 = medium leaf eating; 3 = high leaf eating; 4 = damage plant and 5 = dead heart. Percent reduction in infestation was estimated using Henderson and Tilton (1955) equation to determine the effect of the tested insecticides.

At harvest time thirty ears were picked at random from each treatment plot. Each ear was husked, inspected for presence of European corn borer O. nubilalis and rated for feeding damage as follows: 0 = no damage; 1= damage to silks only; 2 = damage to unfilled tip only; 3 = damage to kernels > 3.8 cm from tip and 4 = damage to kernels < 3.8 cm from tip (Hazzard et al., 2003). The means of ears infestation were calculated and subjected to statistical analysis by ANOVA (Costat, 1990).

To calculate the grain yield, all ears (infested and uninfested) from one row randomly selected from each plot were separately, collected, dried and shelled. The grain yield was exposure to sunlight for two months to remove its moisture content (Metwally and Barakat, 2003). The grain yield was calculated and subjected to statistical analysis by ANOVA (Costat, 1990).

RESULTS

Efficiency of Tested Insecticides Against Sesamia cretica
Data in Table 1 showed that the initial and residual effect of tested insecticides against S. cretica infestation during the early season. Concerning the initial effect (after three days of spraying), carbaryl was the most effective toxicant giving 68.61% reduction in infestation. The other insecticides arranged descendingly according to their initial effect as follows: Chlorfluazuron (66.13%), spinosad (50.97%) and malathion (29.34%). On the other hand carbaryl showed the longest mean residual effect (during the next four weeks post treatments) recording 90.24% reduction in S. cretica infestation. The descending order of the mean of residual effect of the rest toxicant was chlorfluazuron (75.82%), spinosad (70.33%) and malathion (29.48%).

Regarding the effect against S. cretica in the late summer season data in Table 2 showed that the Spinosad recorded the highest initial reduction (40.37%) and no differences were showed between carbaryl and chlorfluazuron they were recorded the same value (30.52%), malathion gave the lowest effect (21.98%) reduction. Meanwhile chlorfluazuron was recorded the highest mean residual effect (83.78%) followed by descendingly carbaryl (60.38%), spinosad (60.26%) and malathion (33.19%) reduction.

Efficiency of Tested Insecticides Against Ostrinia nubilalis
Results in Table 3 showed that all treatments reduced O. nubilalis infestation damage compared with untreated plots in the early and the late summer seasons. Concerning the early season spinosad gave the best effect (76.50% reduction) followed by descendingly chlorfluazuron (66.50%), carbaryl (60.00%) and malathion was recorded the lowest effect in reduction 10.00%. On the other hand in the late summer season chlorfluazuron gave the best effect, it recorded 77.06% in reduction followed by spinosad (69.72%), carbaryl (70.64%) and malathion (66.06%) reduction.

The efficiency of the tested insecticides against O. nubilalis can be arranged according to the mean of reduction percentage during both seasons in a descending order as follows: Spinosad, chlorfluazuron, carbaryl and malathion they were 73.11, 71.78, 65.32 and 38.03%, respectively.

Effect of Tested Insecticides on Grain Yield
Data presented in Table 4 showed that all tested insecticides caused increase the grain yield of maize in the early and the late summer seasons. In the early season, Spinosad gave the highest grain yield 15.71 ardab/feddan (1 ardab = 180 kg of maize grain yield) with an increase of 89.05% comparing with control, no differences could be detected between chlorfluazuron and carbaryl they gave nearly the same grain yield 14.16 and 14.05 ardab/feddan, the corresponding increase were 70.40 and 69.07% increase in grain yield as compared with the control. While malathion gave the lowest grain yield 10.13 ardab/feddan with an increase of 21.90% as compared with the control. Concerning the late season, Spinosad gave the highest grain yield 12.55 ardab/feddan with an increase of 108.82%, followed by descendingly Chlorfluazuron 11.90, Carbaryl 10.30 and Malathion 9.55 ardab/feddan with an increase of 98.00, 71.38 and 58.90% as compared with the control, respectively.

The general means of change with an increase in grain yield of the tested insecticides during both seasons were arranged in a descending order as follows: Spinosad, chlorfluazuron, carbaryl and malathion giving 98.94, 84.20, 70.23 and 40.40%, respectively.

Table 1:	Initial and residual effect of tested insecticides against Sesamia cretica infesting maize during the early season 2005
I = Degree of infestation in 30 plants, R (%) = Percent of Reduction

Table 2:	Initial and residual effect of tested insecticides against Sesamia cretica infesting maize during the late season 2005
I = Degree of infestation in 30 plants, R (%) = Percent of reduction

Table 3:	Mean of ears infested damage by Ostrinia nubilalis as affected by tested insecticides during the early and the late summer seasons 2005
Means followed by the same letters are not significantly different, Reduction of infestation (%)= (Test - Control)/Controlx100

Table 4:	Means of grain yield (ardab/feddan) as affected by tested insecticides on the early and the late summer seasons 2005
Means followed by the same letters are not significantly different, Change of control (%) = (Test - Control)/Controlx100 Ardab = 180 kg of grain yield - (1feddan = 4200 m2).

DISCUSSION

Present results reported successful the spinosad and chlorfluazuron as safety environmental compounds more than the malathion and carbaryl as conventional insecticides for controlling the two corn borers S. cretica and O. nubilalis. Muresan et al. (2001) assessment the efficacy of chemical and biological products for the diminution of European corn borer and found that, the efficacy of chemical control ranged from 25 to 71% and the biological products 45 to 75%. Musser and Shelton (2003) found that Spinosad was able to provide control of O. nubilalis. Ahmed et al. (2002) studied the field efficacy of some bioinsecticides (spinosad one of them) against jower stem borer Chilo partellus (Pyralidae: Lepidoptera) and found that in Spinosad treated plots, the infestation was reduced from 10.72% before spray to 3.05% after seven days of first spray and to 0.74% on the seventh day of second spray, which was done one week after first spray. Ascher et al. (1987) found that more than 90% mortality in eggs and larvae of O. nubilalis when sprayed infested maize seedlings by 5 insect growth regulators (chlorfluazuron one of them).

Two corn borers S. cretica Led. and O. nubilalis Hub. which are regarded among the major factors affecting the productively of growing maize plants and causing great damage and yield loss. Corn ears may be damaged by caterpillar infestation that begin before or during silk stage. Many researchers have attempted to quantify the relationship between infestation level and corn yield reduction. Guthrie et al. (1975) reported that 11.4 to 34.8% grain yield reduction for some maize single crosses corresponding to different levels of larval survival. Sobek and Mun Kvold (1999) reported that O. nubilalis larvae act as vectors of fusarium, causing kernel rot. Bohn et al. (1999) found that O. nubilalis infestation reduced average maize grain yield by 0.28% for each 1% of damaged plants and by 6.05% for each O. nubilalis larvae per plant. Also, Szoke et al. (2002) found that, losses caused by O. nubilalis ranged 250-1000 kg ha^-1 depending on the degree of infestation year and yield averages. Metwally and Baracat (2003) cited that, the yield losses caused by O. nubilalis larval depended mainly on larval survival, extent of activity, timing of tunneling and their within plant distribution. Sabra et al. (2005) found that the act losses of grain yield caused by O. nubilalis was 0.38 and 0.31 kg/100 plants. Muresan et al. (2000) found that, the biological products and chitin inhibitors significantly reduced the attack frequency and number of O. nubilalis larvae and increased the yield of maize by up to 10.2%.

Becerra et al. (1999) cited that spinosad this a new pesticide is convenient in order to protect the beneficial entomofauna and to reduce the risk of contamination of human food. Musser and Shelton (2003) found that in field trials, Spinosad is less toxic to the must abundant predators in sweet corn (Coleomegilla maculata, Harmonia axyridis, Orius insidiosus). Subramanyam et al. (2003) mentioned that, the activity against a variety of stored product insects, persistence in farm-stored grain and low mammalian toxicity make spinosad a viable alternative to currently registered organophosphate grain protectants, such as malathion, chlorpyrifos-methyl and pirimiphos-methyl. So it is concluded that it can be replacing the conventional organophosphorus insecticides like Malathion and carbamate insecticides like carbaryl with biological products like spinosad and chitin synthesis inhibitors like chlorfluazuron as safety environmental compounds for controlling the two corn borers S. cretica and O. nubilalis in pest management strategies.