Abstract: Efficacy of Dimlor 662 EC was monitored against corn leaf aphid, Rhopalosiphum maidis (Fitch) under three different micro-climatic conditions i.e. seedling of maize sown on plastic pots covered with glass chimneys, seedling in test tubes, and maize leaves placed on petri dishes. Maize leaves and seedling were painted with serial dilutions i.e. 0.0003, 0.0006, 0.0012, 0.0025, 0.0050 and 0.01 percent. It was found effective and statistically non significant in all micro-climatic conditions. A liner regression model was developed between concentration and percentage mortality. The LC90 were calculated as 0.0024, 0.0041 and 0.005 percent, respectively. This study will help in determining appropriate dosage of insecticide for the aphid control would be beneficial in resistance monitoring, IPM development, and product screening.
Introduction
Laboratory bioassay to determine the toxic effects of pesticides on target or nontarget species are carried out for product registration (Guillebeau, 1994), toxicity rankings to elute which pesticides to use as per of integrated pest management (IPM) programs (Hassan, 1989) and the primary screening of putative products for use as novel crop protection agents. Bioassay involve exposing test species to chemicals via topical or residual routes (Hall and Adams, 1990), dietary exposure (Thacker and Hickman, 1990; Wiles and Jepson, 1993) and exposure resulting from inhalation.
Corn leaf aphid R. maidis (Fitch) is problem on maize in Barani area (Ghouri et al., 1978), where it appears in early stage of crop i.e before tasseling and sibling whereas, in plain area its incidence on autumn maize after tasseling and sibling which has no importance in crop maturity and field potential (Bolt et al., 1983).
Dimethoate has been proved effective systemic insecticide against sucking pest as maize fruit fly (Halling, 1983). The joint action of mineral oils with some organophosphorus insecticides were more effective against wheat aphid R. padi (EL Deeb, 1993). High mortality of English grain aphid and oat-birdchery aphid was observed when treated with dimethoate and carbaryl (Neil et al., 1997). It has been suggested that comparisons between laboratory concentration response data for insect pest might indicate how to use pesticides selectively as a part of IPM programs (Wiles and Jepson, 1992).
It is difficult to control insect without chemicals. However pesticides also have the side effects by destroying the ecology. Therefore, judicious use of chemical is recommended. This involve right formulation, right dosage, appropriate timing and effective application. An attempt is made to evaluate toxic of insecticide under different micro climatic conditions at similar rate against corn leaf aphid. This study would provide appropriate dosage for the aphid control.
Materials and Methods
Dimlore 662 EC (a mixture of chlorpyrifos and dimethoate provided by Dow Chemical and Pacific Ltd (Pvt), Karachi. Serial dilutions i.e. 0.0003, 0.0006, 0.0012, 0.0025, 0.0050 and 0.01 percent were prepared from stock solution (1%) in distilled water. Corn leaf aphid (R. maidis) were collected from maize field at National Agricultural Research Center (NARC), Islamabad.
Serial dilution of Dimlore i.e. 0.0003, 0.0006, 0.0012, 0.0025, 0.0050 and 0.01 percent were tested under three micro climatic conditions. In the first condition seedling of maize were sown on plastic pots covered with glass chimneys, in the second condition seedling were placed in test tubes and in third condition maize leaves were placed on petri dishes. After drying 20 aphids were released in all cases and each concentration had three replicates. A batch of check was placed with each method of treatments. Mortality data were recorded after 24 hours of treatment. After treatment they were placed under control temperature 26±2°C and relative humidity 60±5 percent. DATA were analyzed by Duncan’s Multiple Range Test and regression models were developed calculate LC90.
Results and Discussion
The dose-response relationship is the most important single principle in toxicological studies which leads to efficient use of insecticide with minimal disruption to the ecosystem (Anwar et al., 1990).
| Table 1: | Percent mortality of corn leaf aphid under different micro climate condition by application of Dilore 24 |
| The figure followed by the same letter in column are not significant different at P<0.01. | |
Dimlore was found to be equally effective against corn leaf aphid when tested under different micro climatic condition. Significant mortality (85-100%) was recorded at higher concentration (0.0025-0.01%) in three different conditions of treatment compared with after 24 hours of treatment. Test at low concentrations (0.0003-0.001 percent) gave significant mortality results (Table 1). Dimethoate controlled the english grain aphid and oat bird cherry aphid with nine percent increase in wheat yield (Neil et al., 1997). It was also found effective at 0.03 percent in reducing corn leaf aphids population and increasing the yield (Grandhale et al., 1986). Similar results have also been reported by Geeshler (1989). Chlorpyrifos (0.025%) was also reported effective when applied after treatment against this pest (Verma et al., 1988).
| Table 2: | Regression model of Dimlor against corn leaf aphid under three different micro climatic conditions after 24 hours. |
LC90 were calculated for each test condition (Table 2) were 0.0021, 0.0041 and 0.0055 percent respectively at test procedures 1, 2 and 3 after 24 hours of treatment followed by slopes of regression equation which were 5.74, 5.19 and 5.87 respectively, indicating the same variation of susceptibility of corn leaf aphid to Dimlor. Statistical analysis gave similar results in all procedures. Russian wheat aphid was found to be more susceptible to chlorpyrifos than dimethoate in contact toxicity experiment by Dip-test method, whereas the systemic toxicity of dimathoate were found to be more than chlorpyrifos (Bayoun et al., 1995). A linear relationship between concentration of insecticides and percentage of mortality was observed. Similar results were observed when dimethoate, methamidophos and primicarb were tested against cereal arthropod, e.g. R. padi (Heyer, 1995).
Bioassay methods needs refinement for reproducibility (Leeper et al., 1988). A FAO recommended method (FAO, 1970) determining resistance in peach aphid was replaced by two methods due to time consumption and ambiguity in results. Topical application methods incl a rapid dip test that has been used successfully in UK Myzes persicae, M. solanin and Apis fabe, a spray re test that is applicable to wide range of compound ind systemic insecticide (Busvine, 1980).
Dimlor, a mixture of systemic and contect insecticid be used for the control of aphid successfully because are exposed to both mode of action of insecticide field. Moreover any one of these procedure can be us a rapid bioassay technique when it is desirable to q determine the cause of failure and for determination appropriate dosage.
Acknowledgment
We thank to Mr. M. Irshad, Senior Scientific O Intergated Pest Management Institute, National Agriculture Research Center, Islamabad for technical suggestion.