Abstract: With a view to potentiate malathion by other insecticides, experiments were carried out with eight insecticides like, dichlorovos, pirimiphosmethyl, phenthoate, monocrotophos, diazinon, cypermethrin, lambda-cyhalothrin, and propoxur. All tests were done by the topical application of the insecticide mixtures on adult housefly, Musca domestica L. Toxicity of malathion was determined when applied alone and low dose of each insecticide was mixed with malathion in ratios of 1:1, 1:2, 1:5, 1:10 and 1:20. Separately. The mortality data after 24 hours of application showed that nearly all insecticides offered increased toxicity of malathion. But when it was applied jointly with lambda-cyhalothrin and cypermethrin the potentiation results were maximum.
Introduction
Resistance to pesticides is one of the most serious problems facing agriculture today. Resistance within or between whole classes of insecticides is an ever-increasing problem for control of major crop pests (Kanga et al., 1999; Bues et al., 1999; Graham and Janet, 2000; McKenzie, 2000). Resistance can lead to increased application rates, increased frequency of pesticide use, and ultimately loss of efficacy. We cannot assume there will be an unending supply of new insecticides to replace current compounds, due to the increased difficulty in discovering pesticides and the tremendous cost of their development (Georghiou, 1986; Hammock and Soderlund, 1986). The present way of dealing this problem is just to increase the dose of the insecticide to eliminate the more resistant populations. However, the result of this is to apply a greater selection pressure on the population. This ultimately enhances the evaluation of resistance in the population and obviously this procedure can not go on infinitely, since increasing the dose and thus the presence in the environment of a pesticide not only increases the non-target organisms mortality, but level could reach where mammals, ultimately man would be seriously affected. Therefore, the introduction of synergists to these systems could be of great benefit both economically and ecologically, especially since tests have shown that synergistic increases in toxicity of insecticides is only towards insects and not mammals (Metcalfe, 1992).
These interactions may be viewed as beneficial if the control of target pests is accentuated by unintended joint action. It has also been suggested that the advent of resistance to insecticides may be forestalled through the use of potentiations (Knowles and El-Sayed, 1985), that the control of recalcitrant pests may be increased through the use of joint action (El-Sayed and Knowles, 1984a), and that resurgence of certain pests could be prevented with potentiations (El-Sayed and Knowles, 1984b). Insecticide resistance in the housefly, Musca domestica (L.), has been recognized for many years. The rapid life cycle of houseflies in livestock farms gives a high potential for the selection of resistance when insecticides are applied intensively; such as through the repeated use of residual sprays (Keiding and Jespersen, 1986). The objective of this study was to assess the ability of malathion to act as a potentiation with a series of insecticides belonging to the organophosphorus, pyrethroids and carbamate classes.
Material and Methods
A consistent quality of common housefly was reared in the Crop Protection Laboratory, Department of Zoology, Rajshahi University at an economical cost following the methods of Morgan (1981), Morgan and Patterson (1978) and Morgan et al. (1978). The adults were provided with a medium to lay eggs. The medium was presented in plastic cups approximately 10cm deep. The medium consisted of 9 gm milk powder and 5 gm fresh yeast dissolved in 100 ml of water and added to 100 gm bran (Wilkins and Khalequzzaman, 1993). The mixture was then thoroughly stirred and put into the pots leaving 3 cm from the top. The pots were placed in the fly rearing cage (50 × 30 × 20cm3) for 24 h. The adults were provided with petridish (90 ml) containing cotton wool soaked in the solution of milk powder with sugar for a food source. After that time the females had laid batches of eggs. Batches of approximately 100 eggs were separated out and transferred to similar pots containing the same mixture and fitted with plastic lids with gauze centers. They were then placed in an incubator at 28°C ± 0.5°C. Pupa were collected and kept in separate pots (kept in the incubator under the same temperature) for emergence of the adults.
The experiments were carried out during July to December 1999. Nine insecticides, viz., malathion (Limithon 57 EC of ACI Ltd.), dichlorovos (Nogos 100 EC of Novartis Bd. Ltd.), pirimiphosmethyl (Actellic of ACI Ltd.), phenthoate (Cidial of ACI Ltd.), monocrotophos (Novacron 40 EC of Novartis Bd. Ltd.), diazinon (Diazinon 60 EC of Novartis Bd. Ltd.), cypermethrin (Cymbush 10 EC of ACI Ltd.), lambda-cyhalothrin (Karate 25 EC of ACI Ltd.) and propoxur (Acekro 20 EC of McDonald Ltd) were used for this study. The insecticides were diluted in acetone and the treatments were done by topical application with 1 μl of solution of insecticide on thoracic notum of each fly with the help of a micro syringe (Hamilton gas tight micro syringe no. 630). The actual dose was calculated from the amount of active ingredient present in 1 μl of the solution. At first malathion was applied singly. Then the low doses of dichlorovos (6.25 ng/fly), pirimiphosmethyl (62.5 ng/fly), phenthoate (62.5 ng/fly), monocrotophos (25 ng/fly), diazinon (25 ng/fly), cypermethrin (1.25 ng/fly), lambda-cyhalothrin (1.25 ng/fly) and propoxur (50 ng/fly) were mixed with malathion in mass ratio of 1:1, 1:2, 1:5, 1:10 and 1:20 and applied. The treated flies were kept in the food cup with cotton soaked in glucose solution. Each trial was repeated 3 times to obtain more uniform results with a smaller error. The mortality of the flies was recorded after 24 hours of treatment.
The mortality percentage was corrected using Abbott’s formula (Abbott, 1925) and observed data was subjected to probit analysis according to Finney (1947) and Busvine (1971) using a software developed in the Department of Agricultural and Environmental Science, University of Newcastle Upon Tyne, UK. The co-toxicity coefficient values were calculated by the method of Sun and Johnson (1960a,b). When the co-toxicity coefficient of a mixture is 100, the effect of this mixture indicates probability of similar action. If the mixture gives a coefficient significantly greater than 100, it indicates a synergistic action. On the other hand, when a mixture gives a co-toxicity coefficient less than 100, the effect of the mixture indicates an antagonistic action. In those cases where potentiation occurred the co-toxicity coefficient of mixture was divided by one hundred to obtain the degree of potentiation of malathion in that mixture (Gera and Gupta, 1978).
Results
The combined LD50 with 95% confidence limits and regression equations of log doses of different mixtures on probit mortality of the adult housefly after 24 hours of topical application are presented in Table 1.
| Table 1: | LD50, confidence limits (CL) and regression equation of combined doses of different insecticides and malathion on adult housefly (M. domestica) after 24 hours of treatment. |
| Table 2: | LD50 of the combined doses and co-toxicity coefficient at different ratios of insecticides and malathion applied topically on adult housefly (M. domestica) after 24 hours of treatment |
The LD50 of malathion was 433.655 ng/fly when applied singly on adult housefly but the combined LD50 showed variation in different insecticide-malathion mixtures. The combined LD50 was the lowest indicating high toxicity in all ratios of lambda-cyhalothrin:malathion and they were 4.585, 6.415, 10.880, 15.020 and 23.239 ng/fly in 1:1, 1:2, 1:5, 1:10 and 1:20 ratios respectively. The second highest toxicity was observed for cypermethrin:malathion mixtures followed by mixtures of dichlorovos, diazinon, monocrotophos, pirimiphosmethyl and propoxur with malathion. Phenthoate: malathion showed the highest LD50 value indicating less toxicity to flies and the LD50 values were 250.997, 296.625, 305.139, 295.337 and 341.583 in the above mentioned ratios respectively.
Table 2 contains segregated LD50 as per ratio in mixture of insecticides, co-toxicity coefficient and degree of potentiation of malathion by other insecticides. Here also lambda-cyhalothrin showed the highest co-toxicity coefficient and degree of potentiation. It may be noted that lambda-cyhalothrin and cypermethrin when mixed separately with malathion in ratios of 1:1, 1:2 and 1:5 gave the maximum co-toxicity coefficient value. It was 18920.3, 10139.2 and 4782.78 for lambda-cyhalothrin and 8423.76, 5170.56 and 3423.50 for cypermethrin with malathion in the above mentioned ratios respectively. The lowest values were observed for phenthoate: malathion mixtures indicating low level of potentiation.
Discussion
Interactions between chemicals in insects are well known. A variety of pesticides, applied in binary mixtures, cause a toxic effect in insects, which is different than the sum of the two alone. For instance, the formamidine chlorodimeform while itself less toxic has been shown to potentiate the activity with a variety of insecticides belonging to other organophosphorus, pyrethroid and carbamate classes (Scott et al., 1990; Rahim et al., 1994). In this case, the toxicity of the mixture generates a response which is greater than the additive effects of the two compounds alone (Sprague, 1969). Joint interactions are also important from an environmental standpoint. Pesticide residues are found both at the site of application and, due to dissemination by leaching, runoff, and erosive events, in remote destinations which include ground and surface waters. Some contaminants found in surface waters include parathion, malathion, carbaryl, carbofuran, atrazine, simizine, and 2,4-D (Crathorne et al., 1984). It is not known whether these chemicals will interact jointly in nontarget biota in terrestrial and aquatic environments or how environmental quality parameters can affect such interactions.
Toxicities of two triazine herbicides (atrazine and cyanazine) and an organophosphate insecticide (chlorpyrifos) were evaluated individually and in binary combinations using the fourth instars of the aquatic midge, Chironomus tentans by Jin-Clark (2001) and observed that atrazine and cyanazine produced significant synergistic effects on the toxicity of chlorpyrifos. Although atrazine and cyanazine were not effective inhibitor of acetylcholinesterase (AChE) in vitro, the synergism of atrazine and cyanazine to chlorpyrifos was associated with increased in vivo inhibition of AChE in the midges. Parimi et al. (2001) observed that the joint action seen between atrazine and methyl parathion, carbaryl, fipronil and indoxcarb may be due to atrazine induction of detoxification enzymes involved in biotransformation of the insecticides. The mixture of Delfin with conventional insecticides also killed diamondback moth Plutella xylostella faster than Delfin alone (Saranthoy et al., 1997).
Eighteen tris-substituted derivatives of phosphoric acid were evaluated as synergists for malathion and for several other organophosphate insecticides against resistant strains of the housefly (M. domestica) and the mosquito Culex trasalis (Coqulilett). Many of the materials markedly synergized malathion against resistant insects, but caused little or no increase in its toxicity to susceptible insects. In most cases with the insect populations studied the synergists did not overcome resistance present to organophosphates other than malathion (Plapp et al., 1963).
It was found that nearly all insecticides offered increased toxicity of malathion when applied jointly on housefly and cypermethrin and lambda-cyhalothrin were the best in this regard. It can safely be said from the present study that pyrethroid insecticides like lambda-cyhalothrin and cypermethrin can be used to potentiate malathion significantly for its toxicity.
Acknowledgment
The first author acknowledges the necessary facilities provided by the Department of Zoology, Rajshahi University during his Ph.D. programme.