Introduction
Mulberry (Morus spp) is infested by several pests. The insecticides applied for the control of mulberry pests have greater impact on silkworm. Pesticides leave residues in mulberry leaves which inturn affect the sensitive silk worm. Perusal of literature in this aspect reveals that the loss of body weight was noticed in silk worm, when exposed to pesticides (Pant and Katiyar, 1983; Venkata et al., 1989; Surendranath, 1993). Subacute exposure to pesticides is reported to affect the reproduction (Kuribayashi, 1980, 1981; Yamonoi, 1980). Pesticide residues were also thought to be transmitted to the silk worm eggs, embryos and pupae affectd later generation (Watanabe and Takano, 1966a; Kuwana et al., 1968). Along with chemical pesticides neem based insecticides are known to possess several adverse biological effects on insects like repellency, antifecundity and inhibitory activity (Schmutterer, 1990; Jothi et al., 1999; Medina et al., 2004; Tiwari et al., 2006; Kodandaram et al., 2008). Azadirachtin affect food intake by exerting strong antifeedant effects against a wide range of insect species (Meisner et al., 1981 a, b; Ley et al., 1989; Warthen, 1989; Blaney et al., 1990; Mordueluntz and Blackwell, 1993). Biological effects on insects like repellency, antifecundity and inhibitory activity (Schmutterer, 1990; Jothi et al., 1999; Medina et al., 2004; Tiwari et al., 2006; Kodandaram et al., 2008). Azadirachtin affect food intake by exerting strong antifeedant effects against a wide range of insect species (Meisner et al., 1981a, b; Ley et al., 1989; Warthen, 1989; Blaney et al., 1990; Mordueluntz and Blackwell, 1993).

Since Dichlorovos is widely used against a variety of agricultural pests, particularly mulberry pests which may attack silkmoth through their feeding on pesticide sprayed leaves during development. Neem pesticide also caused more than 80% larval mortality in many insects. Hence in the present investigation an attempt has been made to study the effect of Vijay neem and Dichlorovos on the metamorphosis of mulberry silk worm, Bombyx mori. L.

1 Results and Discussion
Silkworm larvae fed with pesticide treated foliage resulted in persistent growth inhibition at different levels during metamorphosis. The silkworm larvae fed with pesticides treated foliage caused growth disruption in pupal and adult morphogenesis in a dose dependent manner. When compared to Dichlorovos, Vijay neem showed much morphogenetic inhibition effect (Table 1). The present findings showed that pesticide is effective in inhibiting larval growth and inducing pupal and adult deformities in silk worm (Table 1). Such growth inhibitory effect of pesticide ranging from delay in moulting with the production of deformed insects, to the complete inhibition of growth at high doses had been reported in various insect species (Schmutterer, 1990).
 

Table 1 Effect of pesticides on metamorphosis of silkworm bombyx mori (Percentage (%) change over control given in parenthesis)

Dose dependent inhibition of larval growth and morphogenesis in silkworm Bombyx mori could be accounted due to reduced physiological change brought about by reduced food intake and weight gains as reported in Heliothis virescens (Barnby and Klocke, 1989, Joseph, 2000). The present study revealed that 4.04% of larval mortality was due to 0.0001% concentration of Dichlorovos. In the neem pesticide sprayed leaves fed larvae the mortality rate was 9.09% for a concentration of 0.001% and 35.35% for a concentration of 0.005. Similarly a dose dependent reduction was noticed in the moth emergence. Moth’s emergence rate was reduced to 11.45% for a concentration of 0.001% and a reduction of 57.29% was noticed for a concentration of 0.005%. From the present study it was found out that among the two pesticides, neem pesticide has a more lethal effect than Dichlorovos (Table 1). The two pesticides had an impact on the development of normal pupae also. The reduction in the development was found to be dose dependent. The development rate was reduced to 4.25% for a concentration of 0.0001% Dichlorovos and 28.57% for a concentration of 0.0005% Dichlorovos. The development of normal pupae was more affected in the case of larvae fed with neem sprayed leaves. A maximum of 42.85% reduction in the development rate was noticed for a concentration of 0.005 neem pesticide (Table 1).

In the present study, pesticides caused a lower percentage of pupation and adult emergence in treated insects in conjunction with increased pupal and adult abnormalities. This was in acceptance with the findings of Nagapasupathy et al. (2005) (Schmutterer, 1990; Tiwari et al., 2006). The silkworm larvae fed with pesticides treated foliage resulted in persistant growth inhibition at different levels of metamorphosis when compared to Dichlorovos Vijay neem showed much morphogenetic inhibition effect.

2 Materials and Methods
Eggs of the silk worm breed LXCSR2 were procured from Government sericulture grainage at Tenkasi, Tamilnadu and were raised up to 3^rd instar in a rearing house. Controlled temperature (24?-27?) and humidity (75%-80%) were maintained and appropriate measures were taken to prevent parasitization by uzifly. The silkworms were fed with mulberry leaves and standard rearing practices were followed (Krishnaswami et al., 1971). Two pesticides (Dichlorovos, Vijay neem) were used for this experiment. III instars (soon after the second ecdysis) of uniform size from a pooled batch were divided into eleven groups of 100 larvae each. Sublethal concentrations of Dichlorovos (0.0001%, 0.0002%, 0.0003%, 0.0004% and 0.0005%) and Vijay neem (0.001%, 0.002%, 0.003%, 0.004% and 0.005%) were given to the III instar larvae by feeding with pesticide sprayed leaves one time on the first day. Larvae were transferred to rearing tray containing fresh mulberry leaves. Simultaneously control was also maintained. Surviving larvae were kept for pupation and observations on various parameters like larval weight, pupal weight, larval duration, and larval, pupal and adult abnormalities were recorded. The recorded data were subjected to analysis to determine the significant difference between the various parameters of the treated and untreated insects.

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