الفهرس 
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Abstract
The Egyptian cotton plants have been infested by many pests specially cotton leafworm Spodoptera littoralis. Insecticides with novel modes of action with high selectivity towards invertebrates and fewer hazards are required. The use of the photochemical sensitization as a tool to control the population of different types of insects has been frequently examined such as porphyrin derivatives (pds) and methylene blue, which have been tested in this work as photoinsecticides on the cotton leafworm S. littoralis as larvicidal agents, and study of joint action between the two photosensitizes methylene blue (MB) and hematoporphyrin (HP) and the two tested insecticides (chlorpyrifos and cypermethrin), as will as study the effect of these compounds on the activity of some enzymes involved insecticidal resistance. Also, studied the biological effect of the tested compounds on larval stages. The data showed gradual increase in the mortality with increase time interval after treatment. The level of mortality was also increased generally with concentration of the tested compounds but it was pronounced in MB treatment. This may simply reflect differences in consumption of the compound with consequent effect on mortality.The possible effect of insect age on susceptibility toward photoinsecticides compounds was investigated by comparing the mortality of second, third and fourth instar larvae of S. littoralis treated with the same concentration (1×10-2 M/L) of each MB and HP and exposed to light at fluency rate 425 W/m2. The results suggested that, for the particular age, the younger larvae (2nd instar) were more susceptible to the tested photosensitizer compounds than the oldest larvae. The results described in this work pointed out that methylene blue could be considered as a useful photoinsecticidal agents against the cotton leafworm larvae than the hematoporphyrin. The lethal time (LT50) obtained from the 2nd instar larvae fed on 1×10-2 M/L of methylene blue was 184.97 hrs, while it was 1286.76 hrs in the case of HP for the same instar and concentration. The data resulted in the joint action between the two photosensitizes (MB and HP) and the two tested insecticides (chlorpyrifos and cypermethrin) revealed that, at the level of LC25 + LC25 mixing MB with the two tested insecticides resulted in antagonism effect on the 2nd instar larvae especially cypermethrin (-60), additive effect took place with MB/chlorpyrifos mixture on 3rd instar larvae. While potentiation effect was exhibited only in MB/cypermethrin mixture on the same instar regarding to hematoporphyrin (HP), a high level of potentiation with chlorpyrifos and gave low level of potentiation with cypermethrin was observed on the 2nd and 3rd instar larvae. At the level of LC50 + LC50 data showed that MB/chlorpyrifos mixture or MB/cypermethrin mixture produced high level of potentiation on the 3rd instar larvae and antagonism on the 2nd instar larvae. While HP/chlorpyrifos mixture produced high level of potentiation on the 2nd instar larvae and antagonism on the 3rd instar larvae. On the other hand, remarkable level of antagonism with HP/cypermethrin mixture on the 2nd instar larvae and high level of potentiation on the 3rd instar larvae was recorded. It is obvious, as could be noticed from the above data that decreasing the titer of the amount of HP compound in the mixture increase the toxicity of the mixture.