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Background The indiscriminating and intensive use of insecticides to control the cotton leafworm (CLW), Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae), usually induces high levels of resistance. The insecticides are the principal method for controlling this pest because of its critical role in reducing insects when the economic threshold (ETL); therefore, the effectiveness of these insecticides should be maintained. So, the target of the present work was directed to focus on studying the change in the activities of some important enzymes as a result of sublethal treatment concentrations (viz. LC25 values) of tested new insecticides (profenofos, cyfluthrin, emamectin benzoate, lufenuron, and spinetoram). The expected results could offer better understanding and more specific information about the resistance development in field populations of CLW because resistance is a significant challenge to pest control workers and these results may contribute to making the right decision at the right time. Materials and methods Bioassays were performed on fourth instar larvae of S. littoralis field populations compared with the laboratory strain to assess the activity of the emamectin benzoate, lufenuron, and spinetoram by LC25 and study the biochemical activities of some detoxification enzymes, like acetylcholinesterase (AChE), glutathione S-transferases (GST), alkaline phosphatase (ALP), and acid phosphatase (ACP) in fourth larval instar which was treated with LC25; protein content is also determined. Results The emamectin benzoate was the most toxic compound followed by lufenuron, cyfluthrin, spinetoram, and profenofos with the corresponding LC50 values of 0.05, 49.18, 70.99, 130.26, and 156.78 ppm respectively. The results showed that all the tested insecticides at LC25 value gave a slight inhibition of the acetylcholinesterase (AChE) activity, except profenofos was the most potent one. The activity of glutathione S-transferase (GST) of larvae treated with spinetoram has significantly increased, whereas the enzyme activity was significantly inhibited following cyfluthrin and non-significantly inhibited following profenofos, lufenuron, and emamectin benzoate application. There were no significant differences between treated and untreated larvae in ACP activity. In contrast, the alkaline phosphatase (ALP) activity of larvae treated with tested insecticides significantly increased, while the activity was inhibited following cyfluthrin application. On the other hand, spinetoram, emamectin benzoate, and lufenuron exhibited significant increment in the protein content, whereas there was no significant effect following either cyfluthrin or profenofos treatments. Conclusions In summary, the present results suggest that not only the lethal effects but also the sublethal effects of newly tested insecticides could have a negative influence on the dynamics of CLW; thus, these sublethal effects can be integrated into pest control to reduce the overuse and misuse of insecticides. This effect appears reduction in the activity of detoxification enzymes, resulting in response to the tested insecticide by the lowest concentration of (sublethal doses). Also, the inhibition of detoxification enzymes, which represents defensive reactions in insects, is playing important role in reducing resistance in S. littoralis, which is one of the most dangerous pests of all agricultural crops in the world. Therefore, these results were valuable for the practical use of these insecticides in IPM programs.

The toxicity of azadirachtin, pyridalyl and quercetin compounds were evaluated against the immature stages of the cotton leafworm, Spodoptera littoralis, field strain andlaboratory susceptible strain.The results showed that azadirachtin was the most potent tested compound flowed by pyridalyl and the least one was quercetin. Azadirachtin at 15.0 ppm achieved 30.4% mortality of treated egg masses, while pyridalyl and quercetin at the same concentration caused 28.8% and 21.2% mortality of treated egg masses, for lab strain, respectively, while field strain at the same concentration caused 23.4%; 20.4% and 16.4% mortality of treated egg masses respectively. The toxicity of tested compounds were evaluated against 2nd and 4th larval instar of S. littoralis, cleared that the toxicity of the tested compounds increased with increasing the exposure time, decreased by increasing the stage of larval instars. The larvae stop eating after two days of treatment. Also, the tested compounds shows no pupal mortality after 24; 48, and 72 hrs of treatment, but significantly increased percentages of deformed of adults. Results proved that the three natural compounds (azadirachtin, pyridalyl and quercetin), are potentially potent for control of S. littoralis, as antifeeding. Generally, natural compounds, will produce a new trend to increase toxicity of the newly hatched larvae of S. littoralis, enhance the role of beneficial insects. These compounds could be used in the IPM programmes, in order to minimize the negative effects of conventional insecticides on environment components and to protect the natural enemies.

Different strains of cotton leafworm, Spodoptera littoralis larvae were collected from heavily sprayed fields or recently cultivated fields in different areas in Alexandria, and El-Boheira, proviance governorate. Chitinase activity was determined in laboratory susceptible strain of cotton leafworm and compared with enzyme activity for other collected strains which cleared that highest level of chitinase activity was found in Alexandria strains. The sensitivity of chitinase activity to chlorfluazuron and teflubenzuron was measured by I50 values, values of I50 in the case of chlorfluazuron were 0.23, 0.34, 0.41, 0.60, and 0.69 µM for lab strain; Borg El-Arab; West of Nobaria; Abou El-Matamir, and Edko strains of Spodoptera 2nd larvae respectively, while I50 values were 0.31, 0.40, 0.46, 0.66, and 0.74 µM for lab strain and four field strains of Spodoptera 4th larvae respectively, similarly, the teflubenzuron were 0.40, 0.50, 0.57, 0.74, and 0.82 µM for lab strain and four field strains of Spodoptera 2nd larvae respectively, the I50 values were 0.47, 0.59, 0.65, 0.88, and 0.93 µM for lab strain and four field strains of Spodoptera 4th larvae respectively. Also, the inhibition constant (Ki) values were determined, the obtained data proved that compounds competitive inhibition of chitinase activity. The significant high mortality percentages were observed at all tested concentrations with chlorfluazuron than teflubenzuron and the result clearly showed that the 2nd instar larvae were more sensitive to the compounds tested, compared to those of the 4th instar, so when IGRs used for S. littoralis larvae control, dosage and timing of application should be carefully considered. The results of the present study may add some forward steps to use IGRs as alternative to conventional insecticides especially against this insect, so, the IGRs can be involved in important steps necessary for successful IPM programs applied against S. littoralis.