Efficacy of Three Insect Growth Regulators on Growth and Development of the Egyptian Cotton Leafworm Spodoptera littoralis (Boisd.)

摘要

The direct and latent effects of the three commercial insect growth regulators lufenuron, methoxyfenozide and teflubenzuron on the 2nd and 4th instar larvae of susceptible strain of cotton leafworm Spodoptera littoralis were studied. Different concentrations of each insecticide (0.01,0.1,1 and 2.5 mg (a.i)/Kg diet) were incorporated into the artificial diet. The newly moulted 2nd and 4th larval instars were fed for 6 days on the treated diet in order to investigate the biological activity of these compounds under controlled laboratory conditions. Activity of chitinase and polyphenol oxidase (PPO) in surviving larvae after treatment was carried out in order to investigate the biochemical influences of these compounds. The insecticides proved to be high toxic to the test insect larvae and they showed adverse effects on pupae, and emergence of adults. The larvae suffered from more mortality when they were fed for a longer period on the diet that treated with concentrations of 1 and 2.5 mg (a.i)/Kg diet. The significant high mortality percentages were observed at all tested concentrations with teflubenzuron followed by lufenuron and then methoxyfenozide and the result clearly showed that the 2nd instar larvae were more sensitive to the compounds tested, compared to those of the 4th instar. Larval weight gain was considerably decreased as concentration increased and the reduction in weigh was highly effective in case of teflubenzuron followed by lufenuron and then methoxyfenozide. The affected larvaeceased feeding within 48h and most deaths occurred during moulting to the next instar. Some larvae failed to pupae successively. The insecticides varied in their influences on chitinase and PPO activity, and these enzymes could have relation with their toxicity against S. littoralis larvae. The susceptibility of the fourth larval-stage S. littoralis to these insecticides evidenced a similar pattern however^ the activity was lower than that obtained with the second larval-stage. So, when these insecticides should be used for S, littoralis larvae control, dosage and timing of application should be carefully considered.