Lower Threshold of Development and Thermal Requirements of Cotton Leafworm, Spodoptera littoralis (Lepidoptera: Noctuidae), from Emamectin Benzoate-Susceptible and -Resistant Colonies

Lower threshold of insect development and thermal requirements were estimated in the laboratory at five constant temperature regimes (15-35^oC).  The two biological parameters were established for different developmental stages of cotton leafworm from two colonies, emamectin benzoate-susceptible and -resistant strains.  From this study, the lower threshold of development (T₀) was only species dependent.  The least value of zero development was 9.1^oC (for larval stage) and the greatest was 11.77^oC (for adult stage); however, the range of variations from 9.10 to 11.77^oC was insignificant.  In contrast, thermal requirements (day-degrees or degree days) were mostly colony, stage, sex and temperature dependent.  When the two colonies were compared, larval stage, pupal stage, generation time and life cycle for individuals from resistant colony required greater thermal units than those from susceptible colony and the difference was highly significant.   For all immature stages from each of the two tested colony, statistically, the greatest thermal requirements were for larval stage followed by pupal stage and the least were for the development of egg stage. In general life cycle required significantly greater heat constant than insect generation.  Regardless the tested colony and temperature regimes, thermal requirements were highly significant greater for oviposition period than for the two other physiological periods of adult female longevity.  Variations in thermal requirements were also compared between male and female at all temperature regimes tested and for both of the two tested colonies. Thermal units were greater for female pupae (ranged from 144.97 d-d at 15^oC to 166.51 d-d at 20^oC) than those for male pupae (129.49 d-d at 15^oC to 157.41 d-d at 20^oC), however this variation was significant at 15^oC and was insignificant at 20^oC.