APPLICATION OF THERMAL REQUIREMENTS TO PREDICT SEASONAL ACTIVITY OF THE MOST IMPORTANT COTTON INSECT PESTS

Abstract

The present work is an attempt to study some biological and ecological aspects of the cotton leafworm, Spodoptera littoralis (Boisd.) and pink bollworm Pectinophora gossypiella (Saund.) and determining the thermal requirements needed for development of different stages of the two insect pests, consequently estimating seasonal activity and number of field generations based on degree days (DDs) All experiments were carried out in the Laboratory of Ecological Studies, Department of Plant Protection, and experimental stations, affiliated to Faculty of Agriculture, Ain Shams University. Biological studies of Spodoptera littoralis revealed the following The most favorable temperature for egg incubation was 35oC, while 23.5oC was the best for hatching. The estimated threshold of development for egg stage was 11oC and the average of thermal units required for development was 48.9 degree-days. For larval stage, the shortest larval duration was 13.5 days at 35oC, the estimated threshold of development was 9oC and the average of thermal units required for larval development was 351.73 degree-days. Data revealed that pupal periods decreased as temperatures increased, the threshold of development was 13.1oC and the average of thermal units required for development was 121.09 degree-days. The longevity of both sexes of adult moths (male and female) tends to be reduced at high temperatures. Females emerged from pupae reared at 15oC were few and didn’t complete its life cycle, on the other hand, females emerged at 35oC were malformed and couldn’t mate with healthy males. Female oviposition periods of S. littoralis reached its maximum at 23.5oC; while post-oviposition periods reached its minimum at 30oC. Generally Females lived longer than males at the five tested temperatures (20, 23.5, 25, 30 and 35oC), the highest fecundity occurred at 25oC, while the highest female ratio was at 23.5oC. Regarding total generation, the shortest duration occurred at 30oC being 25.74 days. The threshold of development was 13.19ºC and the average of thermal units required for completing one generation was 444.16 degree-days. For Pectinophora gossypiella, the shortest egg incubation period was 4.0 days at 30oC, while the highest percent of hatchability (98%) occurred at 25oC. The estimated threshold of development for eggs, larvae, pupae and adult moths (male and female) were 12.10, 11.60, 12.93, 9.80 and 9.50oC, respectively. Generally, females lived longer than males at the four tested temperatures (18, 20, 25 and 30oC) and the highest fecundity was obtained at 25oC, while the highest ratio of females (1.44) obtained at 18oC, The threshold of development for one generation was 12.29oC and the average of thermal units required for completing one generation was 567.81 degree-days. Ecological studies of S. littoralis and P. gossypiella were carried out at three governorates viz, Qalyubia, Sharkia and Monufia to estimate annual field generations of both insect pests, in addition to determining time of peaks occurrence. Three methods were used for this purpose, namely, fluctuation curve, accumulated heat units and regression lines (Scale Gauss). The obtained results by the three methods showed that S. littoralis had 7-8 annual field generations at different governorates depending on the prevailed temperature, generation peaks nearly occurred in the same dates with maximum ±2-4 days deviation. Regarding to P. gossypiella data revealed, the presence of 4-6 annual field generations at Sharkia Governorate, while 5-6 annual field generations were detected at Monufia Governorate, the differences between number of generation may be due to the prevailed temperature, specially early season and appearance date of infestation receptors.