Effect of temperature on development and reproduction of Spodoptera frugiperda (Lepidoptera: Noctuidae)
Spodoptera frugiperda (Lepidoptera: Noctuidae) is a polyphagous pest with a preference for crops which belong to the Poaceae family. Spodoptera frugiperda is native to the tropical and sub-tropical regions of South America and is one of the most serious maize pests in the Americas. This pest recently invaded the tropical regions of Africa, where it is considered to be a serious threat to food security. Due to the absence of diapause in S. frugiperda, its biology and distribution is strongly influenced by low temperatures. The aim of this study was to evaluate the effect of temperature on the development and reproduction of this species. The effect of temperature on the development and reproduction of S. frugiperda was studied at five different temperature regimes, namely 18, 22, 26, 30 and 32 ± 1 °C, at 65 ± 5 % relative humidity (RH) and a 14L: 10D photoperiod. Fertility was found to be high with all eggs that hatched at temperatures ranging from 18 to 32 °C. Development of eggs at a constant temperature of 18 °C was, however, slow and the percentage of eggs that survived very low. Continuous low temperatures, although above the lower thermal limit, will therefore slow development down and may reduce population dynamics as a result of high mortality. The optimal range for egg, larval and egg-to-adult development of S. frugiperda in South Africa was determined to be between 26 and 32 °C. The development rate of S. frugiperda increased linearly with increasing temperatures between 18 and 30 °C and larval survival was also the highest between 26 and 30 °C. The optimum temperature with the most rapid development rate and lowest mortality for larvae was at 30 °C. Pupal development time varied from 7.82 to 30.68 days (32 - 18 °C) with a mean pupal development time of 17.06 days at 22 °C, but only 11.43 days at 26 °C. The development period of the egg-to-adult stage decreased from 71.35 days at 18 °C to 20.27 days at 32 °C. Based on linear regression analysis of development rate at all temperatures, a minimum temperature threshold of 13.01 °C was calculated for egg development and 12.12 °C for larvae, 13.24 °C for pupae and 12.57 °C for egg-to-adult development. Degree-day requirements for S. frugiperda egg and larval development was determined at 35.72 ± 1.30 °D and 202.67 ± 4.45 °D respectively when larvae were reared on sweet corn kernels. Pupae needed 147.06 °D for development and development of the life cycle (egg-to-adult), 391.01 ± 1.22 °D. The number of larval instars was determined by using head capsule widths that ranged from 0.30, 0.46, 0.80, 1.40, 1.90, and 2.60 mm. All successive instars increased in size according to Dyar’s ratio. The threshold temperatures determined in this study can be used in a model to estimate the number of generations at specific localities where the crop host plants are cultivated. It can also be used in a model to determine areas suitable for cultivation to which S. frugiperda can migrate from its overwintering sites, as well as areas with suitable environmental conditions for persistent occurrence. Oviposition occurred at all the temperatures and the mean number of eggs laid by S. frugiperda was 224.4 and 979.2 at 32 and 22 °C respectively. There was a strong negative correlation between temperature, oviposition period and longevity of moths. The optimum temperature for oviposition was determined to be between 18 and 26 °C. Results from this study on the thermal constants and lower and upper threshold temperatures of S. frugiperda can be used to predict the impact of climate change on the distribution and population growth of this pest. This knowledge can contribute to the development of integrated pest management strategies for this pest in Africa.