Effects of temperature on life-history traits of the newly invasive fall armyworm, Spodoptera frugiperda in Southeast China

Computational biogeographic distribution of the fall armyworm (Spodoptera frugiperda J.E. Smith) moth in eastern Africa

The fall armyworm (FAW), Spodoptera frugiperda J.E. Smith, has caused massive maize losses since its attack on the African continent in 2016, particularly in east Africa. In this study, we predicted the spatial distribution (established habitat) of FAW in five east African countries viz., Kenya, Tanzania, Rwanda, Uganda, and Ethiopia. We used FAW occurrence observations for three years i.e., 2018, 2019, and 2020, the maximum entropy (MaxEnt) model, and bioclimatic, land surface temperature (LST), solar radiation, wind speed, elevation, and landscape structure data (i.e., land use and land cover and maize harvested area) as explanatory variables. The explanatory variables were used as inputs into a variable selection experiment to select the least correlated ones that were then used to predict FAW establishment, i.e., suitability areas (very low suitability - very high suitability). The shared socio-economic pathways, SSP2-4.5 and SSP5-8.5 for the years 2030 and 2050 were used to predict the effect of future climate scenarios on FAW establishment. The results demonstrated that FAW establishment areas in eastern Africa were based on the model strength and true performance (area under the curve: AUC = 0.87), but not randomly. Moreover, ∼27% of eastern Africa is currently at risk of FAW establishment. Predicted FAW risk areas are expected to increase to ∼29% (using each of the SSP2-4.5 and SSP5-8.5 scenarios) in the year 2030, and to ∼38% (using SSP2-4.5) and ∼35% (using SSP5-8.5) in the year 2050 climate scenarios. The LULC, particularly croplands and maize harvested area, together with temperature and precipitation bioclimatic variables provided the highest permutation importance in determining the occurrence and establishment of the pest in eastern Africa. Specifically, the study revealed that FAW was sensitive to isothermality (Bio3) rather than being sensitive to a single temperature value in the year. FAW preference ranges of temperature, precipitation, elevation, and maize harvested area were observed, implying the establishment of a once exotic pest in critical maize production regions in eastern Africa. It is recommended that future studies should thus embed the present study's modeling results into a dynamic platform that provides near-real-time predictions of FAW spatial occurrence and risk at the farm scale.

Fecundity and Longevity of Spodoptera frugiperda (Lepidoptera: Noctuidae) at Constant Temperatures and Development of an Oviposition Model

Fall armyworm, Spodoptera frugiperda (J.E. Smith), is a notorious invasive pest native to subtropical and tropical regions in the Western Hemisphere. It has recently invaded and established in south Asian countries and in South Korea only seasonally. Longevity, survival, and fecundity of fall armyworm were examined at different temperatures (16, 20, 24, 28, and 32°C) and an oviposition model was developed. The maximum observed fecundity was 1,485 eggs per female at 22.0°C, which decreased to ca. 815 eggs at 32.0°C. Female longevity decreased as the temperature increased up to 24°C, and then was constant around 13-14 d until temperature reached 32°C, ranging from 33.1 d at 16.0°C to 13.1 d at 32.0°C. Temperature-dependent total fecundity (TDF) was well described by the extreme value function. Age-specific cumulative oviposition rate (AOR) and age-specific survival rate (ASR) curves were fitted to logistic and sigmoid functions, respectively. The model of female adults' aging rate (1/mean longevity) as a function of temperature was used to calculate the physiological age of fall armyworm females in AOR and ASR models. Three temperature-dependent components of TDF, AOR, and ASR were incorporated to construct the oviposition model, and it was simulated to project corn damage with tentative parameters. When 10 fall armyworm females were assumed, a total of 68-74 corn ears with kernel damage were predicted. Such loss was estimated to be US$75-83 currently in the Korean market.

Temperature-Dependent Development Models Describing the Effects of Temperature on the Development of the Fall Armyworm Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae)

The fall armyworm Spodoptera frugiperda (J.E. Smith) is an economically important pest that recently invaded Africa and Asia; however, information regarding its biological capacity to establish itself in newly invaded environments is largely unknown. We investigated the effects of temperature on the development and survival of the invaded populations of S. frugiperda and selected mathematical models to evaluate its development in a new environment. S. frugiperda exhibited optimum survival and growth at temperatures between 28 °C and 30 °C. The lower and upper thermal thresholds for the egg-to-adult life cycle were 13.51 °C and 34.13 °C, respectively. We compared seven mathematical models and found that the Shi model was the most suitable for describing the temperature-dependent development rate of S. frugiperda. Therefore, the Shi mathematical model may be used to predict both the occurrence of particular developmental stages and the geographic distribution to implement measures for the management of S. frugiperda in agricultural fields.

Examining the Effects of Induced Plant Defenses on Spodoptera frugiperda Performance

Knowing the duration insect pests are in the environment is vital for growers to determine management schemes and apply treatments. Unfortunately, experiments to determine long-term insect performance across plant cultivars are infrequently conducted. With that in mind, we report here the performance of Spodoptera frugiperda, the fall armyworm, on jasmonic acid (JA) induced/non-induced soybean cultivars. JA induction increases plant defensive compounds and can be considered an equivalent to a plant’s response to herbivory. S. frugiperda is a global pest, with infestations in soybeans becoming an emerging problem, making information on this pest’s performance on soybeans warranted. Thus, we reared larvae on two different soybean cultivars with contrasting defensive strategies when induced with JA and measured 7-day survival, development time to pupation, and pupal mass. Plant cultivar and JA induction were both important causes of mortality. Although plant cultivars varied in their amounts of constitutive/inducible defenses, this did not cause an interactive effect between plant cultivar and induction. Insect development to pupation was also extended when fed on induced plants regardless of cultivar, while pupal mass was not affected. Overall, induced plant defenses lowered larval survival and extended development time which would alleviate pest pressure and extend the period growers have for detecting infestations.

The Effect of Temperatures and Hosts on the Life Cycle of Spodoptera frugiperda (Lepidoptera: Noctuidae)

The interactions between ambient temperatures and host plants are central to the population dynamics of invasive animal species. Despite significant research into the effects of temperatures, the performance of invasive species is also influenced by host plants. The effects of different temperatures (20, 25, and 30 °C) and host plants (maize, sorghum, and coix seed) were tested on the mortality, development, reproduction, and population parameters of the fall armyworm (FAW), Spodoptera frugiperda (J E Smith) (Lepidoptera: Noctuidae), using an age-stage, two-sex life table. The results support the hypothesis that temperature and the species of the host plant significantly influences the performance of FAW. Feeding on maize at 30 °C resulted in a lower mortality rate, a shorter developmental time and longevity, a higher fecundity, intrinsic rate of natural increase (r), finite rate of increase (λ), and net reproductive rate (R₀). However, at 20 °C, the host plant could eliminate temperature-mediated synergism in FAW performance, which did not reach statistical significance at 20 °C. Similar results induced by a relatively low temperature (20 °C) on different host plants were also found in the age-stage specific survival curves (s_xj), fecundity (m_x), maternity (l_xm_x), and reproductive value (v_xj) curves of FAW. Consequently, we also need to pay more attention to FAW outbreaks on different host plants mediated by relatively low temperatures.