Sub-optimal host plants have developmental and thermal fitness costs to the invasive fall armyworm.

The fall armyworm (FAW) Spodoptera frugiperda (J.E. Smith) is a global invasive pest of cereals. Although this pest uses maize and sorghum as its main hosts, it is associated with a wide range of host plants due to its polyphagous nature. Despite the FAW's polyphagy being widely reported in literature, few studies have investigated the effects of the non-preferred conditions or forms (e.g., drought-stressed forms) of this pest's hosts on its physiological and ecological fitness. Thus, the interactive effects of biotic and abiotic stresses on FAW fitness costs or benefits have not been specifically investigated. We therefore assessed the effects of host plant quality on the developmental rates and thermal tolerance of the FAW. Specifically, we reared FAW neonates on three hosts (maize, cowpeas, and pearl millet) under two treatments per host plant [unstressed (well watered) and stressed (water deprived)] until the adult stage. Larval growth rates and pupal weights were determined. Thermal tolerance traits viz critical thermal maxima (CTmax), critical thermal minima (CTmin), heat knockdown time (HKDT), chill-coma recovery time (CCRT), and supercooling points (SCPs) were measured for the emerging adults from each treatment. The results showed that suboptimal diets significantly prolonged the developmental time of FAW larvae and reduced their growth rates and ultimate body weights, but did not impair their full development. Suboptimal diets (comprising non-cereal plants and drought-stressed cereal plants) increased the number of larval instars to eight compared to six for optimal natural diets (unstressed maize and pearl millet). Apart from direct effects, in all cases, suboptimal diets significantly reduced the heat tolerance of FAWs, but their effect on cold tolerance was recorded only in select cases (e.g., SCP). These results suggest host plant effects on the physical and thermal fitness of FAW, indicating a considerable degree of resilience against multiple stressors. This pest's resilience can present major drawbacks to its cultural management using suboptimal hosts (in crop rotations or intercrops) through its ability to survive on most host plants despite their water stress condition and gains in thermal fitness. The fate of FAW population persistence under multivariate environmental stresses is therefore not entirely subject to prior environmental host plant history or quality.

Comparative assessment of the thermal tolerance of spotted stemborer, Chilo partellus (Lepidoptera: Crambidae) and its larval parasitoid, Cotesia sesamiae (Hymenoptera: Braconidae).

Under stressful thermal environments, insects adjust their behavior and physiology to maintain key life-history activities and improve survival. For interacting species, mutual or antagonistic, thermal stress may affect the participants in differing ways, which may then affect the outcome of the ecological relationship. In agroecosystems, this may be the fate of relationships between insect pests and their antagonistic parasitoids under acute and chronic thermal variability. Against this background, we investigated the thermal tolerance of different developmental stages of Chilo partellus Swinhoe (Lepidoptera: Crambidae) and its larval parasitoid, Cotesia sesamiae Cameron (Hymenoptera: Braconidae) using both dynamic and static protocols. When exposed for 2 h to a static temperature, lower lethal temperatures ranged from -9 to 6 °C, -14 to -2 °C, and -1 to 4 °C while upper lethal temperatures ranged from 37 to 48 °C, 41 to 49 °C, and 36 to 39 °C for C. partellus eggs, larvae, and C. sesamiae adults, respectively. Faster heating rates improved critical thermal maxima (CTmax ) in C. partellus larvae and adult C. partellus and C. sesamiae. Lower cooling rates improved critical thermal minima (CTmin ) in C. partellus and C. sesamiae adults while compromising CTmin in C. partellus larvae. The mean supercooling points (SCPs) for C. partellus larvae, pupae, and adults were -11.82 ± 1.78, -10.43 ± 1.73 and -15.75 ± 2.47, respectively. Heat knock-down time (HKDT) and chill-coma recovery time (CCRT) varied significantly between C. partellus larvae and adults. Larvae had higher HKDT than adults, while the latter recovered significantly faster following chill-coma. Current results suggest developmental stage differences in C. partellus thermal tolerance (with respect to lethal temperatures and critical thermal limits) and a compromised temperature tolerance of parasitoid C. sesamiae relative to its host, suggesting potential asynchrony between host-parasitoid population phenology and consequently biocontrol efficacy under global change. These results have broad implications to biological pest management insect-natural enemy interactions under rapidly changing thermal environments.

Costs and benefits of thermal acclimation for codling moth, Cydia pomonella (Lepidoptera: Tortricidae): implications for pest control and the sterile insect release programme.

Sterile insect release (SIR) is used to suppress insect pest populations in agro-ecosystems, but its success hinges on the performance of the released insects and prevailing environmental conditions. For example, low temperatures dramatically reduce SIR efficacy in cooler conditions. Here, we report on the costs and benefits of thermal acclimation for laboratory and field responses of codling moth, Cydia pomonella. Using a component of field fitness, we demonstrate that low temperature acclimated laboratory-reared moths are recaptured significantly more (∼2-4×) under cooler conditions in the wild relative to warm-acclimated or control moths. However, improvements in low temperature performance in cold-acclimated moths came at a cost to performance under warmer conditions. At high ambient temperatures, warm-acclimation improved field performance relative to control or cold-acclimated moths. Laboratory assessments of thermal activity and their limits matched the field results, indicating that these laboratory assays may be transferable to field performance. This study demonstrates clear costs and benefits of thermal acclimation on laboratory and field performance and the potential utility of thermal pretreatments for offsetting negative efficacy in SIR programmes under adverse thermal conditions. Consequently, the present work shows that evolutionary principles of phenotypic plasticity can be used to improve field performance and thus possibly enhance pest control programmes seeking increased efficacy.