Physiological Mechanisms of Variable Nutrient Accumulation Patterns Between Diapausing and Non-Diapausing Fall Webworm (Lepidoptera: Arctiidae) Pupae

Abstract

Diapause is a highly advantageous strategy for winter survival for insects living in temperate environments. However, insects typically do not feed during diapause and are therefore presented with a complicated energetics problem. The fall webworm, Hyphantria cunea, overwinter as diapausing pupae, but adults of this species lose their ability to feed due to the degeneration of their mouthparts. Thus, the energy reserves stored before diapauses contribute to the survival rate and fitness of the adults after emergence. In this study, we tested the hypothesis that diapause-destined larvae of H. cunea reserve more energy by increasing feeding rate, feeding efficiency, or both, during the diapause preparation phase compared with non-diapause-destined larvae. We observed higher digestive efficiency, increased lipase and amylase activity, and lower protease activity in diapause-destined larvae compared to non-diapause-destined larvae. These differences in digestive physiology during diapause preparation lead to greater body size and mass, increased lipid and carbohydrate content, and lower soluble protein content in diapausing pupae, relative to non-diapause pupae – results consistent with our hypothesis. We conclude that diapause-destined fall webworm reserve more energy than non-diapause-destined individuals by increasing feeding efficiency, and that this increase in efficiency is at least partially driven by increased lipase and amylase activities in the midgut. This is in contrast to non-diapause-destined larvae, which likely reserve greater protein than diapause-destined larvae to help maintain their physiological function.

Difference in nutrient accumulation patterns between diapause‐destined and non‐diapause‐destined larvae of Hyphantria cunea

“…Although no significant difference in food intake was detected between the DD and NDD larvae, the ECD and ECI of DD sixth and seventh instars were significantly higher than those of NDD sixth and seventh instars. The present findings are consistent with the results of Zhao et al (2021a). This indicates that DD larvae increased their nutrient reserves by increasing the ECD and ECI.…”

“…A difference in body size between diapausing and non‐diapausing individuals has been reported in other insect species (Hahn & Denlinger, 2007). For example, the diapausing pupae of H. cunea and Sericinus montela Gray were significantly larger and heavier than non‐diapausing pupae and the lipid and carbohydrate reserves of diapausing pupae were significantly greater than those of non‐diapausing pupae (Wang et al, 2007; Zhao et al, 2021a). Insect diapause is not simply a pause in development and pre‐diapause individuals increase their nutrient accumulation at a specific time within the diapause preparation stage (Koštál, 2006).…”

“…Each Petri dish contained a layer of absorbent cotton and a layer of moist filter paper was laid on the cotton for moisture retention before incubating with egg masses at 25 ± 1 °C and a L16:D8 h photoperiod. After hatching, the larvae were transferred to a plastic container (5 cm diameter, 4 cm high) and reared on poplar leaves using the methods described by Zhao et al (2021a,b). The leaves were changed daily until the larvae entered the pre‐pupation stage.…”

“…The remaining diet and feces were dried in a 65 °C oven until they reached constant weight before weighing. The initial dry mass of diet and larvae was calculated as described by Zhao et al (2021a,b). Regression analysis was performed on the wet and dry weights of diet and larvae, followed by calculating the dry mass of diet and larvae according to the linear regression equation of wet weight and dry weight (diet: dry mass = 0.2692 × wet mass + 0.0054, r 2 = 0.99; NDD larvae, fifth instar: dry mass = 0.1681 × wet mass + 0.00002, r 2 = 0.99; sixth instar: dry mass = 0.1622 × wet mass – 0.00003, r 2 = 0.99; seventh instar: dry mass = 0.1479 × wet mass + 0.0011, r 2 = 0.98; DD larvae, fifth instar: dry mass = 0.1842 × wet mass + 0.00004, r 2 = 0.99; sixth instar: dry mass = 0.1805 × wet mass + 0.00001, r 2 = 0.99; seventh instar: dry mass = 0.1683 × wet mass + 0.0005, r 2 = 0.98).…”

“…Therefore, the energy required for basic metabolism, resistance to harsh environmental conditions, metamorphosis, flight, and reproduction after diapause all come from the nutrient reserves accumulated during diapause preparation. Our previous research showed that lipid and carbohydrate reserves of diapausing pupae are significantly greater than those of non‐diapausing pupae, and diapause‐destined (DD) larvae can store more nutrient by improving their food utilization efficiency (Zhao et al, 2021a). However, it is unclear whether, and when, the nutrient accumulation patterns of DD and non‐diapause‐destined (NDD) larvae change during diapause preparation.…”