Accumulation and excretion of zinc and their effects on growth and food utilization of Spodoptera litura (Lepidoptera: Noctuidae)

Abstract

By exposing larvae of the holometabolous insect Spodoptera litura to the artificial diets supplemented with a range of Zinc (Zn) contents, we investigated Zn ingestion, excretion and accumulation in the insect throughout its life cycle. The effects of Zn stress on the survival, growth and food utilization of S. litura were also determined. Zn concentrations in the body (larvae, pupae, and adults), faeces, exuviates, puparium, eggs increased with the increasing Zn concentrations in the diets, while Zn excretion and accumulation by S. litura in 750 mg/kg Zn treatments was lower than the 600 mg/kg Zn treatment. In the 450 mg/kg Zn treatment, the Zn accumulation in S. litura at different developmental stages differed as follows: larvae > pupa > adult. S. litura ingested Zn via feeding and could excrete most of the Zn via faeces (compared with Zn excretion via exuviates) to reduce its internal Zn accumulation (compared with Zn ingestion). Survival and weight were significantly inhibited, and the prolonged period of development (larvae, pupae) and shortened longevity of adults were found in S. litura exposed to Zn stress greater than 450 mg Zn/kg. In the 150–450 mg/kg Zn treatments, the 6th instar larvae increased their relative consumption rate (RCR) and approximate digestibility (AD) (namely, food eaten) to gain weight, which resulted in greater Zn accumulation in the body. Therefore, below the threshold level (being close to 450 mg/kg Zn), S. litura seemed to have a strong homeostatic adjustment ability (increase the amount of food eaten, thereby increasing AD, RCR and Zn excretion via faeces and exuviates) to sustain their weight, and Zn was beneficial and harmless. Although larvae treated with 750 mg/kg Zn had a similar RCR and AD as the control, a reduced weight gain and prolonged larval period resulted in significantly lower relative growth rate (RGR), which indicated surviving insects may allocate more energy from foods for detoxification than for growth.