Different factors, such as starvation and metal exposure, may affect development and cause oxidative stress in insects. Some host plants may contain a high concentration of cadmium due to their hyperaccumulating property. The negative effects of metals and hunger may be manifested by low availability of energetic substrates. This study aimed to assess whether the insect population with a history of long metal exposure may better manage metal stress or/and starvation at different developmental stages, with the use of energetic substrates. Two strains of Spodoptera exigua model organism were tested: control strain and cadmium strain (treated continuously for over 200 generations with subtoxic amounts of cadmium). The effects of different factors, individually and in combination, on the tested strains were assessed, first by determining the body weight of larvae and pupae and then by estimating the concentration of biomolecules (proteins, carbohydrates, lipids, or glycogen) in the 4th and 5th larval stages and in pupae, and the total antioxidant capacity and lipid peroxidation level in the 4th larval stage. Compared to control strain, cadmium strain individuals exhibited changes in the concentration of soluble carbohydrates and protein. This was partly related to earlier 1-day starvation. In particular, changes in carbohydrate concentration seemed to be a sensitive biomarker of metal stress, independent of the age of individuals and period of starvation. However, the increase in the total antioxidant capacity and the concentration of lipid peroxidation products in the 4th larval stage under the effect of cadmium was dependent on strain origin.
Related to industrial and urban pollution, high load of cadmium in soil has been commonly noticed. That may cause an increase of metal amounts in herbivore food, host plant, among the others, in the hyperaccumulators of cadmium. Various environmental factors such as short period of starvation and metal exposure may affect differently detoxification processes in insects. The one of the indicative of metal toxicity may be oxidative stress, and enhanced cost of antioxidant defence. Glutathione is the main thiol in living cells. It plays an important role in antioxidant defense. About 90% is present in reduced form. Excessive oxidation can result in the formation of more disulfide, oxidized glutathione [1]. In a study carried out on Spodoptera exigua, the activity of the detoxifying enzyme glutathione S-transferase was checked in larvae from the first and second generation of these insects treated with zinc and cadmium added to the medium. Increase in the activity of the GST enzyme in the fat body and Malpighi tubules caused by zinc and its reduction under the influence of cadmium have been observed [2]. The presence of cadmium in the diet of the last instar S. exigua larvae resulted in lower larval survival rate and longer larval stages duration. Cadmium also contributed to an increase of total antioxidant capacity in the larval hemolymph [3].The aim of this experiment was to compare cadmium and starvation effect on the 4 th instars larvae, assessing their survival rate, the level of reduced glutathione (GSH), and activity of sensitive environmental biomarkers: glutathione Stransferase (GST) and nonspecific esterase, between larvae of a common pest S. exigua originated from control and cadmium strains. They were exposed to cadmium during larval period and starving period (lasting 1 or 2 days during the 3 rd instar period). It is appeared that the 4 th instars larvae originated from cadmium strain characterised enhanced amount of GSH, higher activity of EST and GST, but only those which were not starved. Two days starving significantly decreased survival rate of the larvae. Sensitivity to two days lasting starvation, distinguished cadmium strain animals, for whom the lowering GSH level and drop in examined enzymes activity and the lowest number of survived animals were registered. A drop in GSH concentration was noticed for earlier starving larvae, exposed to cadmium and originated from control strain. It seems that in the case of esterases, lowering of the activity was growing with length of starvation period, similarly for control and cadmium strain animals. When, glutathione and glutathione related enzyme role seems differentiated the animals from control and cadmium strain, exposed to metal.
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