Effects of decreased O2 and elevated CO2 on survival, development, and gene expression in cowpea bruchids

Use of modified atmospheres with depleted O2 and/or elevated CO2 is an environmentally friendly alternative to currently used fumigants for control of stored grain insect pests. In the present study, we examined the impact of hypoxia and hypercapnia on cowpea bruchids (Callosobruchus maculatus), a storage pest of cowpea and other legumes. Two O2/CO2 combinations were used; (i) 10% O2 + 10% CO2, (ii) 2% O2 + 18% CO2. In both cases, N2 was maintained at 80%, equivalent to normal atmospheric concentration. In ambient atmosphere, the rate of O2 consumption and CO2 output at different stages (from low to high) was: eggs ≈ 1st instar < 2nd instar ≈ pupae ≈ adults < 3rd instar < 4th instar. When exposed to 10% O2 + 10% CO2, eggs, larvae and pupae were able to complete development and successfully enter the next developmental stage, although developmental time and mortality varied at different stages. In contrast, more severe hypoxic/hypercapnic treatment, i.e. 2% O2 + 18% CO2, led to cessation of development of all stages. Effects on eggs and adults were most dramatic as they could only withstand 2–3 days exposure. Further, eggs at early (4–6 h old) and later stages (102–104 h old, black-headed) were more susceptible compared to those at intermediate stage (52–54 h old). The 3rd and 4th instar larvae were least sensitive and could survive up to 20 days treatment. To gain some insight into molecular mechanisms underpinning the hypoxic/hypercarpnic response, we performed qPCR reactions on selected metabolic genes involved in TCA cycle and in protein digestion, as well as genes encoding stress-responsive heat shock proteins. Patterns of gene expression and proteolysis suggest that cowpea bruchids suppress their metabolic activity and increase stress tolerance when challenged by O2 deprivation. Transcript abundance as well as proteolytic activity recovered once normoxic conditions resumed. Taken together, cowpea bruchids were found able to cope with hypoxic and hypercapnic stress. This ability was particularly strong in the late larval stage.