Changes in plant physiology during cultivation periods influence the hydrogen isotope ratio of the water-insect relationship

Abstract

1. The hydrogen stable isotope ratios (δ2H) in tissues of terrestrial insects are widely applied to estimate natal origins in field populations. The hydrogen isotopes of insect tissues incorporate those of environmental waters through the insects′ metabolic processes. Water sources and abiotic environmental factors reflect changes in plant physiology, as indicated by the δ2H values of plants. However, the influence of plant physiology on the assimilation of hydrogen in insect tissues derived from water through feeding diets is still unknown. 2. We experimentally examined the influence of water on the δ2H values of maize leaf and of forewings ofMythimna separata(Lepidoptera: Noctuidae) andSpodoptera frugiperda(Noctuidae). We prepared five specific water samples for cultivating maize, aiming to replicate the gradient of δ2H values observed in environmental waters across the Japanese archipelago. 3. The mean-percentage contribution of water to hydrogen in maize leaves was 17.6% (July-August) and 25.1% (September-October). Linear analyses indicated that 17.4% and 32.7% of hydrogen inM. separataandS. frugiperdaforewings were derived from water through the consumption of maize leaves. The slope values of linear regression between the insect forewings and the maize leaves supplied in the final instar were closest to 1.0. These results indicated that the δ2H values of maize leaves and insect forewings reflected those of water resources. 4. This study presented the seasonal changes in climate conditions that affected the δ2H values of host plants and insect tissues. Changes in plant physiology with seasonal variations may influence the interpretation of the linear relationship between water and insect tissues on the estimation of natal origins.