Decline ofEulia ministrana(Lepidoptera: Tortricidae) in polluted habitats is not accompanied by phenotypic stress responses

Abstract

Environmental pollution is currently identified as one of the major drivers of rapid decline of insect populations, and this finding has revitalized interest in insect responses to pollution. We tested the hypothesis that the pollution-induced decline of insect populations can be predicted from phenotypic stress responses expressed as morphological differences between populations inhabiting polluted and unpolluted sites. We explored populations of the brassy tortrix Eulia ministrana in subarctic forests along an environmental disturbance gradient created by long-lasting severe impacts of aerial emissions of the copper-nickel smelter in Monchegorsk, northwestern Russia. We used pheromone traps to measure the population densities of this leafrolling moth and to collect specimens for assessment of three morphological stress indices: size, forewing melanization, and fluctuating asymmetry in wing venation. Wing length of E. ministrana increased by 10%, and neither forewing melanization nor fluctuating asymmetry changed from the unpolluted forest to the heavily polluted industrial barren. However, the population density of E. ministrana decreased 5 to 10 fold in the same pollution gradient. Thus, none of the studied potential morphological stress indicators signaled vulnerability of E. ministrana to environmental pollution and/or to pollution-induced environmental disturbance. We conclude that insect populations can decline without any visible signs of stress. The use of morphological proxies of insect fitness to predict the consequences of human impact on insect populations is therefore risky until causal relationships between these proxies and insect abundance are deciphered.

The Fluctuating Asymmetry of the Butterfly Wing Pattern Does Not Change along an Industrial Pollution Gradient

“…However, we found that the FA values in the two wing spots were not related to each other. This finding, which is in line with earlier studies of FA in multiple traits of butterflies and moths [30,58,59], seemingly contradicts the outcomes of meta-analysis, which confirmed the existence of the organism-wide response in FA [25]. However, the among-trait correlation in FA is so small (an average effect size of 0.05 [25]) that it can hardly be detected in an individual study, keeping in mind the obvious constraints of sample size.…”

“…Contrary to optimistic expectations [51][52][53], the accumulated evidence does not confirm that FA consistently increases in living beings facing unfavourable conditions during their development [22,54]. The studies of insects detected both positive associations between pollution and FA [55] and absences of correlation between these variables [30,56]. We estimate that, across organisms and traits, the support for this hypothesis was found in no more than half of the examined data sets [57].…”

“…At the same time, the wing length of the autumnal moth Epirrita autumnata (Bkh.) did not change along the Monchegorsk pollution gradient [41], and the wing length of the brassy tortrix Eulia ministrana (L.) increased by 10% [30]. Thus, although metal toxicity is a likely reason for the recorded decrease in the size of butterflies accompanying the increase in pollution, experimental data are required to uncover the immediate reason behind the observed effect.…”

“…We considered butterfly species and sex as fixed effects, the log-transformed distance from the smelter (a proxy of pollution load: Kozlov et al [9]) as a covariate, and the site as a random effect. Following the practice commonly accepted in observational studies [28][29][30], we preferred to use the distance from the polluter rather than the concentration of one of the main pollutants (nickel or copper) in plant foliage or in a litter. This was done to avoid misinterpretation of our results, because we do not know which of the pollution-related factors (which all change with the distance from the smelter in a coordinated manner) may have affected the butterflies.…”

Population dynamics of herbivorous insects in polluted landscapes

“…[30]. Combined with a strong positive correlation between concentrations of trace elements (nickel and copper) in butterflies and in plants within the Monchegorsk pollution gradient [32], these results suggest that migration to or from polluted sites contributes little to herbivore population dynamics compared with pollution effects on insect fecundity and mortality.…”