Inferring origins of migrating insects using isoscapes: a case study using the true armyworm, Mythimna unipuncta, in North America

Abstract

1. Many important insect pests undertake seasonal migrations at continental scales in response to changes in resource quality and availability. The frequency and timing of these events could be influenced by the impact of climate change on the suitability of the different sites exploited throughout the year, yet, in many cases, little is known about the origin of seasonal populations, as tracking insect movements is extremely challenging due to their small body size.2. The use of stable isotope measurements in insect tissues combined with the development of tissue-specific 'isoscapes' of modelled geographic isotope patterns presents a potentially valuable but rarely used approach for obtaining such information on important pest species. In this paper it is illustrated how stable hydrogen isotope analyses ( 2 H) in wing chitin of the true armyworm (Mythimna (Psuedaletia) unipuncta Haworth), a seasonal migrant, clearly delineated between 2016 spring immigrants and later locally produced moths in southern Ontario, Canada.3. It is shown that adult moths captured in early fall in Texas were immigrants from farther north, the first direct confirmation of a southward return migration of this species. Stable carbon isotope ( 13 C) measurements indicated that spring immigrants in Ontario and autumn immigrants in Texas were from exclusively C3 biomes. Stable nitrogen isotope ( 15 N) measurements also provided information on the probability of individuals coming from agriculturally intensive (i.e. higher 15 N) sites. Finally, several recommendations are provided regarding future research that could improve the Bayesian assignment models and thus improve assignment accuracy.

Expanding the Isotopic Toolbox to Track Monarch Butterfly (Danaus plexippus) Origins and Migration: On the Utility of Stable Oxygen Isotope (δ18O) Measurements

“…Metabolically inert tissues such as animal keratins and chitins are related to local growing season averaged δ 2 H p and can be later measured to infer natal origins once the relationship between δ 2 H p and tissue δ 2 H is established (Hobson, 2019). This principle has been used effectively to infer origins of numerous taxa including insects, birds and mammals on several continents (Hobson et al, 2018a;Hobson and Wassenaar, 2019).…”

Contribution of larvae developing on corn and dry beans to the adult population of western bean cutworm in Michigan

“…This is possible due to the strong isotopic differences of the stable isotopes of carbon (! 13 C) found between C3 and C4 plants (Adams et al 2016, Hambäck et al 2016, Hobson et al 2018). Tissues lacking large amounts of cellular turnover can be used to determine if the insect developed on C3 (dry beans) or C4 (corn) photosynthesizing plants.…”

Background and the use of isoscapes in the Brazilian context: essential tool for isotope data interpretation and natural resource management

“…Assignment models use tissue isoscape as a probability surface, where an unknown origin sample value may be assigned to a region as a function of a normal distribution, given its isotopic value and the expected standard deviation (Wunder, 2012;Reed et al, 2018;Hobson et al, 2018). For that reason, the construction of tissue isoscapes is a priority for studies on animal migration.…”

“…Studies about migratory species using isoscapes in countries with few available data should elaborate ground-validated models for the species in question (Gutiérrez-Expósito et al, 2015). The tissue-calibrated isoscape can be made by using the residual from a regression analysis between the environment isoscape and the target tissue isotopic value (Hobson et al, 2018). The tissues to calibrate the isoscapes must have known origins.…”