Not all isotopes make good geolocators: trophic transfer reveals contrasting geolocation potential in eastern spruce budworm

Scientists have long sought a way to tell where an animal has been just by looking at its body. The idea relies on a simple truth: the water, soil, and air in different places contain unique chemical signatures. When plants drink from local water or pull nutrients from local soil, they absorb these signatures. Animals that eat those plants then carry those same chemical marks in their own tissues. By measuring these marks, researchers can often trace an insect or a bird back to its birthplace or migration path. This method, known as stable isotope geolocation, has become a standard tool for tracking wildlife, especially for creatures too small to carry electronic tags. However, the entire approach rests on a critical assumption: that the chemical signals move from the plant to the animal in a predictable, reliable way. If the animal changes the signal too much while digesting its food, or if the signal gets lost in the process, the map becomes useless.

A team of researchers set out to test this assumption using the eastern spruce budworm, a moth that feeds on trees in eastern Canada. They collected moths and the needles of their host trees from known locations across the region. In the lab, they measured five different chemical markers in both the foliage and the moths to see how well each one traveled from the plant to the insect. The results revealed that not all chemical markers are created equal. Some transferred with remarkable precision, while others failed almost completely.

The study found that two specific markers stood out as excellent tools for tracking these moths. One marker, related to the way sulfur behaves in nature, moved from the tree needles to the moth with almost no change in its value. It showed a near-perfect match between the plant and the insect, and because the sulfur patterns across eastern Canada are already well mapped, this marker offers a clear path to pinpointing where a moth came from. A second marker, which reflects the age of the rocks in the local soil, also performed well. While individual moths were sometimes too small to give a clear reading on their own, when the researchers combined samples from several moths, the signal from the soil rocks shone through clearly, matching the local geology with high accuracy.

Another marker, tied to nitrogen, also transferred very faithfully from the tree to the moth. The researchers observed a consistent shift in the value, meaning they could predict exactly how the moth's body altered the signal from the plant. This makes it a powerful tool, provided scientists can first map out the nitrogen patterns in the trees across the region. However, the study also uncovered significant limitations. A marker based on carbon showed only a moderate connection between the plant and the moth, with too much variation from one spot to another to be useful for precise tracking. Most surprisingly, the marker based on hydrogen, which is currently the most common tool used for geolocating insects, performed very poorly. The hydrogen values in the moths were barely related to the values in the trees they ate, with so much random variation that the signal was effectively lost.

These findings challenge the idea that a chemical marker is inherently good or bad for tracking animals. Instead, the study shows that the usefulness of a marker depends on a complex mix of factors: how the environment distributes the chemical, how the plant absorbs it, and how the animal processes it. For the eastern spruce budworm, sulfur and strontium offer a reliable map, while hydrogen, despite its popularity, fails to tell the story. This means that scientists cannot simply apply a standard list of markers to every species. To track any animal accurately, researchers must first understand how that specific animal interacts with the chemistry of its local food and environment.