This post contains only the summary of a longer research post, written by Meghan Barrett and Hannah McKay. The full post can be found at the above link on Rethink Priorities website.


One aim of wild animal welfare research is to identify situations where large numbers of wild animals are managed by humans in ways that have significant welfare impacts. If the number of individuals is large and the welfare impacts significant, the issue is important. As humans are managing these animals, it is possible the welfare impacts could be moderated to reduce their suffering. The massive scale of invasive (e.g., non-native) Lymantria dispar dispar (spongy moth) outbreaks represents an unappreciated wild animal welfare issue, and thus deserves further attention from a welfare (not simply an invasive species-control) perspective.

The spongy moth is not endemic to North America. The species experiences localized three year-long outbreaks of half a billion or more caterpillars/km2 every 10-15 years in regions where they are well established (including their native range). Spongy moths currently occupy at least 860,000 km2 in North America, only ¼ of their possible range (though most of the occupied area is not experiencing outbreak conditions, most of the time). L. dispar continues to spread slowly to new areas each year despite multi-million dollar efforts to stop expansion. Assuming spongy moth caterpillars are sentient, methods for actively controlling them during outbreaks cause substantial suffering. The aerial spray (Btk) ruptures the stomach, causing the insect to die from either starvation or sepsis over two to seven days. However, because outbreaks are so large, most caterpillars are not actively targeted for control, and ‘natural forces’ are allowed to reduce the outbreak. The most prominent natural forces to rein in an outbreak are starvation and disease. The accidentally introduced fungus, Entomophaga (meaning “insect eater”) maimaiga, digests caterpillars’ insides before pushing through the exoskeleton to release spores, usually within a week. LdNPV virus is also common in the spongy moth population, but only causes high levels of mortality during outbreaks when larvae are stressed from extreme competition. A symptom of severe LdNPV infection is “larval melting,” or the liquefaction of the insect’s internal organs.

The scale of spongy moth outbreaks is tremendous, though notably these outbreaks are not necessarily higher-density than numbers of other insect species (e.g., 740 million to 6.2 billion individual wireworms/km2; Smithsonian, n.d.). However, spongy moths are one of the best tracked non-native insects (Grayson & Johnson, 2018; e.g., Stop the Spread program), providing us with better data for analyzing the scale of the welfare issue (both in terms of caterpillar density within outbreaks, and the total area affected by outbreaks). In addition, there is potential for significant range expansion by spongy moths that would increase the scope of the welfare concern, and there appears to be extreme suffering1 induced by both active and natural outbreak control. As a result, spongy moth welfare during outbreaks could be an issue of concern for animal welfare advocates. Further research could improve spongy moth welfare by: 1) identifying the most promising long-term interventions for preventing/reducing the occurrence of outbreaks behind the invasion front, 2) contributing to halting the spread of spongy moths into new areas, and 3) identifying the highest-welfare outbreak management strategies where outbreaks do occur.


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