Moths: some food for thought

By Athayde Tonhasca

A bear in a blog about pollinators? Well, read on.

A grizzly bear ©Tony Hisgett, Wikipedia Creative Commons

As any fan of Yogi Bear & Boo Boo knows, food is the main preoccupation of brown bears (Ursus arctos). They need to accumulate calories to survive the winter, so most of their waking hours are spent searching for something to eat. Almost anything of nutritional value would do: flowering plants, grasses, roots, tubers, berries, insects, small rodents, carrion, roadkill or human refuse. For the grizzly bear subspecies (Ursus arctos horribilis), finding food in the cold, harsh mountainous areas of the American states of Montana and Wyoming can be particularly tricky. But those bears can rely on one abundant and nutritious menu item: army cutworm moths (Euxoa auxiliaris).

Grizzly bear home in the mountains of Montana/Wyoming © Wikipedia Creative Commons, and a bear searching for moths among rocks ©

This moth, also known as miller moth, breeds in open grassland and farmland across the western United States. The adults migrate to higher elevations during the summer, where they feed on nectar from several plant species at night and gather under rocks during daytime. These moth aggregations are easy meals for a grizzly bear, which can put away 40,000 moths/day. A moth meal, however large, may sound insufficient for a creature weighing 180 to 360 kg (males). Until we learn that army cutworm moths are the fattest animals on record: up to 72% of their body weight is fat (the blue whale comes in at a distant 35%). Moths need this much fat as an energy source for their migration flights, which can cover 100 km/day. Grizzlies are very appreciative of these flying titbits of fat: their energy content average 7.0 kcal/g dry matter, which is more than blueberries (4.5), ground squirrels (5.3), trout (5.7) or pine nuts (6.5) can offer.

An army cutworm moth: a greasy meal to put any chippy to shame © Robert Webster, Wikipedia Creative Commons

Grizzlies are just a tiny minority of the animals that feed on insects. Insectivorous bats consume 30–100% of their body weight in food each night. Tits (Parus major and P. caeruleus) bring hundreds of caterpillars to their nestlings per day, at rates that reach almost one a minute during peak demand. Most birds are insectivorous or eat insects and other arthropods at some point in their development, and they are hungry: worldwide, birds eat roughly 400 million metric tons of prey biomass/year. This mindboggling figure corresponds to the annual energy consumption of a city the size of New York City. It’s not only birds and bats: other mammals, frogs and lizards feed on a variety of insects and spiders. Caterpillars, flies and beetles are particularly important in temperate forests and agricultural habitats. 

When we talk about biodiversity, we usually think about the variety of species, their distribution and interactions. But numbers of individuals are important too, as the size of a population dictates its productivity and determines whether a species has what it takes to make a difference for ecological services such as nutrient cycling, flood regulation, water purification and pollination. But simply counting life on Earth gives us a warped view of species’ relative importance for the obvious reason that size varies tremendously: lots of tiny organisms have much less impact than fewer but bigger ones. For a more useful comparison, life could be quantified as biomass. That’s exactly what a group of researchers did: they put together hundreds of studies to estimate the amount of carbon – the building-block of life and a proxy for biomass – produced by each of the major taxonomic groups. 

Unsurprisingly, plants came first, accounting for more than 81% of all of the planet’s biomass. Not so unsurprisingly, bacteria came second, with ~13%. Animals amount to just 3.6%. Plants rule the Earth, but arthropods (mostly insects, spiders and crustaceans) dominate animal life. Take a good look at the figure below. If you put together each human being, elephant, whale, cow, bird, and every other domestic animal, wild bird or mammal in the planet, it’s not enough to trump terrestrial arthropods. These figures help us put our importance in perspective.

Estimated relative biomass of some animal taxa. Data from Bar-On et al. 2018. The biomass distribution on Earth. PNAS,

We don’t have data specifically for insects, even less so for moths. But considering their species richness, abundance and relative large size, it is reasonable to assume that sustaining some grizzlies in American mountains is just a small part of their contribution to the functioning of ecosystems around the world. With time and more data, we eventually should discover other secrets from the secretive moths.