The first global picture of insect activity at night and day has been revealed thanks to a meticulous study, say scientists from the University of Western Australia.
Led by Dr Mark Wong and published in Nature Communications, the research included the observation of more than three million individual insects.
And the "definitive answer"?
“Our global analysis not only shows that insect activity rises by a third (31.4%) during the night on average," says Dr Wong, "but crucially reveals key ecological factors driving patterns in insect activity across the Earth.”
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How to spot insect patterns
Insect activity across the day-night cycle is a surprisingly understudied area in ecology. This is because patterns are hard to detect using common insect sampling methods; sweep netting, for example, captures inactive insects; and light traps don't really work in the daytime.
Noting the difficulties, Dr Wong and his team identified previous field studies that used pitfall traps, flight interception traps and aquatic drift nets – all methods that measure insects' movement during the day and night.
They found 99 suitable studies on which to base their research, dating from 1959 to 2022, with sample sites ranging from tropical jungles to temperate forests, and arid grasslands to aquatic ecosystems.
In all, the researchers observed more than three million individual insects.
Why are some insects nocturnal and others diurnal?
The results of the study reveal multiple ecological factors that influence insect activity patterns.
Certain insect groups were found to be more abundant during the night (nocturnal), such as mayflies, caddisflies, moths and earwigs. While others, including thrips, bees, wasps and ants, were found to be more abundant during the day (diurnal).
Nocturnal activity was also more common in rivers and streams. In some instances, in these aquatic habitats, twice as many insects were recorded being active during the night.
Conversely, land-based insects generally had higher diurnal activity. This proved particularly pronounced in savanna and grassland areas, where the number of active insects sometimes tripled during the day.
“Fish tend to hunt aquatic insects during the day, whereas nocturnal animals such as bats make life on land more hazardous at night,” says Dr Wong, suggesting that the contrasting patterns in terrestrial and aquatic ecosystems may be the result of insects trying to avoid predation.
“Our global analysis not only shows that insect activity rises by a third (31.4%) during the night." Dr Mark Wong
The paper also highlights the influence of temperature on insect activity. “We found peak levels of nocturnal activity in the warmer regions of the world, as more insects likely sought refuge from the heat of the day,” says Dr Wong.
“Our findings point to the threat of global warming. In the hottest regions in the world such as the tropics, the warming trend may further reduce the activity of nocturnal insects that are already struggling to cope with heat.
"On top of that, artificial lighting is detrimental to nocturnal insects, disrupting their natural behaviours in ecosystems.”
Dr Wong highlights the importance of ecological fieldwork conducted over the years:
“Such research can be exhausting, as traps need to be installed and replaced multiple times over days and nights, across many different locations. But it is absolutely indispensable.
"Insects are among the most diverse and important organisms on our planet, and studying their intricate rhythms represents not just a scientific endeavour, but an imperative for conserving biodiversity on a rapidly changing planet.”
Find out more about the paper: Wong, M. K. L. & Didham, R. K. (2024). Global meta-analysis reveals overall higher nocturnal than diurnal activity in insect communities. Nature Communications.
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