Nearly a Third More Insects Come Out After Dark, Study Finds

While evening and nighttime hours are generally known as a time of rest and sleep for humans and many animals, a great number of insects actually become more active after dark.

A new global study has found that insect activity increases by an average of 31.4 percent at night.

In their research, scientists from The University of Western Australia (UWA) looked at 99 studies comparing insect activity during the day and at night, published between 1959 and 2022, reported The Conversation. They found that, while an average of nearly a third more insects came out after dark, the numbers varied greatly depending upon location.

“There are many reasons for this. In some cases, it relates to the availability or enrichment of resources, such as flowers that bloom during specific times. In other cases, it involves avoiding competition or predation. For instance, many caterpillars of moth and butterfly species are nocturnal to avoid day-active predators such as ants, reptiles, and birds. Additionally, it has been suggested that many aquatic insects are nocturnal to avoid being eaten by fish, which are primarily day-active visual hunters,” Mark K. L. Wong, a Forrest Fellow in UWA’s School of Biological Sciences, told EcoWatch in an email.

“Environmental temperatures also play a significant role: some insects can tolerate the heat, while others need to take refuge from it and only come out at night. We still lack a complete understanding of the activity patterns of many insects and the underlying mechanisms driving them. This represents an exciting and important frontier for future research.”

^{A monkey grasshopper. Nicky Bay}

After looking through studies that had conducted day and night sampling of insect communities, the researchers excluded studies that had used methods like beating branches or sweep nets because they could result in the capture of both active and resting insects. They also excluded studies employing traps that were colored or lighted because they only attract insects in low light, hampering their effects during daylight hours.

The research team instead accepted studies that had used techniques that wouldn’t influence the results. These included those that had taken insect samples at night or during the day with traps that only caught moving insects — flight interception traps, pitfall traps and aquatic drift nets, for instance.

The team also referred to studies that had used food baits like dung for some beetles and honey for ants.

“One of the most memorable studies we encountered sampled mosquitoes using (unfortunate) human subjects as bait. Another had devised innovative automatic time-sorted pitfall traps to minimise the labour required, as the specimens collected would automatically be delivered into different compartments at different times of the day,” the researchers wrote in The Conversation.

In most of the studies they used, entomologists set traps before dawn and new ones before sunset, collecting samples at both times of day.

In order to improve their insect activity estimates, many of the researchers reported data from multiple field sites and days.

“High nocturnal activity may come as no surprise to entomologists and nature photographers. Many of us prowl through jungles wearing head torches, or camp next to light traps hoping to encounter these jewels of the night,” the researchers wrote in The Conversation. “But this is the first time anyone has been able to give a definitive answer to this universal childlike question. And now we know for sure, we can make more strident efforts to conserve insects and preserve their vital place in the natural world.”

In their analysis, the researchers found that more moths, caddisflies, mayflies and earwigs came out at night, while more bees, wasps, thrips and ants made appearances during daylight hours.

^{A mayfly. Nicky Bay}

They found nocturnal insect activity was more common in waterways and wetlands, with as many as two times the number of insects becoming active in these aquatic environments after dark.

“We found high numbers of insects out during the day in open habitats such as grasslands and savannas, and high nocturnal activity in wetland habitats such as river and stream systems. In general, we also found high nocturnal insect activity in warmer parts of the world, such as the tropical regions, which are also a hotspot for insect diversity,” Wong told EcoWatch.

Land-based insects tended to be more active during the day, the entomologists found, with triple the numbers coming out in daylight.

“This may have something to do with avoiding predators. Fish tend to hunt aquatic insects during the day, whereas nocturnal animals such as bats make life on land more hazardous at night,” the researchers wrote in The Conversation.

Warmer areas of the planet had more active nocturnal insects. Being “ectotherms” — unable to regulate body temperature — insects are very sensitive to temperature extremes.

“Insects are ectotherms, meaning their activity is heavily influenced by external temperatures. As the climate warms, there is growing concern about increasingly warm nights. Many insects may be nocturnal because daytime temperatures are too high for them. Further warming of nighttime environments could push these insects closer to their thermal limits or reduce their period of activity, limiting their ability to obtain crucial resources,” Wong told EcoWatch.

This highlights the importance of climate change’s impact on insect activity.

Higher maximum temperatures could promote increased nighttime activity as insects try to avoid heat stress.

“Our research also points to the threat of global warming. In the hottest regions of the globe such as the tropics, the warming trend may further reduce the activity of nocturnal insects that struggle to cope with heat. To this end, we hope our study motivates day-loving ecologists to embrace night-time ecology,” the researchers wrote in The Conversation.

Many of the essential ecosystem services performed by insects — pollination, “pest” control and nutrient cycling — are provided at night when they are active. This means humans must reduce some of their activities and lower levels of artificial light at night, which is detrimental to nocturnal insect activity.

Insect numbers have dwindled globally, but there are things individuals and communities can do to help.

“Insect populations are increasingly threatened by human activities such as land conversion and land use (e.g., deforestation and agricultural intensification), the extensive use of pesticides, and climate change. Advocating for the protection of natural habitats and making informed choices with our groceries can go a long way. For those with gardens, allowing them to grow naturally — instead of using weedkillers or mowing them into bare lawns — can create rich habitats for insects. Additionally, there is compelling evidence that artificial lights disrupt the behaviors of nocturnal insects. To mitigate this, we can minimise our use of nighttime lighting by simply switching it off when not needed,” Wong told EcoWatch.

The study, “Global meta-analysis reveals overall higher nocturnal than diurnal activity in insect communities,” was published in the journal Nature Communications.

“Insects are among the most diverse and important organisms on our planet. Studying their intricate rhythms represents not just a scientific endeavour, but an imperative for preserving wildlife,” the researchers wrote in The Conversation.

^{A hood mantis. Nicky Bay}