Biological control does not stop at sunset. A new study explores current knowledge on nocturnal predator insects and arthropods in row crops, finding that activity among nocturnal predators can be comparable to or even greater than in the daytime. Common among nocturnal predators in crop fields at night were harvestmen, in the arachnid order Opiliones, the researchers say. (Leiobunum vittatum photo by Martha O’Kennon via iNaturalist, CC BY-NC 4.0)

By Grant Bolton, Ph.D.

Hannah Stowe, Ph.D.

When night settles on a corn field, to the human eye, the field has gone quiet.

But throughout the canopy, the night is alive with unseen activity. Whether it’s an egg clinging to a leaf or a caterpillar feeding in the dark, insects and arthropods are busy. Some of these are pests, and some are predators are searching for their next meal. During the day, pest insects and their natural enemies are easier to see and study. After sunset, a different cast moves through the field.

That hidden nighttime world is the focus of a recent article by Hannah Stowe, Ph.D., and colleagues at the University of Nebraska-Lincoln, that examined existing knowledge and research on nocturnal predator insects and arthropods in row crops, published in April in the Annals of the Entomological Society of America.

The article reviews research showing that biological control does not stop at sunset. Nocturnal predation may match or even exceed daytime predation in some systems. But because researchers and growers often observe crop fields during the day, some predators helping suppress pests may be overlooked.

For Stowe, now with the University of California, Riverside, the topic brings together insects, behavior, ecology, and agriculture.

Row crop fields are often described by what has been removed from them. They are intensively managed, disturbed, and usually planted to a single crop across many acres. But Stowe says that framing can make it too easy to miss what is still happening inside those fields.

“In Nebraska, farmland is different from [the ecosystem] it was before. It’s been highly disturbed. It’s been changed from what we would have seen if it were pristine prairie,” she says. “But it’s still an active and functioning ecosystem.”

That functioning ecosystem includes predators, parasitoids, detritivores, weeds, field margins, crop residue, shifting weather conditions, and daily cycles of light and darkness. Those cycles matter because arthropods do not all work on human schedules. The predator community a researcher observes at noon may not be the same as the one active at midnight.

Stowe says this is especially important for generalist predators, which may not specialize on a single pest but can still contribute to background pest suppression.

“We, as entomologists, do a lot of work with parasitoids or specific kinds of predators that can control the focal pest,” she says. “But I think there’s a lot of this background predation that’s occurring from these generalist predators and the predatory complex.”

That background predation is easy to undervalue because it can be difficult to see. A parasitoid may leave behind an aphid mummy. A pollinator may leave behind a better seed set. But a predator may leave behind nothing more obvious than a pest that never hatched, never fed, and never appeared in a scouting report.

“Predation is like an active thing,” Stowe says. “So, you have to be able to catch it when it’s happening to be able to measure it.”

Researchers can use sentinel prey, cameras, direct observation, gut content analysis, and DNA-based tools to study these interactions. As part of her dissertation work, Stowe used time-lapse cameras with sentinel prey and infrared lights to document predator activity after dark.

Biological control does not stop at sunset. A new study explores current knowledge on nocturnal predator insects and arthropods in row crops, finding that activity among nocturnal predators can be comparable to or even greater than in the daytime. As part of her dissertation work at the University of Nebraska-Lincoln, Hannah Stowe, Ph.D., used time-lapse cameras with sentinel prey and infrared lights to document predator activity after dark. (Photo courtesy of Hannah Stowe, Ph.D.)

Biological control does not stop at sunset. A new study explores current knowledge on nocturnal predator insects and arthropods in row crops, finding that activity among nocturnal predators can be comparable to or even greater than in the daytime. As part of her dissertation work at the University of Nebraska-Lincoln, Hannah Stowe, Ph.D., used time-lapse cameras and infrared lights to observe nocturnal predators in front of sentinel prey placed in the field to lure in predators. (Photo courtesy of Hannah Stowe, Ph.D.)

The review found that predation by nocturnal predator insects and arthropods can be comparable to, and in some cases greater than, daytime predation. It also showed that day and night predator communities may differ. Stowe says arachnids stood out as she worked through the literature.

“It seems like there are more spiders active at night than there are during the daytime,” she says.

She also points to the arachnid order Opiliones, or harvestmen, as one group that may deserve more attention. “We have a lot of the Opiliones that are active in the crop fields at night and seem to be doing something,” she says. “They have to be eating something.”

For integrated pest management, this gap is more than an academic curiosity. If nocturnal predators are suppressing pests in ways that are not being measured, researchers and practitioners may be working with an incomplete picture of the system.

That does not mean growers can simply rely on nocturnal predators instead of managing pests. Stowe says farmers face real economic pressure.

“You’re never going to convince somebody that the bugs are more important than their bottom line,” she says.

Stowe sees nocturnal predation as especially timely because researchers now have tools that previous generations did not.

“We already have all of this technology that we can then bring to bear on this problem that’s been here forever,” she says. “Now we can address this.”

By morning, the field looks much the same as it did the day before. The work of the night is almost impossible to see. But that may be exactly the point. If a predator removes an egg before it hatches, there may be no damaged leaf, no larva to count, and no obvious trace of the interaction. There is only a pest that never became a problem.

Grant Bolton, Ph.D., is a freelance writer and technical marketing consultant with a Ph.D. in entomology based in the Missouri Ozarks. Email: [email protected].