As Global Insect Populations Decline, Crop Pests May Not

The insects that survive the coming decades may not be the ones we want

Insects are on the decline. A 2017 study from Germany found that between 1989 and 2016, total insect biomass declined by 77 percent, and a recent global survey found that 40 percent of worldwide insect species could be in danger of extinction in the coming decades.

photo of a aphid infestation
Aphids infest a field in Limuru, Kenya. Photo by Tamzin Byrne.

Already there has been some pushback on these numbers. The global survey, published in Biological Conservation, looked at 73 insect studies, which is a relatively small sample size. In addition, both the German study and the global survey were based on insect counting done mostly in Europe and North America, leaving out the myriad of biodiversity in the global south. 

Still, no matter the exact extent of the problem, entomologists agree that the situation is serious given insects many ecological roles, including providing food for birds and bats; pollinating plants; and facilitating decomposition. “The biggest takeaway is that there’s a body of evidence that’s telling us that as a group, insects are experiencing rapid declines and this has significant implications for the functioning of natural systems as well as food security and agriculture and human health,” says entomologist Helen Spafford, who specializes in invasive species.

Even though insect populations overall are decreasing, that doesn’t necessarily mean that every insect species is declining. In fact, the loss of some insect species could mean an increase in others, and the species that thrive may not be the ones we want — species like crop pests, for example.

“Crops pests on the whole tend to be creatures that to some extent just got lucky and their favorite food plant is something that just got cultivated as a crop,” says Dave Goulson, an entomologist at the University of Sussex. “Pests also tend to have other characteristics that make them prone to being pests, particularly the ability to breed fast.”

Fast breeding means they can produce many generations in the span of a year, making their population numbers high. It also means that they develop pesticide resistance faster because each new generation gives the species a chance to evolve and increase resistance. Pests also tend to be generalists and can feed on a number of different plants, so if they lose one source of food, they can usually find another. Specialists, on the other hand, are the insects that tend to be disappearing because they can’t easily adapt to the changing world. When their habitat is lost, or their preferred food becomes scarce, they are more likely to disappear. While the 2017 study found that insect species are declining overall, specialists and generalists alike, the specialists, according to Goulson, “tend to be the ones that actually go extinct.”

For similar reasons, Spafford says that invasive insect species, many of which are also crop pests, will also likely thrive as overall insect populations go down.

“Invasive species as a group tend to be highly adaptable and have a broad tolerance of different environmental conditions,” she says. “Some invasive species might experience some declines, but yes we’ll probably see invasive species generally doing better, expanding their ranges, and having greater impact.”

Spafford adds that there will probably be new invasive insects colonizing areas they haven’t before, because there will be less local competition as environmental stressors cause native species to decline, making hosts and resources more accessible to nonnative species.

Aphids and diamondback moths are two insect species that could do especially well as overall insect populations decline. Aphids are generalists and can be highly damaging to crops, both by feeding on them and also by acting as plant disease vectors. They reproduce quickly because they don’t need sex to do so — female aphids can just churn out new female aphids. The diamondback moth feeds on brassica crops, like cabbage and broccoli. It is a worldwide pest, highly mobile, can survive in many different climates, and has a high level of resistance to insecticides. “This is an extremely adaptable pest,” Spafford says.

Meanwhile, the insects that might prey on these pests tend to be far more vulnerable than the species they prey upon. According to Goulson, some important predators of crop pests reproduce less frequently, perhaps only once a year. Earwigs only have one generation per year as do almost all species of ground beetles. And lady beetles, which feed on aphids, have fewer generations each year than their prey. This gives them less ability to build resistance to pesticides. If a pesticide is sprayed, the pests will be able to rebuild their populations far more quickly than the predators.

photo of a lady bug eating aphid
Lady beetles, also known as lady bugs, reproduce more slowly than the aphids they feed on. Photo by Elizabeth Weller.

“You tend to get what’s called ‘resurgence’ of pests after you spray a pesticide,” says Goulson, “because you take out the natural enemies and the pest is the first thing to recover and it often comes back stronger.”

This counter-productive effect is one reason that both Goulson and Spafford recommend minimizing the use of chemical pesticides. Other arguments against heavy pesticide use include the impacts of these chemicals on human health, run-off into water sources, and of course, the negative impact of pesticides on biodiversity and insect populations as a whole.

Integrated pest management (IPM), the practice of controlling insect pests by using a combination of natural enemies, bio-pesticides, cropping systems, beneficial fungi, and other techniques to reduce the use of chemical pesticides, offers an alternative, fostering biodiversity as a means of protecting crops. As insect populations continue to decline, however, IPM could become more difficult. Because predator populations are likely to face steeper declines than pest populations, it will become harder to control crop pests without the use of chemical pesticides — the natural insect enemies will no longer exist. And then as more chemical pesticides are used, insect populations will continue to decline, perpetuating a vicious cycle.

Exactly how the decline of insect species will impact individual crops and pests remains unknown at this point. Spafford would like to see more studies, looking not just at sheer insect numbers but at the more specific changes to ecosystem relationships as well, along with examination of biodiversity changes in different parts of the world. And of course, science needs financial support, so she hopes that there will be more consistent and comprehensive funding to support this work.

“What we’re seeing is kind of the tip of the iceberg in my opinion in terms of insect declines,” says Spafford. “I’ve described it before as this is the alarm clock that goes off in the morning and it’s that little blaring buzzing noise that’s really annoying and you just want to turn it off, but we can’t afford to ignore it.”

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