Farmers are spraying more chemicals on their crops than ever, and the insects are doing better anyway.
Pesticide use in agriculture more than doubled worldwide between 1990 and 2024, reaching 3.92 million metric tonnes of active ingredients, according to the United Nations Food and Agriculture Organization. Pests and diseases still reduce global crop yields by an estimated 20-40% every year, causing $220bn in losses.
Warming has expanded the range of many pests, extended their breeding seasons and, in some species, made them harder to control, Yale Climate Connections reports.
Warming does not help every pest. Where a species is already close to the highest temperature it can tolerate, extreme heat can make it less successful. But "in temperate latitudes, warming is more likely to increase pest developmental rates, making them more difficult to control", said Sanford Eigenbrode, an entomologist and professor emeritus at the University of Idaho.
Faster development also accelerates resistance, because insects that mature and breed more quickly give resistant individuals more chances to survive exposure and pass the trait to their offspring.
"In regions experiencing frequent heat waves, we could certainly see a decline in insecticide efficacy over time," said Aline Guidolin, an entomologist and professor at the University of São Paulo's Luiz de Queiroz College of Agriculture.
The diamondback moth, a major pest of cabbage, cauliflower and related crops, shows the pattern at scale. The past 50 years of warming have expanded the area where it can survive winter by about 2.4 million square kilometres. Researchers found that resistance levels were, on average, 158 times higher in areas where the moth could overwinter than in areas where it occurred only seasonally.
In Brazil, fall armyworms were tested between 18C and 32C. At 28C, killing resistant insects required a concentration of the insecticide spinetoram 340 times higher than was needed to produce the same mortality in susceptible ones. Guidolin, who co-authored that study, said higher temperatures speed the insects' metabolism, letting them break down and detoxify insecticides faster.
In China, researchers studying Tetranychus truncatus, a spider mite that attacks more than 100 crops including maize, cotton and soybean, identified five genes involved in producing heat-shock proteins, which protect cells from stresses such as extreme heat and pesticides. Three became much more active at 38C to 42C; two also became more active after exposure to sublethal doses of three acaricides. When the team switched those two off, mortality rose by up to 35% under heat and by up to 38.8% after pesticide exposure.
Entomologists recommend integrated pest management: monitoring fields and spraying when it makes economic sense rather than on a calendar, using natural predators, adjusting planting windows, choosing resistant varieties and rotating chemicals. "As temperatures rise, chemical rotation alone won't be enough," Guidolin warned. The approach is harder for smallholders with limited access to training and resources.
The mite study is the one that settles the shape of the problem. The same two genes answer to heat and to acaricide. Switch them off and the animal becomes easier to kill with either.
So the weather and the sprayer are not two separate pressures on a pest population. They are the same examination, marked by the same gene, sat twice a season across most of the world and continuously in the places that no longer have a winter.
Our analysts accordingly record a global selection programme of unusual scale and consistency. Every warm night is a heat trial and every application is a chemical trial, and because the two trials share a pass mark, each sitting improves the candidates' performance in the other.
The Department of Unnecessary Research notes that the programme has produced a measurable result. In the places where it has run longest without interruption, the diamondback moth is 158 times better at it.