Exposed to very low doses of glyphosate, fruit fly larvae (Drosophila melanogaster), they hunched over, increasing the frequency with which they pitched and altered their movements. When they were exposed to dodine, also in very low concentrations, they saw that the structure of certain worm proteins changed. The problem is that neither glyphosate nor dodine are insecticides. The first is a herbicide and the second is a fungicide. It is one of the findings of a series of experiments with more than a thousand agrochemicals published today Thursday in Science. Most of these compounds do not kill the fly, they are not designed to do so, but they alter its behavior and development, compromising its survival. This discovery could help explain why world is running out of insects.
“It is very disturbing,” says researcher at the European Molecular Biology Laboratory (EMBL), Lautaro Gándara, about these effects. “A molecule that is labeled insecticide and sold as an insecticide is not that different from a herbicide or a fungicide; chemically they have a very similar structure. So it makes sense that, despite what the label says, that they sell it as something different, if it shares such a similar chemical identity, it is expected that they will have the same effects,” he adds. Gándara is the first author of the work published in Science in which, with a robust series of experiments, they have studied the non-lethal effects of 1,024 agrochemicals on the fruit fly. Among the compounds there are well-known insecticides such as neonicotinoids or pyrethroids, but there were also herbicides, acaricides, fungicides, plant growth inhibitors and even rat poisons. “There are previous works that tried to do this, but comparing a couple of molecules. No one had ever tried it for such a large library of a thousand molecules,” completes the molecular biologist.
To measure the effects of the different agrochemicals, they exposed groups of larvae in their third stage (the one prior to the pupal phase) to three different dosages. Two of them (20 micromoles, and 200 micromoles, µM) correspond to the usual range for the application of the pesticide. The third, 2 µM, is an estimated value of the presence of these compounds in the environment some time after use, actually measured in water from lakes and ponds far from crop fields. Most of the hatchlings exposed to the first two doses of insecticides died. This was expected in compounds designed to kill insects. But they observed something else, regardless of the dose, 57% of the agrochemicals affected the behavior and development of future flies. What is striking is that 384 of these molecules were not insecticides. Hence Gándara’s disturbance.
“In control populations (not exposed to substances), the frequency in which they slouch is very low, the time in which they nod, searching for food, is different, and so are their ‘movement patterns’,” he highlights. the EMBL biologist. “Now, what do each of these behaviors mean? We don’t know, it’s not clear what it means that they slouch more. What we do know is that it is not natural.” And that is one of the messages of this work: “An insecticide is supposed to kill insects. Even herbicides, fungicides… are molecules designed to kill life forms. But what we are detecting are sub-lethal effects at concentrations well below those that kill organisms,” he adds.
In this case, what doesn’t kill doesn’t make you fat: “What we show is that even in conditions several orders of magnitude below lethal concentrations, the physiology and behavior of these insects can be affected so profoundly that it can compromise long-term survival at the population level. For example, in the real world, less mobility exposes the larva to its main predator, a wasp, for more time. Another effect has to do with the following generations: “In the concentration that we used them, the agrochemicals did not kill a single fly. However, those flies are laying half as many eggs,” Gándara concludes.
Heat enhances non-lethal effects
In another set of experiments, they manipulated the temperature during the night that the fly larvae were exposed. From the 25º of the initial condition, they rose to 27º, where no changes were observed. But when doing so up to 29º, a whole series of aberrant behaviors were unleashed and formal changes were produced in certain insect proteins. Although they do not know why this happens, their hypothesis is that the thermal increase would affect a series of biochemical reactions in animals that are ectotherms, considered cold-blooded. This result has great relevance in the present context of climate change that is already affecting the insect behavior.
The last thing the researchers did was study whether what they had discovered with fruit fly larvae could be happening with other insects. The D. melanogaster It is a basic research model, among other things, because it is very easy to breed and manipulate. But doing it with other species and doing it with thousands of specimens is much more complicated. Even so, they repeated part of their experiments with a pollinator, the thistle vanessa, one of the most recognizable butterflies, and with a mosquito, the Anopheles stephensiknown vector of malaria. They exposed several populations to three agrochemicals, the aforementioned diodine and two insecticides, a neonicotinoid, which is not supposed to harm non-target species, and a pyrethroid. Without killing any of the larvae of the former, they showed less mobility or strange movements (see video). As for the butterfly worms, only one of the insecticides killed some, but all of them slowed their movements.
For Francisco Sánchez Bayo, associate professor at the School of Environmental and Life Sciences at the University of Sydney (Australia), the merit of this work is in demonstrating that “the exposure of insects to non-lethal residues of pesticides is more important than what you could think.” Sánchez, not related to the study, told SMC Spain that the most worrying thing is the negative effects of the mixtures on the animal’s reproduction. “This confirms what we already indicated a few years ago, saying that pesticides—not just insecticides, but all other phytosanitary products on the market—are an important cause of insect decline, even more so than climate change.” If what was discovered by Gándara and his colleagues became widespread, one of the causes of the widespread decline in populations would have been solved. Until now, the focus was on habitat destruction, the lethality of insecticides and climate change itself. The non-lethal effects of pesticides would complete the cavalry of the insect apocalypse.
The researcher from the agricultural production department of the Higher Technical School of Agricultural Engineers and Biosciences of the Polytechnic University of Madrid, Ana Belén Muñiz goes beyond the alarm that works like this may cause: “the results obtained may serve to improve precision in the selection and application of agrochemicals”. It is not about pesticides yes or pesticides no: “It seems evident that the next generation of pesticides should be subjected to more exhaustive tests focused on sublethal effects in different representative species and not only focus on their possible lethality, since it could be camouflaging the long-term impact on these key organisms.”
The senior author of the experiments, EMBL researcher Justin Crocker, recalls that agrochemicals are essential to maintain crop yields and food security. “The goal is not to eliminate them, but to use them more carefully,” he says. In fact, safer alternatives and more specific products are already being developed. With work like yours, it will be possible to improve environmental risk assessments and adopt integrated pest management, which will, he concludes, “reduce damage to insects and, at the same time, guarantee agricultural productivity; “It’s about balance, not fear.”
