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Night shift, or when pesticides are harmful

Night shift, or when pesticides are harmful

There have recently been viral online calls for not spraying crops during the day, when bees visit the plants to gather food. Instead, it is suggested that the spraying should take place in the evening, when bees are already safe inside their hives. Refraining from spraying during the day is by all means right and can certainly limit the damage done by pesticides to some pollinators, but, unfortunately, will not make them completely harmless.

‘He did night shift in the field’; image by Dr Justyna Kierat showing two bees commenting on a dead moth

‘He did night shift in the field’; image by Dr Justyna Kierat

There are two main reasons why applying pesticides after sunset will not eliminate the problem. Firstly, fields are inhabited by a wide variety of organisms. The abovementioned initiative focuses on protecting honey bees, a vast majority of which don’t live in the wild but are kept by humans. Bee families live in hives, where they spend nights. Worker bees fly far, even several kilometres away from home, so they don’t necessarily live next to the blooming crops they visit to gather food. Anyway, when the evening comes, honey bees leave the fields and go back to their nests. Yet, they are just one of many species of pollinators inhabiting cultivated fields. Wild bees, represented in Poland by nearly 500 species, have a much shorter flight range (usually up to several hundred metres), which means that their nests are located on the fringes of the fields, within the spraying area. Besides, not all bees spend nights in their nests – the males usually leave their homes once and for all, and then spend nights on plants or in various hideouts. Other pollinators, such as hoverflies and butterflies, don’t build any nests, and some of them are most active during the night. Moths – the most important nocturnal pollinator insects – are certain to suffer from nightly spraying.

The second problem is related to the stability of pesticides. Applied in the evening they won’t miraculously vanish until morning. Event if the remaining amount does not instantly kill the insect, it can still inflict some considerable damage. The so-called sublethal effects, that is, harmful but not causing death, can weaken the immune system, making the insects more vulnerable to diseases, degrade their navigation skills, preventing them from returning to their nests, inhibit fertility, or cause memory and learning disorders. Some of the pesticides don’t decompose immediately, but tend to be very stable and cumulate after subsequent applications, which means that they are still present in the environment many years after use. It should also be noted that not all of the applied substance remains where it was used – it may be washed away and travel to other places, posing danger even to those insects that stay away from cultivated areas.

To sum up, acting in accordance with recommendations and good practices related to the use of pesticides is very important and commendable, but the only fully effective way to protect pollinators from harmful substances is not using them at all. Research has shown that reducing the use of pesticides in agriculture is really possible and instead of increasing the costs of agricultural production, it can actually reduce them (by cutting the expenses of plant protection and increasing crop yield of insect-pollinated plants). Using less pesticides will bring best effects in cultivated areas with significant amount of uncultivated land, such as buffer strips with trees, shrubs, and flowering plants.

The article authored by Dr Justyna Kierat from the JU Institute of Botany was initially published on the Science for Nature website.

Common carder bee - Bombus pascuorum. Photo: Justyna Kierat

Common carder bee - Bombus pascuorum. Photo: Justyna Kierat

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