The unanswered questions on impact of pesticides on the health of ecosystems

There is growing evidence that pesticides are implicated in declines of everything from songbirds to bug life in rivers and streams

Switching to a plant-based diet is an essential component in the transition to climate and nature-friendly food production. However, a downside to the current system of plant agriculture is the volume of chemical sprays required for their cultivation.

Insecticides, herbicides and fungicides are toxins designed to kill off problem organisms and are euphemistically referred to by industry as “plant protection products”. The most famous of these is glyphosate, the active ingredient in the widely-used weedkiller marketed as Roundup, but this is only one of hundreds of pesticides which are currently licensed for use in Ireland.

Defenders say that growing the quantity of food needed for a burgeoning population is not possible without pesticides. Grain producers, in particular, will point to the need for glyphosate as a necessary concession to protect their soil. Ploughing, despite its long history, damages soil structure, practically eliminates earthworm populations and leads to erosion and loss of carbon. Ploughing has historically been popular as it eliminates the wild plants (generally referred to as “weeds”) that would otherwise compete with the crop plants. Farmers that have pioneered cultivation that protect their soil through methods such as “no till” or “min till” argue that glyphosate is essential for eliminating the weeds.

The goal of “organic no-till”, where neither chemicals nor ploughing are required, is being pioneered in some quarters using specially designed machinery or grazing animals to eliminate the weeds prior to sowing. Nevertheless, it is not yet widely adopted.


Minister for Agriculture Charlie McConalogue has said he wants to see more tillage in Ireland and yet the sector is shrinking. This explains why, to the dismay of many, he recently supported the extension of the licence to use glyphosate in the European Union for another 10 years.

More glyphosate was just one of a number of wins for the agri-farm lobby in their assault on the EU’s green deal. Another is the abandonment (for now) of the “sustainable use regulation” which had aimed to reduce the use and risk of pesticides across the EU by 50 per cent by 2030. In Ireland, the key indicator in measuring pesticide risk fell by 48 per cent in 2021 when compared with the level in 2011-2013, according to the CSO. Nevertheless, across the EU, sales in pesticides have barely budged in this period.

In order to use pesticides, Irish farmers are required to undergo training, maintain records and seek to minimise use through what’s known as “integrated pest management” (IPM), ie practices that can help to control pests without the use of chemicals, such as cleaning equipment, crop rotations or encouraging natural predators of pests, such as ladybirds that eat aphids.

Con Trass is an apple farmer in Co Tipperary, one of about 30 in Ireland, only two of which are organic. He says fungal diseases are a real problem for producers while selective breeding for resistance to one pest can increase vulnerability to another. “It will be quite a while until we have varieties of apples that really have solid resistance that will enable us to reduce the number of fungicide applications ... I’d love if we had an alternative solution.”

Applying chemicals in wet weather, the ideal conditions for fungal growth, requires heavy machinery on soft ground. “It creates ruts in the orchard, a lot of mud, it’s not good for the soil conditions ... all that damage, if we didn’t have to use the products it would be fantastic.”

Trass, who lectures in pest management in the University of Limerick, believes that the training in pesticide use is adequate while farmers are subject to inspections by An Bord Bia (if they are a part of their quality assurance programme), as well as the Department of Agriculture and the Health and Safety Authority. Trass feels the regulatory framework works well, even if streamlining of inspections would make life easier for farmers on the ground.

Methods for minimising the requirement for pesticide sprays are “well established” for permanent crops like apples, Trass adds. This started out with the use of natural predators of pests. “Nowadays we’re controlling aphids, red spider mites and caterpillars from certain moths using purely IPM systems”.

Aphids can be controlled by limiting nitrogen applications, an important fertiliser in agriculture. “Aphids’ breeding rates go up a lot when there are high levels of nitrogen in leaves” says Trass. “We also have high numbers of earwigs in the orchard, which are great for controlling aphids”. He also uses a technique called “mating disruption”, which uses scents to fool moths, eg by convincing males to mate with other males. These scents can be manufactured to target particular species, therefore avoiding broader effects to insect life.

These methods also work for larger field systems such as grains, but Trass points out that IPM can be expensive and so the economic incentive can be much stronger to turn to a chemical application. He feels there needs to be more research and investment in IPM and pesticide reduction but also there needs to be market incentives for those farmers putting in that effort.

Dara Stanley, assistant professor in applied entomology at UCD School of Agriculture and Food Science, was co-ordinator of the Government-funded Protects project which ran from 2018-2022 involving researchers from UCD, DCU, TCD and Maynooth University. It was charged with studying the effects of pesticides on soils and pollinators.

“One of the interesting things we found was that for one famous group of pesticides called neonicotinoids, which were banned across the EU a decade ago due to the risk they posed to bees, we are still finding residues of them in soils and in the nectar and pollen in crops. We also found pesticide residues not just in crops but wild plants that are flowering around the fields in which they’re applied,” she says.

The research found evidence that wild bees are returning to hives with pollen containing pesticide residues and that exposure may be affecting their behaviour. But getting a full picture is difficult. Risk assessments may not take into account the formulation of the chemicals as they are applied in the field, rather than just the active ingredient; studies tend to focus disproportionately on domesticated honey bees, and not the great diversity of wild bees, which can have very different biology, never mind wild insects and other invertebrates that are critical for working ecosystems; while effects to bee behaviour or reproduction that fall short of killing them (referred to as ‘sublethal’ effects) may not be tested at all.

This leaves important unanswered questions, particularly around the nature and scale of the impact of pesticides on the health of ecosystems. Nevertheless, there is growing evidence pesticides are implicated in declines of everything from songbirds to bug life in rivers and streams but, says Stanley, “it’s an incredibly hard thing to measure at the scale needed. The focus therefore is making sure that what we do use is as safe as possible.”

An additional approach is to develop mitigation measures, such as directed spraying, or limiting spraying to particular times of the day. The Protects project looked at the recommended measures listed on the sides of chemical containers. According to Stanley, “We trawled the academic literature to see what evidence there was for their effectiveness, and unfortunately we didn’t find much.” A key recommendation is that authorities should not rely on these measures when giving approval for their use, until their effectiveness can be demonstrated.

Can we hope for a world without these chemicals? “Pesticides are important for food production and it’s simplistic to say we should just eliminate them,” Stanley says. “Therefore, the two important things to me are to improve the risk assessment process and improve the mitigation measures, which includes reducing usage.”

Increasing the level of organic farming would help and progress is being made in this regard. But, she says, if there is low-hanging fruit it must be the availability of pesticides to the general public. “That, to me, is a lot more concerning. We’ve people out there using pesticides that they’re buying off the shelf, they don’t have any training and they’re not using it for something crucial like food production, but more for aesthetics in gardens or golf courses.”

Regulation, or even outright banning of over-the-counter sales of these toxic chemicals to non-professionals, could bring significant benefits to human health and biodiversity with no impact to food production, Stanley adds.