Benefits and risks of neonicotinoid insecticides to Australian agriculture
AFTER years of concern about the impact of a widely used insecticide group known as neonicotinoids on insects like honey bees, in April 2018 the European Union decided to ban all outdoor uses of three active ingredients.
Though the industry debates the scientific evidence informing the EU ban, there is a rapidly growing number of publications reporting effects on bees and other insects. Worryingly, effects are also being displayed in birds, bats, aquatic organisms and related ecosystem services.
In response to these concerns, a group of 232 scientists published an open letter to policy makers in the journal Science, calling on other countries around the world to also restrict the use of neonicotinoids.
The story behind neonicotinoids
CSIRO entomologist Dr Sarina Macfadyen says neonicotinoids are registered in Australia for a very wide range of crops, including cotton, canola, cereals, sunflower, potato, many vegetable crops and fruits.
“They are a really important pesticide in the farmer’s crop protection tool box,” Dr Macfadyen says.
While they are often applied as a spray, they are more commonly used as a seed coating, with the active ingredient absorbed into the plant tissue and protecting newly emerging seedlings from potentially costly insect damage for a number of weeks.
However, plants only take up as little as five per cent of the active ingredient. Once inside the plant, the insecticide can be found in the leaf tissue, pollen, nectar, extra-floral nectaries, and water secreted by the plant. Any insect that feeds on the crop, including pollinators feeding on flowers, will be exposed to the toxin.
The toxicity of neonicotinoids stems from interfering with transmission of neural messages in insects.
So is there any evidence of impact on the Australian environment, and would a ban for use in Australian agriculture be appropriate?
Non-target impacts: more than just honey bees
What is not taken up by the plant remains in the soil and can persist from several months to several years. These compounds can move through the soil after rain, and therefore insects that feed on plants in field margins may also take up neonicotinoids. In addition, exposure has been reported via dust coming off the seed at planting.
Dr Macfadyen says exposure can also impact beneficial natural enemies (predators and parasitoids) that attack the crop pests that the neonicotinoids are targeting.
“This can cause significant problems for farmers, with crop yield reductions due to a lack of biological pest control,” she says.
“So on the farm, the negative impact to beneficial insects can sometimes negate the positive effects that insecticides can have.”
The wider insect food chain can also be impacted indirectly. For instance, neonicotinoid seed treatments can poison carabid beetles that eat slugs that have fed on treated plants.
Furthermore, insects serve as food for a wide range of other insects, birds, bats, reptiles and amphibians in farming landscapes.
Can we apply findings of overseas studies to Australian systems?
Neonicotinoids have been used in Australia since 1994. While no data exists on the actual quantities used on different crops and in each region, neonicotinoids have become widely adopted for use both as a seed treatment and spray in agricultural and horticultural landscapes. One substance, imidacloprid, is a commonly used insecticide in Australia.
A few scientific studies have been conducted in Australia to assess the efficiency of neonicotinoids, and non-target impacts. One study demonstrated a marginal yield benefit in canola when using a seed treatment, and some inconsistent impacts on natural enemies.
Additionally, the timing of crop planting, the pest and beneficial species present in our fields, and the behaviour of species when neonicotinoids are present, are all different to other parts of the world.
Neonicotinoids are also broken down in the environment, and this occurs quicker in soils that are moist, have low pH and high organic matter. However, these conditions do not apply to large areas of Australia’s low rainfall broad-acre cropping regions.
“So while some processes associated with the breakdown of neonicotinoids in the Australian environment can be estimated from overseas research, the impacts on our unique farmland biodiversity are just that: estimates,” Dr Macfadyen says.
Have pollinator impacts been seen in Australia?
Anecdotally there appear to be limited declines in European honey bee populations in Australia, although this has not been properly quantified.
Dr Katja Hogendoorn, a University of Adelaide scientist with expertise in crop pollination by insects says that of all crop pollinating species, only the health of European honey bees in managed hives is monitored to some extent in Australia. The impact of neonicotinoids on feral honey bees and solitary native bees are unknown.
While pesticides are involved, declines in bee numbers have not been directly linked to neonicotinoids alone. Declines in insect biodiversity in agricultural areas in Europe and the USA, have been related to habitat decline and wider pesticides use (including neonicotinoids). Overseas, additional stressors causing poor hive health include loss of floral diversity, and disease transmitted by the Varroa mite.
“We cannot say if similar patterns are occurring in Australia, as we are not monitoring the populations,” Dr Katja Hogendoorn says.
“However, absence of evidence is not the same as evidence of absence, and recent concerns raised by a large group of professional entomologist indicate that many Australian insect communities are under threat from a wide range of stressors.”
Plotting a course ahead
The absence of scientific research quantifying non-target impacts of neonicotinoids makes it challenging to suggest the best way forward for Australian farming systems.
On one hand, neonicotinoids are an important group of insecticides that provide an alternative mode of action to other insecticide groups. This means that, when rotated with other insecticides, they can be used to reduce the risk of resistance developing in insect pests. But many insecticides can also pose a risk to honey bees and other insects, so why single out neonicotinoids?
Farmers have comprehensive guidelines on strategies to reduce the risk to bees when they spray insecticides, but little advice around seed treatments.
“We think Australian farmers have an opportunity to demonstrate the highest standards of stewardship around the use of neonicotinoids if given the appropriate tools to do so,” Dr Macfadyen says.
The road ahead in Australia requires geographic documentation of the use of seed treatments, and conducting research to pinpoint when and where neonicotinoids (and other insecticides) can be used.
This will demonstrate where they can provide the greatest benefits for crop protection with the least risk to Australia’s unique and useful agricultural biodiversity, and the ecosystem services they provide.