This neurotoxin is now suspended from use on mangoes and avocados after harvest, so why not oranges?

A range of tropical fruit including mangoes and avocados are now no longer allowed to be dipped in dimethoate, a neurotoxin banned in Europe but still used in some countries as an insecticide. 

This week Australia's chemical regulator, the Australian Pesticides and Veterinary Medicines Authority (APVMA), issued a 12-month suspension notice after testing found dimethoate residue above the maximum permissible limit on the skin of mango and avocado samples.

The announcement said the use of dimethoate as a post-harvest dip to control fruit fly on avocados, mangoes and other tropical fruits like bananas, custard apples, feijoas, guavas, lychees and pawpaw "may not meet the safety criteria".

The APVMA says the residual insecticide levels detected "are unlikely to pose a significant risk to human health", but issued the suspension as a precaution.

Some in the horticulture industry say the move could affect supplies of fruit and impact the price and availability in southern states.

Avocados Australia says the industry is committed to ensuring the wellbeing of consumers.(ABC Rural: Lucy Cooper)

What is dimethoate?

Dimethoate and its breakdown product omethoate are insecticides from a group of chemical substances known as organophosphates.

In Australia, it's been used to control more than 80 insect pests in horticultural and agricultural production since 1956.

In the case of mangoes and avocados, the fruit is either dipped in or sprayed with the insecticide to control Queensland fruit fly, a native pest found throughout eastern Australia.  

There are more than 150 species of native fruit fly in Australia but most don't attack commercial crops.

The suspension only relates to use of dimethoate as "a post-harvest dip for tropical and sub-tropical fruit with inedible peel".

Dimethoate products are still licensed for use until the point of harvest, which includes applying it to crops via a spray.

Since 2004, dimethoate has been under review by the APVMA and, in 2011, its use was banned on a number of products including stone fruit, grapes, figs, guavas and strawberries, and was no longer available for home gardeners.

Dimethoate has a toxic effect on the nervous systems of fruit fly and other insects such as ticks and mites.

The problem is, dimethoate is also toxic to the human nervous system, albeit if ingested at much higher doses.

Until now the APVMA has managed the risk to human health by regulating the maximum residue level (MRL) of the chemical on the skin of fruit.

"Unfortunately, in this last season, levels have been detected which have been above that maximum level in both avocados and mangoes that have been treated post harvest with dimethoate," APVMA executive director Sheila Logan told the ABC's Matt Brann in August.

"With the levels we've detected on the last season, consumption at that level is unlikely to pose a significant risk to human health.

"But it's about the established acceptable levels, and so it's eating into what we'd call the safety factors."

What is the impact for consumers?

The MRL is the highest amount of a chemical residue allowed in a food product.

In Australia, state and territory food regulatory agencies monitor and enforce the Food Standards Code, including MRLs.

The agencies undertake surveillance programs for chemical residues in food and also monitor how chemicals are used by food producers.

The Australian Mango Industry Association (AMIA) has raised concerns that the suspension on dipping fruit in dimethoate could restrict the transport of fruit into states like Western Australia and result in an oversupply in other states.

Other fruit growers in Queensland say that while dimethoate is not preferred, it's an effective back-up pest control. 

There are a number of alternatives to dimethoate, such as fumigation and irradiation, but some of these pest deterrents also carry risks to human health and the environment. 

The mango industry says the ban could result in an oversupply of fruit in some states.(ABC Kimberley: Courtney Fowler)

What does this mean for growers?

There are a range of tropical fruit growers who will be affected by the ban on the post-harvest use of dimethoate.

However, those most concerned are mango and avocado growers in states that have Queensland fruit fly — the Northern Territory, Queensland, New South Wales and Victoria.

Avocados Australia chief executive John Tyas says while the ban on dimethoate will put added financial pressure on producers to change processes, avocado farmers care about the wellbeing of consumers.

"[Growers] will need time and the physical and financial resources required to put in place procedures to meet the requirements of their alternative protocol and complete any registration processes in time for the next season," he said.

"For some there may be new ongoing costs associated with taking on an alternative protocol."

Despite this, Mr Tyas says the industry accepts the decision is about lowering the risk to consumers and is committed to making the change.

A map showing the states where fruit fly is an established pest.(Supplied: Department of Agriculture, Fisheries and Forestry)

Dimethoate is also listed as a post-harvest fruit fly control for bananas, however for almost all the fruit available to consumers, the insecticide is not used. 

"The treatment is not used for Cavendish (about 97 per cent of the banana industry's fresh production) or lady fingers (most of the remaining 3 per cent)," said Dr Rosie Godwin, research and development manager at the Australian Banana Growers' Council.

"Dimethoate is, in fact, not widely used within the banana industry.

"The risks posed by fruit fly are managed by ICAs (interstate certification assurances) which outline strict responsibilities and the procedures required for those sending or receiving mature green bananas interstate."

Edible and non-edible fruit skin

While the suspension relates to dipping mangoes and avocados in dimethoate, it does not extend to citrus fruit.

This is because the APVMA factors in how much the chemical penetrates the edible portion of the fruit, and even how much of the fruit is consumed by different age groups.

"The maximum residue level permitted for citrus fruits is currently higher than that for mangoes," the APVMA's Sheila Logan said.

"That is because based on dietary consumption figures, that is acceptable from a human health basis."

Fruit fly traps are another method of pest control for growers.(ABC: Courtney Wilson)

So if you've eaten mangoes or avocados for decades, should you be worried?

"No at this stage, the suspension is a precautionary measure," Ms Logan said.

"Depending on where you are living in Australia will depend on the level of fruit fly control required for produce available for you."

What does dimethoate do to humans?

There are no notable cases of anyone being poisoned by dimethoate in Australia.

The highest risk is for those applying the chemical because they are exposed to larger quantities.

Following a review, in 2017 the acceptable daily intake (ADI) for humans was lowered to 0.001 milligram of dimethoate per kilogram of body weight per day.

In human studies conducted by the APVMA, male and female volunteers were given up to 1.00 milligram of dimethoate per kilogram of body weight for between 14 and 57 days.

During the trial there were no clinical signs of gastrointestinal effects, but there was some reduction in blood cholinesterase, a family of enzymes involved in neurotransmission.

Dimethoate was banned in Europe in 2019, and while it is approved for use in the United States, its Environmental Protection Agency has classified it as a "possible human carcinogen". 

The US Center for Disease Control (CDC) says the "human health effects from dimethoate or omethoate … from low environmental exposures are unknown" but reports that at high doses dimethoate can cause "nausea, vomiting, weakness, paralysis and seizures".

The APVMA's toxicology report says dimethoate has "moderate-to-acute oral toxicity and low-to-moderate dermal [via the skin] toxicity". The authority has also classified it as a possible human carcinogen.

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