Key points

• Neonicotinoids play an important role in Australian grain crops, but need to be used wisely to prolong their usefulness.
• Resistance to neonicotinoids is already present in green peach aphid.
• Avoid using neonicotinoid seed treatments in consecutive years, unless the pest risk warrants application and no other effective options are available.

The use of neonicotinoid seed treatments has risen considerably over the past decade in Australian broadacre cropping, and particularly in the last few years in response to the threat of Russian wheat aphid (under Permit 82304 only).

Yet, to ensure neonicotinoids remain a viable, long-term control option for grain pests, industry stewardship and good resistance management are vital.

Neonicotinoid seed treatments are highly effective at curbing pest feeding damage and virus transmission in vulnerable establishing crops.

Unlike foliar sprays, they reduce the risk of chemical exposure for growers and spray operators, and in many cases, reduce the risk of damage to non-target organisms.

Beneficial insects can be exposed to agricultural chemicals by eating tainted prey or when they feed on leaves, flowers, pollen and nectar.

Despite the many benefits, there are risks with neonicotinoid seed treatments, especially if used pre-emptively across large expanses of cropping land every year.

Resistance risk

Insecticide management strategies recommend that growers avoid prophylactic use of insecticides (that is, insurance or just in case applications).

But insecticide seed treatments are typically pre-emptive, with the decision made well before sowing and before growers can assess the risk of pests establishing in crops.

Resistance to neonicotinoids in crop pests is common overseas and the number of species with resistance has increased 20-fold in the past two decades.

Some overseas green peach aphid (GPA) populations have high-level resistance that renders neonicotinoid insecticides completely ineffective.

In Australia, low-level resistance was recently detected in some GPA populations, and this is likely to evolve further if selection pressures continue to be high.

Increased use of seed treatments in Australia will increase the risk of resistance evolving in many other pest species as well.

For example, redlegged earth mite (RLEM) is a pest found across large expanses of cropping land and is often exposed to seed treatments during the crop-establishment stage.

RLEM has already demonstrated the evolutionary capacity to develop insecticide resistance to other chemical groups (Modes of Action).

Beneficials

Neonicotinoid seed treatments can be less harmful to beneficial insects than foliar applications of broad-spectrum insecticides and are compatible with integrated pest management programs.

Seed treatments are not benign and need to be used with care to minimise the impact on beneficials. Beneficial insects can be exposed to agricultural chemicals by eating tainted prey.

For example, overseas research has shown that grey field slugs can feed on soy beans grown from neonicotinoid-treated seed without harm, but beneficial predatory ground beetles die when they eat the slugs.

Beneficial insects can also be exposed to neonicotinoid seed treatments when they feed on leaves, flowers, pollen and nectar, while organisms that reside in the soil and provide important ecosystem functions (for example, earthworms and microbial communities) can also be adversely affected.

When to treat

There are very limited opportunities in Australian grain crops to rotate seed-treatment chemicals that do not contain a neonicotinoid, as other insecticide groups do not target the same pest spectrum.

Instead growers need to decide whether a seed treatment is warranted in each situation. Assess the risk of damaging pest infestations (or virus risk) based on the prior paddock and seasonal history.

In the case of RLEM, for example, a high-risk situation would be indicated by:

• Canola or lucerne to be sown
• High mite numbers the previous year
• No Timerite® spray the previous spring.

Unless the pest risk is deemed high, avoid using neonicotinoid seed treatments in consecutive years, preferably using in no more than one in three years in any given paddock.

More information:Dr Paul Umina, cesar, 0405 464 259, pumina@cesaraustralia.com; Timerite