Neonicotinoid seed treatments are a valuable early season crop protection tool, but their indiscriminate use will hasten the evolution of resistance, and as an unexpected consequence, can disrupt beneficial invertebrates and microbes.
The use of neonicotinoid seed treatments (STs) has risen considerably over the last decade in Australian broadacre cropping, and particularly so in 2017 in response to the threat of Russian wheat aphid (Diuraphis noxia).
Why such growth in the use of neonicotinoids?
Coating seeds with a concentrated amount of this chemical class (e.g. imidacloprid, clothianidin and thiamethoxam) has many advantages.
Neonicotinoid STs are highly effective at curbing pest feeding damage and virus transmission in vulnerable establishing crops.
In contrast to foliar sprays, they reduce the risk of chemical exposure to farmers and spray operators, and reduce the risk of negatively impacting non-target organisms.
If used wisely, they can delay or negate the need to apply foliar insecticide sprays, as well as reduce the impacts on non-target species.
Despite these advantages, neonicotinoid STs are not a panacea for early establishment pest control – there are risks that come with their use, especially if used pre-emptively across large expanses of cropping land every year.
In this article, we discuss the potential impacts of neonicotinoid STs on beneficials in crops and the increased risk of resistance evolution.
Impacts on crop beneficials
Neonicotinoids are very soluble in water, so when applied as a ST, they are readily absorbed and efficiently transported by growing plants to provide systemic control against a variety of pests.
While less likely to impact beneficial species than foliar applications of broad-spectrum insecticides, and generally regarded as IPM-compatible, they are not completely benign.
One topic that has received considerable media attention, and remains the subject of controversy, is the negative effects of neonicotinoids to honeybees and the impact on pollination services.
Fewer empirical studies have examined the effects of neonicotinoid STs on other beneficial species, but they are also worth considering. This includes ants, soil biota, and those species that play an important biological control role in cropping systems (these are often referred to as ‘natural enemies’).
One exposure route through which natural enemies are exposed to neonicotinoid STs is by eating tainted prey.
For example, scientists showed that after grey field slugs (Deroceras reticulatum) feed on soya beans grown from thiamethoxam-treated seed, these slugs delivered a lethal dose of the insecticide to predatory ground beetles. This is despite the insecticide having no (or very little) harmful effect on the slugs. In this case, the reduction in predators due to neonicotinoids resulted in an increase in slug numbers and lead to a reduction in crop yield.
Natural enemies are also exposed to neonicotinoid STs because the adults of some species feed on plant nectar.
Neonicotinoids are not only transported to the leaves but also other parts such as the flowers, pollen and nectar.
While the larvae of many parasitoids attack agricultural pests, the adult stage often supplements its diet with nectar, and can be exposed to sub-lethal doses of neonicotinoids.
The same can be true for some predatory invertebrates.
Individuals may survive this type of exposure, but can experience sub-lethal physiological and behavioural effects, that can decrease population sizes.
For example, scientists have shown a reduction in host foraging ability and longevity of the parasitoid, Microplitis croceipes, after feeding on extra-floral nectar from cotton plants treated with neonicotinoids. Microplitis is a common parasitoid of diamondback moth larvae, native budworm, cotton earworm, armyworm, cutworm and other caterpillar pests of grain crops.
Other crop beneficials that are potentially affected are the soil engineers. These are critical to stubble retention and minimum tillage systems. Organisms such as earthworms, collembola, protozoans, fungi and bacteria play key roles in these environments by improving soil structure and fertility, waste matter breakdown, delivery of nutrients to plants, and carbon storage. The activity and/or survival of these beneficials has been shown to be impacted through the application of neonicotinoids and other insecticides.
Risk of resistance evolving to neonicotinoids
The risk of a given pest developing resistance towards an insecticide class increases with the regularity of exposure.
For this reason, a mainstay of any sound insecticide resistance management strategy is to avoid the prophylactic use of insecticides (i.e. insurance or ‘just in case’ applications).
But where do STs fit into this picture?
The way STs are used in broadacre crops is by its very nature pre-emptive. The decision to use a ST is typically made well before sowing and before the opportunity arises to assess the risk of pests establishing in crops.
Within Australia, there is very limited opportunity to rotate STs with products that do not contain a neonicotinoid as there are, in effect, no other classes of insecticide STs targeting the same pest spectrum as the neonicotinoids.
Resistance to neonicotinoids in crop pests is common overseas, and has reached a level at which efficacy in controlling some major pests, such as the green peach aphid (GPA, Myzus persicae) and whitefly (Bemisia tabaci) is considerably reduced. In the last 2 decades, the number of species with resistance to neonicotinoids has increased 20-fold.
Resistance has now arrived locally, with the first signs of resistance to neonicotinoids recently detected in GPA. Fortunately, the resistance in Australian populations of GPA confers only low-level resistance to neonicotinoids.
Overseas populations are known to possess another form of resistance that renders neonicotinoid insecticides completely ineffective, as is the case for synthetic pyrethroid (e.g. alpha-cypermethrin) and carbamate (e.g. pirimicarb) insecticides in Australia. This high-level resistance may very well evolve in Australian GPA if selection pressures continue to be high.
The recent rise in ST usage in Australia has no doubt increased the risk of resistance evolving in multiple pest species.
For example, one species that is consistently found across large expanses of cropping land is the redlegged earth mite (RLEM, Halotydeus destructor). This mite attacks pastures, pulses, canola and cereal crops, and has already developed resistance to pyrethroid and organophosphate (e.g. dimethoate) insecticides. Resistance in RLEM is now found across the WA grain belt, and was recently detected in south-eastern Australia for the first time. RLEM are found in very high densities during the crop establishment stage of multiple crops, and therefore subject to considerable selection pressure for resistance to neonicotinoid STs.
So, what can we do?
Neonicotinoids STs have numerous advantages and play an important role in early season crop protection. However, continued wide-scale use of neonicotinoid STs will select for resistance and is likely to impair ecosystem services provided by some beneficial invertebrate and microbial communities.
Industry stewardship and good resistance management are paramount to ensure neonicotinoids remain a long-term viable control option for grains pests.
Before making a management decision we should stop and ask ourselves, is a neonicotinoid ST warranted in this paddock, in this year?
- Wherever possible, 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: (i) canola or lucerne to be sown (ii), high mite numbers the previous year, and (iii) no Timerite® spray the previous spring.
- Unless the pest risk is deemed high, avoid using neonicotinoid STs in consecutive years, preferably no more than 1 in 3 years in any given paddock.
The challenge, of course, is our ability to accurately forecast the timing and severity of pest (and virus) occurrences well ahead of time.
Predictive tools may provide useful information here, but are currently not being used for such purposes, or simply do not exist for species of interest.
This article was prepared by Paul Umina, Julia Severi, James Maino, Garry McDonald and Ary Hoffmann.