By John Holland, Head of the GWCT Farmland Ecology Unit
- Increased pyrethroid use for aphid control may affect crop pest predators especially carabids and money spiders.
- Many early drilled cereal crops that use neonicotinoid seed dressing also have a follow up pyrethroid use. Resistance risk increases if repeated applications are used.
- Reducing drift is important to reduce the effects on over-wintering invertebrates in field margins and buffers will help.
- Later drilling and reduced cultivations may reduce pest attacks on cereal, but previous crop trash may cause a green bridge for pests.
The EU has now extended the ban on the use of neonicotinoid insecticides to all field crops which includes cereals and sugar beet. Neonicotinoids were used on these crops to protect them from aphid borne viruses such as Barley Yellow Dwarf Virus (BYDV) and virus yellows. To protect crops from now on the first reaction is likely to be to reach for a can of pyrethroid insecticide.
This is understandable given the potential risk to yield that these viruses can cause. Moreover, this is more likely to be a prophylactic spray because of the difficulty, especially in cereals, of accurately assessing cereal aphid infestation levels and the proportion that are infected with the virus. With resistance to pyrethroids already widespread in peach-potato aphid and present in grain aphid, an increase in frequency and extent of spraying will only speed up further resistance.
Here I consider what the impact of the ban may be for other invertebrates that are vital for sustainable farming, especially the natural enemies of cereal pests. Long-term impacts are also possible, threatening levels of chick food insect vital to our farmland birds.
Impact on non-target insects and spiders
Many different invertebrate species reside within fields in the autumn, some of which are pest natural enemies that help prevent pest outbreaks, others helping to break down organic matter. Unlike the case for oilseed rape, there is some evidence of the negative impact of applying pyrethroid insecticides in the autumn.
GWCT supported a PhD study (Pullen, 1990) that looked into this almost 30 years ago and the products tested, cypermethrin and deltamethrin, are still available. Both products were found to have a substantial impact on the numbers of money spiders (Linyphiidae) that were detectable into late spring of the following year. It is also worth bearing in mind that these trials were conducted in 1-4 ha plots within a field and reinvasion would have occurred from surrounding unsprayed areas within the field. The impact of whole field/blocks of fields is likely to be greater.
Money spiders are one of the most useful natural enemies in the autumn because they can quickly invade newly sown crops. This can be seen in cereal fields in the autumn, where their extensive network of webs that can be seen glistening in the early morning, almost covering the entire field, trapping aphids as they arrive. Moreover, there are also hunting species of money spiders which seek out aphids that escape the webs.
Cypermethrin and deltamethrin (incidentally both also highly toxic to bees) also caused an initial 70-80% reduction in the activity of carabid beetles, but their numbers recovered within 1-2 months as also found in other studies conducted in the 1980s, although again carabid beetle declines could persist for longer if the whole field was sprayed.
Many insects only have one generation per year and therefore the impact of an insecticide can affect populations into the following year and beyond. This is more likely with the less mobile species, such as carabid beetles than with spiders that can balloon across the landscape. For the last 50 years the GWCT has been monitoring the abundance of invertebrates in cereal crops every summer across approximately 100 cereal fields in Sussex.
Interpretation of this data revealed that indeed foliar applications of insecticides in the autumn were having an impact on a range of invertebrates in the following summer. These effects were seen even in the following year for spiders and carabid & click beetles, and some other non-predatory species – effectively a carry-over effect (Ewald et al, 2016).
Controlling for the effect of foliar insecticide applications, neonicotinoid seed dressings reduced the abundance of carabid & click beetles by 30% in the following summer. Rather surprisingly, winter-sown cereal crops treated with neonicotinoid seed dressing were more likely to be treated with foliar insecticides in the autumn/winter. This goes against Integrated Pest Management principles as seed dressings should be reducing the need for follow up treatments.
It is likely that this reflects an early sowing date for these crops, with agronomic advice indicating that the efficacy of neonicotinoid seed dressing had expired by the time BYDV- carrying aphids could be a problem in late autumn, early winter. The insecticide usage in Sussex was in-line with national figures, so the findings are applicable nationwide. It was also interesting to find that, over the duration of the Sussex Study - nearly fifty years- that the abundance of aphids in the summer has shown a steady decline that is possibly a result of the widespread use of insecticides (particularly foliar-applied) for many years.
Effects of drift
Spraying insecticides also poses a risk to invertebrates overwintering in the field boundaries because of the potential for drift. The Trust looked at this in 1997 (Longley & Sotherton, 1997) and found spray deposition in the boundaries was higher at this time of year compared to summer because the vegetation was shorter within the field and in the boundary. The levels of deposition increased with wind speeds from 1-4 m/s, from 1 to 7% of the volume applied to the field.
These levels were high enough to cause 25-29% mortality of moth larvae (Spodoptera littoralis) fed grasses collected in the field margins. The use of a 6m buffer helped reduced spray deposition in the boundary and would be recommended for any boundaries along the downwind edge of fields to protect overwintering natural enemies.
The first step is always to determine whether pest control is needed, but monitoring crops and using spray thresholds is not always feasible for busy farmers and agronomists. This should also be done for each field, because they can vary in their pest levels. For aphids that are weak fliers, where they are deposited can depend on the wind currents but also on the colour of the vegetation and plant volatiles that are emitted by the crop. Consequently, aphids are often initially deposited downwind of hedgerows so this is a good place to start looking for them.
There are also other possible methods to help control aphids in the autumn, besides the use of insecticides. Later drilling is recommended as aphids are less likely to fly and secondary spread within the crop is lower in cooler weather, however, with warmer winters this is becoming less reliable and may come with a yield penalty.
The use of minimum tillage has also been identified as a means of providing some protection from aphids and BYDV, because either the aphids are less able to identify suitable crops because they are disguised by the trash or minimum tillage allows more natural enemies to survive and provides better architecture on which spiders can build their webs.
On the other hand, crop residues increase the risk of a green bridge allowing aphids that are already present in a field to survive. Money spiders also rely on grassland outside of the growing period and so having grassland in the landscape can increase numbers within arable crops.
Selective insecticides such as pirimicarb that specifically target aphids, can also be part of an integrated approach, as they that are less toxic to pest natural enemies (Moreby et al., 2001), but unfortunately pirimicarb use on many crops was withdrawn in 2016.
Unfortunately, owing to a lack of funding for field research into IPM over recent decades, many of these alternative approaches to aphid control are unconfirmed and so we will as with flea beetle control in oilseed rape, be stumbling after a bolted horse.
Ewald, J.A., Wheatley, C.J., Aebischer, N.J., Duffield, S.J., & Heaver, D. 2016. Natural England Commissioned Report, NECR182. Natural England, York.
Longley, M. & Sotherton, N.W. (1997). Measurements of pesticide spray drift deposition into field boundaries and hedgerows 2. Autumn applications. Environmental Toxicology and Chemistry, 16: 173-178.
Moreby, S.J., Southway, S.E., Barker, A.M., & Holland, J.M. (2001). A comparison of the effect of new and established insecticides on nontarget invertebrates of winter wheat fields. Environmental Toxicology and Chemistry, 20: 2243-2254.
Pullen, A.J. (1990). Autumn Applied Pyrethroid Insecticides: Consequences for the Natural Enemies of Cereal Aphids. Unpublished Ph.D. thesis. University of Southampton, Southampton.