Gran Bretaña: Wireworm in potatoes is worsening – how growers can tackle it

This worsening problem has prompted plenty of research activity to try to find solutions for growers looking to protect their investment in what is a very expensive crop to grow.

Cambridge University Potato Growers’ Research Association (Cupgra) commissioned the first wireworm research review in the UK since 2001 to help growers combat the problem.

The research was a collaboration by Niab CUF senior research associate Marc Allison and independent agronomist Martyn Cox.

We ask them about the main findings and outline what action growers can take.

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See also: How to control nematodes in potatoes with a trap crop

Risk factors – why is wireworm getting worse?

Withdrawal of persistent soil-applied insecticides

Larger machinery with faster rates of cultivation

Reductions in farm bird populations for predation

Move to minimal or no cultivation in crop rotations

Increased proportion of winter cropping in rotations

More cover crops – less bare soil

More permanent field margins and uncropped areas

What species are important in the UK?

The wireworm species in the UK that are economically important to the potato crop are Agriotes lineatus, Agreotes obscurus and Agriotes sputator, and are found across the potato area.

Other species, including several Agriotes spp, Athous haemorrhoidalis and Ctenicera cuprea are known to exist in the UK.

Some are widespread and can damage potato crops, but the three main species are usually responsible.

Agriotes sordidus is generally a southern European species, but evidence in France indicates that it is moving northwards and becoming a very significant problem for crop producers.

Independent agronomist Martyn Cox, one of the study’s authors, says at present there is no evidence the species is an economic problem in the UK

But A sordidus adults have been trapped in coastal areas and researchers need to better understand its importance to growers.

What are the key differences?

Most agriotes species have a five-year lifecycle, where adults lay eggs between April and June in year one, which hatch and grow from just a few millimetres to 2cm before pupating in the soil in late summer of year four.

Adults then emerge again the following spring to start afresh.

Understanding where field populations are in this lifecycle in relation to the potato crop is key to effective control of the pest, so growers need to think about what is happening in the previous four or five years before planting.

Wireworm larvae are most sensitive and vulnerable just after hatching. Once overwintered, they toughen up and become much harder to kill. This is likely to be the best time to deploy intensive cultivation or a biofumigation crop, for example.

Other species, including A sordidus, are known to have a shorter lifecycle, so damage to crops can occur earlier. This underlines the importance of knowing which species you are dealing with.

Can growers sample soil for wireworm?

Wireworm core sampling has a limit of detection of 62,500 larvae/ha, but crop damage can occur well below this level.

This means soil core sampling, where a large soil core is taken from the field and larvae extracted from the sample, is currently not viable for quantifying a damaging population. The large volumes of soil taken also pose practical challenges.

However, Dr Alison says with the introduction of mechanised automated sampling systems, the use of this method may become advantageous, and fresh research may be worthy of funding.

What are the pros and cons of trapping?

There are two types of traps that can be used for monitoring wireworm – pheromone traps and bait traps.

Pheromone traps use sex pheromones specific to a particular species to lure in male adult click beetles as they try to find the females and mate.

Bait traps contain a pre-soaked cereal seed mix, ideally maize and wheat, and are buried just under the soil surface.

As cereal seed germinates, it gives off carbon dioxide and other volatiles which attract larvae into the trap via small holes.

From his own work in recent years, Mr Cox says the pheromone traps available have improved significantly and are now much easier to use.

They are a tool for monitoring pest activity in a landscape and they can highlight where the pest is moving into fields.

They will not always reliably indicate the presence of larvae, as correlation between A lineatus adults and larvae is weak, but can be better for the other main two species.

At present, there are no pheromones for species such as Athous spp, which is a common click beetle in the UK.

Until more is understood about behaviour, and pheromones are available for all economically important species, there are limitations to pheromone trapping.

Bait traps are much better at providing evidence of wireworm activity in a particular field, and a key factor in their success is soil temperature and moisture.

Both factors strongly influence the activity of wireworm in soil.

In the past, people have used them before potatoes in early spring, when soils are cold and wet.

Despite not catching any larvae, they have subsequently seen damage in the crop planted soon after.