The use of cover and biofumigant crops to reduce wireworm populations in potatoes

The group held a webinar looking at the issue of wireworm and the learnings from the trial. It can be viewed here.

The final report has been published and can be found in the results and reports   section of the field lab page (click on brown banner).

Here are the main Conclusions

This trial suffered due to the extremely wet autumn and winter of 2023-24 and as a result of this, we were unable to carry out all the work at the intended timings or in the case of the biofumigation incorporation, not at all.

The work on the trial was delayed by the wet harvest of the previous years’ oats and this will have allowed juveniles to feed longer than usual thus reducing their susceptibility to starvation (Sufyan 2012). This ensured that all treatments in the trial were ultimately drilled too late in the year. As a result, the mustard was never likely to reach optimal growth stages for incorporation, further limited by overly wet soil conditions. The wet conditions may also have reduced the effects of cultivations on weeds and volunteers and mortality of juveniles.

Although there were few significant effects in this trial, most treatments did behave as expected except for the flame weeder. This raised questions about wireworm preferences for an area ostensibly free of any viable food source, hinting towards a possibility of a preferably warmer environment due to bare soil, or the presence of enough organic matter or previous plant material to reduce need for migration. Anecdotally, larvae were observed feeding on stubble during informal sampling prior to soil-coring, further cementing these organisms’ ability to sustain themselves on minimal or nutritionally poor food sources.

Arable farming is rapidly adopting the use of cover cropping and reduced or zero tillage, but we have not yet managed to predict the longer-term effects of these changes although they are widely considered to increase click beetle populations. To enable these studies, we must develop improved techniques for detecting small larvae in soil from a known area of soil. While core sampling remains the accepted standard for determining population density of small larvae, this study has highlighted its limitations - particularly in detecting low populations and early-instar larvae. Even if they are found, the small size of juvenile larvae is at the lower limit of the extraction methods used. This was evidenced here where core sampling failed to detect early-instar larvae in areas where prior manual sampling had indicated a reasonable wireworm population.

To compound this drawback of the trial, the ground was far too wet for a potato crop in the field following treatments and so post-harvest assessment to quantify any wireworm presence or damage was impossible. The development of more sensitive detection methods, potentially drawing from techniques used in nematology or other areas of soil science, could significantly advance our understanding of treatment efficacy.

The unexpected results from flame-weeded plots warrant further investigation. The increased wireworm populations in these areas could either represent natural variation or indicate previously unknown ecological interactions, such as impacts on predator populations. Although our data is limited here, it does point towards the complexity of wireworm ecology and the need for a comprehensive understanding of soil ecosystem dynamics when implementing control measures.

At T C & N Taylor, Nick remains committed to organic growing techniques, and the implementation of any novel methods that can sustainably solve current pest problems. After the trial, Nick did grow buckwheat within the rotation but found it particularly susceptible to periods of cold weather, a possible barrier to wider UK adoption. TC & N Taylor’s main focus is to overwinter areas with mustard to combat subterranean invertebrate pests and continue to have something in the ground year-round. If the benefit of field-scale implementation was better studied under various conditions, then uptake would be an option moving forward. Planting potentially inconsistent crops and the costs incurred with incorporation of those, such as mustard, currently may not currently be economically viable for all growers.

Huge thanks to all those involved in the trial: