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Soil management matters

Benefits for pest and disease control

TResearch Autumn Winter 2021As public demand for fruit and vegetables increases and European legislation tightens around fertiliser and plant protection use, more information is needed on the differences between organic and conventional soils.

Agronomic practices – practices that farmers use to improve areas like soil quality and water usage – can alter soil macrobiology and microbiology. How these factors may affect a soil’s ability to impact on the survival of crop pests is of interest, particularly if it can help inform and enhance integrated pest management (IPM) strategies.

Research shows that agricultural soils are habitats for many naturally occurring beneficial species that can kill insect pests, and soil management may play a key part in improving pest suppression. To find out more, Teagasc researchers explored these processes in controlled long-term agronomic experiments.

Multiple soil frameworks were assessed during the study, and a model insect (Galleria melonella) was used to work out how effective each soil type was at suppressing insects.

Pest and predator occurrence

To assess the impact of organic and standard agronomic management systems on pest numbers, cabbage root fly (CRF) eggs were monitored over two years at sites in Kinsealy, Dublin (2014-2015) and Nafferton, Newcastle (2015-2016).

Celine Delabre, a Teagasc Walsh Scholar who conducted this research for her thesis, says: “Egg numbers were 29% (Kinsealy) and 52% (Nafferton) lower in organic plots compared to standard plots.

“At Kinsealy, there was no difference in observed above-ground predators. However, when plant root zones were examined, there was a 30% increase in Carabid and Staphylinid predator beetles in the organic plots.”

CRF pupal numbers were also significantly reduced in organic plots, with the presence of root zone predator beetles positively aligned to the number of pupae recovered, independent of the agronomic approach.

Similar results and predator groups were observed at Nafferton, with medium-sized (6-9mm) Staphylinid beetles being the dominant predator found.

While the agronomic approaches and plot size at each site differed (Table 1), there was some consistency in the effect of soil management practices on pest and predator occurrence. When soils from the field were placed in containers at a constant temperature and a Galleria melonella was added to the soils, a significantly higher rate of mortality was seen in the organic soils from Kinsealy than from the standard soil.

Similarly, when CRF eggs were added to the soils with brassica vegetables growing in pots, the root systems from the standard soils were 15% lower in biomass than plants grown in the organic soils. However, when the Nafferton soils were tested contradictory results were found. There was higher mortality of the Galleria in the conventionally managed soils, and the sterilised soils had the largest below-ground biomass.

Widening the scope of research across soil types

“While the results from the controlled experiments were contradictory,” says Celine, “these findings applied to only two sets of experimental soils at Kinsealy and Nafferton.

“To get a broader picture across more soil types and agronomic approaches, the same techniques were applied to 36 paired horticultural fields (organic and standard) across Great Britain and Ireland.”

Initial analysis of the soils sampled indicated observable differences by agronomic approach in microbial activity, mineralisable ammonium, earthworm counts and the presence of free living nematodes (small non-segmented worms).

When baiting with Galleria, the mortality ratio from the 36 soils showed a significantly higher mortality in organic soils after 10 days’ exposure. After 19 days, however, the gap closed and the significance was reduced.

Potential for natural regulation

Individual analysis of data from paired fields indicated that four pairs of fields showed no difference between organic and standard soils. Meanwhile 10 pairs showed higher mortality in the organic soils, and four pairs showed higher mortality in the standard soils.

Of the five Irish pairs, four showed significantly higher mortality in the organic soils. All soils indicated the presence of entomopathogenic nematodes (a group of nematodes that cause death to insects) and there was no significant effect of agronomic approach in their occurrence.

“This study indicates that agronomic management may potentially be tailored towards positive impacts on soil microbial activity, as measured by community physiological profile,” says Celine. “It can also lead to pest suppression at plot level, while improving the presence of natural enemies.

“The presence of natural enemies co-occurring in all pest samples and the related positive correlation of pests and predators suggests the potential for natural regulation occurring in some instances. And while results showed some variability, overall the study does highlight the need to include naturally occurring beneficial species in future crop protection studies.” 


This research was funded through the Teagasc Walsh Scholarships Programme.


We’d like to thank Andy Evans and Alistair Hamilton, SRUC, and Tom Little, University of Edinburgh, for their contribution.


Celine Delabre, Teagasc Walsh Scholar, Scotlands Rural College (SRUC),  Edinburgh; The Institute of Evolutionary Biology, University of Edinburgh.

Michael Gaffney, Senior Research Officer Horticulture Development Department, Teagasc Food Research Centre, Ashtown, Dublin.

Julia Cooper, Senior Lecturer in Soil Science, School of Natural and Environmental Sciences, Newcastle University.

Richard Hopkins, Professor of Behavioural Entomology, Natural Resources Institute, University of Greenwich.