It’s well known that plants grown as mixtures in a ‘polyculture’ suffer less pest damage than if grown in monocultures. If you look in the wild, plants naturally grow as mixtures, and we don’t see evidence of extensive pest damage.

There are many reasons why this happens, and a number of recent review papers explore some of the work that’s been done over the last few decades^1,2. They summarise that mixtures offer protection to plants in the following ways:

Physical barriers – different plants hide the host plants from the pests.
Inappropriate landing – if the pest lands on a nearby non-crop plant too many times, they give up and fly off.
Plant volatiles – these are chemicals that plants release into the air and soil that can disrupt how the pests find the plants, or it could repel them.
Natural enemies – plants provide resources that boost the numbers of predators and parasitoids that attack the pests.

How plant mixtures work in practice depends on the combination of plants and the pests in question – but these mechanisms have been discussed for a long time. More recently, other research has uncovered other, more subtle ways that plants protect each other.

Three sisters

Several pieces of work have centred on the traditional three sisters cropping system – which has prevalence in US food growing. The system requires three crops to be grown together: climbing beans, maize and squash or pumpkin. The maize provides a support for the beans to grow up, while the pumpkin provides some ground cover to suppress the weeds. This cropping system, sometimes also known as ‘Milpa’ has been used by native Americans for centuries³.

A deep dive into the intricacies of this growing method demonstrates it fosters a much more diverse range of arthropods (small segmented creatures, including insects) than when the crops are grown alone⁴. In many instances, this increase in biodiversity was greater than the sum of its parts - suggesting the plants were interacting and helping each other to boost diversity. This diversity helps to protect the plants because each species naturally attracts a distinct set of pest enemies, building a strong population of beneficial insects to defend the plants.

So how do the plants help each other boost populations of defensive insects? A research group tried to answer this, by studying the behaviours of army worms. Army worms are a devasting caterpillar pest that attacks a wide range of crops around the world⁵, especially maize. They are not a regular pest in the UK but could become more widely established in a changing climate.

Insects are often attracted to volatile ‘smelly’ chemicals to locate their food, so researchers decided to test whether this was playing a part in the interplay between plants, pests and predators in the three sisters crop⁶. Their work in both the field and lab showed that parasitic wasps were strongly attracted to secretions from the army worm caterpillars mouth when it was feeding. So the caterpillars were inadvertently sending out a ‘come and get me’ message to the wasps every time they fed.

But the interplay extends further than this. These volatile chemicals not only signal to the parasitic wasps, but the bean plants pick up on the signals too. They start producing more sugar in their ‘extrafloral nectaries’, which are small glands producing sugar on the undersides of leaves. This extra sugar was shown to bring in more of both parasitic wasps and predatory wasps that feed on the pests directly. The extra sugar also doubled the life of parasitic wasps so they could spend more time looking for insects⁷.

Additionally, work in China⁸, showed maize plants in a three sisters system produced higher concentrations of bitter compounds than plants growing alone making themselves less palatable to pests.

Extending the benefits

So can we extend this beneficial system to other crops? The answer to this is not simple. It appears this interplay of partner crops, smelly volatile chemicals and natural enemies are highly specific, so some combinations of plants are far more effective than others.

This complexity was highlighted in a study where they examined the effects of growing mixtures of tomato varieties on populations of natural enemies⁹. They found only some combinations of varieties boosted natural enemies, and it was dependent on the combinations of smelly terpene chemicals given off by the plants. So, if we were trying to design mixtures that ward off pests, we would need to have a better idea of the chemical make-up of the odours given off by each variety.

While we’ve got a good handle on how mixed cropping protects from pests, it seems that we still have a lot to learn about its many intricacies. The three sisters method has been studied the most because it has a proven track record of being used in traditional farm settings for centuries.

From our experience, it’s a tricky balancing act to get it to work productively in the dull, short growing season in the UK. But we’re happy to hear from people who have been successful.

The vast majority of our food production in the UK comes from monocultures, despite the many benefits that can be achieved with mixed cropping. In this respect, it appears that some of the small-scale organic growers are at the forefront with many experimenting with various mixtures of green manures and vegetables. In our own gardens we can make the most of these positive interactions by growing mixtures of different plants and varieties.

Practical tips for combining plants

There’s still much to understand about the interplay between plants and other life in the garden, but that shouldn’t put us off trying out combinations. You could experiment with:

Brassicas planted with flowering dill, coriander and fennel to attract predators and parasites.
Brassicas under sown with small leaved white clover to reduce attack from cabbage root fly.
Growing just climbing beans and squashes together. This can be better suited to the UK than the three sisters method, where the corn often doesn’t compete well.

References

1. Yousefi, M. et al. The effectiveness of intercropping and agri-environmental schemes on ecosystem service of biological pest control: a meta-analysis. Agron. Sustain. Dev. 44, 15 (2024).

2. Altieri, M. A., Nicholls, C. I., Dinelli, G. & Negri, L. Towards an agroecological approach to crop health: reducing pest incidence through synergies between plant diversity and soil microbial ecology. Npj Sustain. Agric. 2, 6 (2024).

3. Ngapo, T. M. et al. Historical Indigenous Food Preparation Using Produce of the Three Sisters Intercropping System. Foods 10, 524 (2021).

4. Grof-Tisza, P., Muller, M. H., Gónzalez-Salas, R., Bustos-Segura, C. & Benrey, B. The Mesoamerican milpa agroecosystem fosters greater arthropod diversity compared to monocultures. Agric. Ecosyst. Environ. 372, 109074 (2024).

5. Allen. Army worms (Spodoptera spp.) Plant Pest Factsheet. https://planthealthportal.defr... (2022).

6. Grof-Tisza, P., Turlings, T. C. J., Bustos-Segura, C. & Benrey, B. Field evidence for the role of plant volatiles induced by caterpillar oral secretion in prey localization by predatory social wasps. Biol. Lett. (2024) doi:10.1098/rsbl.2024.0384.

7. Pennisi, E. How the Three Sisters shrug off pests. Science 385, 920–921 (2024).

8. Liao, H. et al. ‘The Three Sisters’ (maize/bean/squash) polyculture promotes the direct and indirect defences of maize against herbivores. Eur. J. Agron. 155, 127118 (2024).

9. Hauri, K. C., Glassmire, A. E. & Wetzel, W. C. Chemical diversity rather than cultivar diversity predicts natural enemy control of herbivore pests. Ecol. Appl. 31, e02289 (2021).

This article was first published in our spring 239 issue of The Organic Way members' magazine.