Agriculturalists have often commented that fields with a variety of plants tend to suffer less from pest damage, and even sowing two crops together can have an impact on pest activity.

Last summer, Agrovista published the results of two-year trials growing oilseed rape with companions Berseem clover and vetch; researchers noted that companion cropping appeared to reduce pest pressure, but noted that this has not yet been proven and that the mechanism behind any effect isn’t fully understood.

Now though, fresh research from the United States has shed more light on why fields with greater variety seem to attract fewer plant-eating insects than mono cropped areas. Reporting in the journal Nature this week, researchers said the observed reductions in pest pressure in more diverse farm fields may have to do with the nutritional needs of insects. They believe that returning plant diversity to farmland could be a key step toward sustainable pest control.

William Wetzel, who led the study whilst at the University of California Davis, explained, "Insects have a perfect nutrient level that they really like. When it's too high or too low, they do poorly."

Having studied 53 species of insects (including caterpillars, beetles, aphids and other flies), Wentzel and co. found that bugs have narrow ranges of nutrient levels which help them to flourish. If they feed on plants that are too nutrient-rich or -poor, the insects are less likely to thrive, and bugs surrounded by diverse plants are harmed much more by low-quality plants with the wrong nutrient levels than they are benefited by high-quality plants with high nutrient levels.

Wetzel, now an assistant professor at the University of Michigan, elaborated, ”A monoculture is like a buffet for plant-eating insects where every dish is delicious. A variable crop is like a buffet where every other dish is nasty."

"Farm fields can create monocultures where pests may find the perfect nutrition to be healthy and reproduce. Planting fields with higher plant nutrient variability could contribute to sustainable pest control.”

Modern arable fields not only contain very few different species, but also genetic variability within and between fields has also been massively reduced in just a few decades. Crop plants are bred from just a few cultivars, and the demands of modern agriculture require crops to be as identical as possible.

Wetzel suggested that one way around this could be to beed crop varieties with variable nutrient levels in the parts eaten by insects (most commonly leaves or roots), while the parts eaten by consumers (the ears of corn or heads of broccoli for example) remain consistent. Wetzel noted that this sort of genotype mixing for plants is already being done on some rice and wheat fields to reduce the spread of disease among the crops, adding, ”So far people haven't done that in ways to reduce insects, but it shows that it's possible to mix varieties and genotypes together. Now we need to think about how to do that to control insects."

Or else, the professor said, farmers could plant new mixtures of crop varieties or genotypes that differ in nutrient levels.

Speaking to Farming Online in late September, Nuffield Scholar Andrew Howard, who had recently published his report on intercropping in arable farming, said companion planting or intercropping - which played significant roles in agriculture up until the mid 20th Century - could return to the mainstream much sooner than most people think. Given environmental concerns about crops like maize and its effects on soils in the UK, plateauing yields of arable crops and growing resistance in black grass and fungal crop diseases (all of which can potentially be helped by intercropping), returning diversity to farmed landscapes could become a more viable alternative within the next few years.

Prof Wetzel intends to continue looking into reintroducing diversity to fields at Michigan State.