Mixing It Up: Mixed-Crop Fields Can Boost Natural Biocontrol of Insect Pests
By Meredith Swett Walker
Modern intensive agriculture, with its vast monoculture fields of cash crops, may seem rich in terms of yields and profits, but to an ecologist’s eye these fields are rather poor. In terms of species richness, they are downright impoverished. Species richness is a measure of biodiversity, simply the number of species present. It does not take their abundance into account. A carefully managed, weed-less field of wheat has a plant species richness of essentially 1, whereas a similarly sized area of native grassland may have dozens of species of plants.
Agroecology is the study of how ecological principals can be applied to agricultural practices. For instance, ecology’s diversity-stability hypothesis states that the greater the plant diversity in an ecosystem, the more stable it should be. Some agroecologists argue that increasing plant diversity in farm fields via polyculture (growing more than one crop in a field simultaneously) can make yields more stable and farms more sustainable.
But how does polyculture—which increases plant species richness and diversity—affect the populations of insects, including pests, that live in the fields? In a study published in October in the Annals of the Entomological Society of America, Zi-Hua Zhao, Ph.D., of the China Agricultural University, along with colleagues based in China, the United States, and South Africa, test whether polyculture could help farmers manage pests in their fields.
Zhao and colleagues examined how plant species richness in farm fields affects the insect community. The researchers divided the insect community into three groups: pests, which damage plants by feeding on them; natural enemies, insects that prey on crop pests; and environmentally friendly insects, which either do not harm crops or provide beneficial services such as pollination. The researchers wanted to know if plant species richness affects these groups of insects differently. Would increased plant species richness in farm fields stabilize the insect community? Would it boost the effectiveness of natural enemies, helping them control crop pests?
To find out, the team created an ambitious experimental model system, which was essentially a small farm with 50 mini-fields or plots (9 meters by 9 meters), each planted with one, two, four, eight, or 16 species of plant randomly selected from a pool of 20 different crop species. Then, over a period of three years, the researchers sampled insects on the plots and measured both plant and insect biomass. This was a tremendous undertaking. Each plot was planted with 484 individual plants and was weeded by hand (no pesticides or herbicides were applied). And, over the course of the study, more than 250 species of insects were identified.
Zhao and colleagues found that the more species of plants in a plot, the more species of insects found there. This was also true when the three groups of insects—pests, natural enemies, and environmentally friendly insects—were looked at separately. Higher plant species richness was associated with higher species richness in each insect group.
Higher plant species richness was also associated with higher total insect biomass. In other words, more plant species supported greater numbers of individual insects. But the effect of plant species richness on biomass was not the same across the insect groups. The more plant species in the plot, the higher the biomass of natural enemies, but plant species richness had no significant effect on the biomass of pests or environmentally friendly insects.
Crucially, the ratio of natural enemies to pests increased with the number of plant species in the plot. This ratio is an indicator of the biocontrol services that these natural enemies can provide—a valuable pest management tool. If there are more predators around, they can keep populations of pests from booming.
The biomass of environmentally friendly insects was also associated with a higher natural-enemy-to-pest ratio. The researchers suggest that perhaps environmentally friendly insects serve as alternative prey for natural enemy insects when pests aren’t abundant. Having an alternative food resource could stabilize natural enemy populations and help keep their numbers high.
The researchers conclude that increasing plant species richness in fields can stabilize insect communities and enhance biocontrol of crop pests by their natural enemies. Zhao points out that “this is a micro-landscape experiment, which could reveal effects of spatial patterns on biological control function at a large scale.” Planting a mix of crops is popular in China (where this study was conducted,) but Zhao admits that logistical challenges—such as mechanically harvesting polycultures on large scales—are far from solved. Still, a mix of just two or three crops would be sufficient to reap the biocontrol benefits, he says.
Annals of the Entomological Society of America
Meredith Swett Walker is a former avian endocrinologist and wannabe entomologist. She now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. Find a sampling of her work at www.magpiescicomm.com.