New Review Examines the Promise of Habitat Management for Pest Control
By John P. Roche
Habitat management for pest control is part of a discipline called integrated pest management (IPM), which seeks to use multiple ecological strategies to effectively control pest species. In habitat-based pest management, crop growers adjust the community of plants in and around crop fields to discourage pest populations while encouraging the insect predators and parasitoids that are natural enemies of pest species. Plants effective at helping the natural enemies of pests can do so by providing four benefits: shelter, nectar, alternative prey/hosts, and pollen.
The practice gained renewed interest about 20 years ago when insect ecologists Geoff Gurr, Ph.D., and Steve Wratten, Ph.D., proposed a concept called “integrated biocontrol,” which focused on IPM practices that fostered natural enemies of pests. This month, a review published in the Annals of the Entomological Society of America examines research on how farmers can increase the fitness of natural insect enemies of pests by maintaining the functional diversity of plants. The article—authored by Mauricio González-Chang, Ph.D., of the Universidad de Aysén in Chile; Sheela Sharma, Ph.D., of the Nepal Agricultural Research Council; and Wratten and Sundar Tiwari, Ph.D., both of Lincoln University in New Zealand—is part of a special collection in the Annals on trap and cover crops in IPM.
Habitat-based pest management, the researchers explain, can be viewed in terms of two hypotheses: the “enemies hypothesis,” which predicts that pests can be reduced by planting non-crop plants that help the natural enemies of pests; and the “resource concentration hypothesis,” which predicts that pests have a reduced chance of finding their preferred hosts if there is a more diverse assemblage of plant species. These hypotheses are models that can help shape our thinking about factors related to habitat management; they are not meant to be mutually exclusive, and both processes are likely to be operating at the same time.
The practices of integrated biocontrol have been used in agriculture for thousands of years. Gonzáles-Chang and colleagues report that, more than 2,000 years ago, Chinese farmers provided shelters of straw to help spiders that were enemies of crop pests. They also cite that in the 1700’s farmers in Burma connected citrus trees with canes of bamboo to help Oecophylla weaver ants move among trees and reduce citrus caterpillar pests. Despite the long history of utilizing habitat-based pest management, however, research and wide-scale implementation of these techniques have only been pursued very recently.
Habitat-based pest management strategies involve either manipulating non-crop plants in agricultural settings to directly reduce pest populations or manipulating plants to increase the concentration of pests’ enemies, such as predators and parasitoids. Strategies that directly reduce crop pests include trap cropping, multiple cropping, and intercropping. In trap cropping, plants are grown that attract pest insects, which can then be eliminated with targeted pesticides that do not have to be applied to the main food crop. Trap cropping has been used for hundreds of years. For example, in England in the 1800s, farmers planted parsnip to trap parsnip webworms to keep them away from carrot crops. Multiple cropping involves planting multiple species of non-crop plants. Intercropping involves planting non-crop plants that serve as physical barriers to pests and resource providers for enemies of pests.
Strategies that focus on increasing predators and parasitoids of crop pests include cover cropping, the use of insectary plants, and intercropping. In cover cropping, the farmer grows a cover crop plant species before or after growth of the main food crop, and that cover crop benefits natural enemies of the pest. An example the authors cite is the planting of red clover in cucumber fields. The red clover increases concentration of bigeyed bugs, minute pirate bugs (Orius tristicolor), and lady beetles, species that in turn reduce pests of cucumber, such as the striped cucumber beetle (Acalymma vittatum) and the melon aphid (Aphis gossypii). The use of insectary plants involves planting species that provide nectar and pollen to natural enemies of pests. One example is planting the insectary plant sweet alyssum (Lobularia maritima) in apple orchards. Sweet alyssum increases concentration of the parasitoid Trichogramma that reduces pests called apple leaf rollers. Other examples include planting sweet alyssum to help hoverflies that reduce aphid pests, planting buckwheat to help the parasitoid Dolichogenidea tasmanica to reduce leaf roller pests, and planting phacelia to help syrphids that reduce aphid pests.
In their review, Gonzáles-Chang and colleagues explain that proponents of habitat-based pest management need to do more than just increase plant diversity: They need to examine multiple variables of biodiversity. These factors include spatial scale, temporal scale, complexity of the landscape, complexity of the vegetation, how connected crops are to the natural habitat, how closely pests are synchronized with their enemies, the numerical abundance of arthropods, and the species diversity of arthropods.
The use of flowering plants for habitat management can be made more effective, the review concluded, by choosing plants that provide abundant nectar and pollen, that flower for long periods, that are easy to grow, and that provide shelter for enemies of pests. Future research could focus on identifying plants that have the most favorable influence on natural enemies of pests.
The authors point out that understanding the behavior of crop pests and their natural enemies is critically important to the success of habitat-based pest management. They write that more attention should be given to “insect oviposition and feeding preferences, their movement and migratory/dispersal behaviour, and host selection characteristics.”
Another important but rarely studied topic in integrated biocontrol is plant-plant interactions. Gonzáles-Chang and colleagues explain that, when non-crop vegetation is used for trap cropping or flowering strips, they can reduce the crop’s susceptibility to pests, or they can increase the crop’s susceptibility to pests, depending on the particular conditions. Also, plant-plant interactions can alter the behavior of pests and the behavior of the enemies of pests.
There is resistance on the part of many farmers to adopt habitat-based pest management strategies, in part because of the low comparative cost of pesticides and in part because of concern over crop losses if ecological methods of pest control are not effective. But there are ways to successfully encourage adoption of these techniques at the local level. Gonzáles-Chang and colleagues explain that knowledge of habitat-based pest management can be facilitated by observation-based learning mechanisms. “In these learning schemes,” they write, “farmers that are willing to experiment with new farming techniques adapt the locally based scientific knowledge … and, if they succeed, neighbors will try it as well by themselves, leading to a potential widespread adoption of such a technique in the area.”
A key consideration for the future of IPM is that, as the global climate changes, the dynamics of interactions among plants, insect pests, and insect enemies of pests will be constantly changing. The ranges of many species will shift, and conditions affecting community interactions will change. As a result, researchers and farmers will need to adopt a dynamic approach to applying biocontrol in the face of change.
There is a growing demand around the globe for reduced use of chemical pesticides and an increased implementation of IPM strategies to control crop pests. Many nations are calling for reducing or eliminating the use of pesticides, and The International Organization for Biological and Integrated Control is encouraging increased implementation of habitat-based pest management strategies. As the growth of research and adoption of these diverse and powerful techniques continues, myriad benefits should be enjoyed by both farmers and consumers.
Annals of the Entomological Society of America
John P. Roche, Ph.D., is an author, biologist, and educator dedicated to making rigorous science clear and accessible. Director of Science View Productions™ and Adjunct Professor at the College of the Holy Cross, Dr. Roche has published over 195 articles and has written and taught extensively about science. For more information, visit https://authorjohnproche.com.