Does crop diversity affect pest control by natural enemy on an EMS using a microlandscape?

Professor GE Feng and his group from State Key Laboratory of Integrated Management of Pest Insects and Rodents, Institute of Zoology, Chinese Academy of Sciences set out to tackle this problem.

After 4-years of field experiments, they have developed a novel experimental model system using microlandscape to examine resource concentration hypothesis and discovered the ecological mechanism behind the landscape pattern and structure. They found that high crop species richness could suppress the pest population, indicating that crop species richness could enhance biological control services.

Their work, entitled “Effects of crop species richness on pest-natural enemy systems based on an experimental model system using a microlandscape”, was published in SCIENCE CHINA Life Sciences.2013, Vol 56(8).

Agro-ecosystems are composed of a variety of cultured crops grown mostly for human consumption, and simultaneously supply multiple ecosystem services. In these ecosystems, tri-trophic level interactions (crop, pest, and natural enemy) are an important component and have evolved close relationships. Indeed, the effect of species composition and community structure on yields and ecosystem services is a very popular topic in ecology. Some studies have reported that plant species richness directly affects species composition and the abundance of pests, which would influence the biological control services provided by natural enemies. Indeed, a high level of plant diversity (e.g., intercropping, non-tillage, and weeds) could suppress pest populations and reduce yield losses caused by pest damage. Root proposed two hypotheses (the resource concentration hypothesis and the natural enemy hypothesis) to interpret these phenomena. However, Andow found that high plant species richness did not always facilitate the biological control of pests, and only 52%–53% of pest species can be affected by plant species richness. In fact, no consensus has been reached based on this type of research to date. Therefore, the effect of plant species richness on biodiversity and the biological control service of natural enemies in agro-ecosystems is currently an important research topic.

The study region is located in the county of Yishui, Linyi City, Shandong Province, China (35°48'05″N, 118°37'11″E) and has a temperate maritime monsoon climate with an elevation of 101.1–916.1 m. An experimental microlandscape (EMS) was implemented. Twenty primary crop species commonly grown in North China were selected. Five plant richness levels (1, 2, 4, 8, and 16) were designed, and we randomly selected 1, 2, 4, 8, and 16 species to achieve the five crop species richness levels. Every treatment was repeated 10 times. Fifty 9 m×9 m plots were used which were located 1 m apart; the entire experimental site covered 70 m×150 m. For a given plot, the crops were distributed in a matrix of 22 rows and 22 columns. The crop density and amount of fertilizer use was the same in each plot so that the study sites were homogeneous. Herbicide was applied to the unplanted area between the plots to suppress weed growth. No pesticides, herbicides, chemical fertilizers, or other agrichemicals were applied to any of the crops in any of the plots. Weeds were removed by hand. Crop species richness (N) and crop arrangements in each plot remained the same during the four-year study (2007–2010).

Crop species richness had major effects on the biomass of pests and natural enemies, with the pest biomass increasing with increasing crop species richness. However, the differences were not significant. In addition, the natural enemy biomass also increased with increasing crop species richness, although the differences were not significant. The mid-range value of the pest and natural enemy biomass peaked when the crop species richness was at a maximum (N=16); the mid-range value of biomass was 0.12 g/22 plants and 0.04 g/22 plants, respectively. When the crop species richness was at a minimum (N=1), the mid-range value of pest and natural enemy biomass was also low (pest, 0.04 g/22 plants; natural enemy, 0.03 g/22 plants).

The relationship at the tri-trophic level was analyzed by generalized additive models. The effects of sampling time, pest biomass, and crop biomass on the natural enemy biomass were also analyzed. The sampling time had significant effects on the biomass of natural enemies. Although the crop biomass also no significant effects on the natural enemy biomass, the pest biomass did have significant effects. The sampling time had no significant effects on the pest biomass, and the crop biomass also did. In contrast, the natural enemy biomass did have significant effects on the pest biomass.

The ratio of biomass (natural enemy/pest) was used to analyze the relationship between the biological control service and crop species richness. The results showed that the ratio of biomass (biological control service) first increased and then decreased with increasing crop species richness; the differences were significant. The biological control service peaked when the crop species richness was 4, with the median and mean of the natural enemy/pest ratio at the maximum. In contrast, the median and mean of the natural enemy/pest ratio were both at a minimum when the crop species richness was 8. The relationship between the pest and natural enemy biomass was positive and significant: the natural enemy biomass increased with increasing pest biomass. The relationship between the pest and crop biomass was also positive and significant: the pest biomass increased with increasing crop biomass. However, no significant relationship existed between the crop and natural enemy biomasses (as shown in the Figure).

Our finding on the effect of crop species richness on biomass of pest and natural enemy species can be applied in integrated pest management strategies that aim to incorporate crop composition and arrangement into habitat management as they suggest that successful biological control would mainly be effective through the use of well-planned landscape design and the creation of habitat diversity. Combining agricultural landscape design with biological control services may effectively address the ecological function of natural enemies when integrated pest management strategies are used regionally and may provide multiple ecosystem services. Far less attention has been paid to additional ecosystem services that agricultural landscapes could provide as managed landscapes. Our results not only address the biological control of insect pests but also address other ecological services received through landscape design and habitat manipulation.

See the article: ZHAO ZiHua, SHI PeiJian, MEN XingYuan, OUYANG Fang, GE Feng. Effects of crop species richness on pest-natural enemy systems based on an experimental model system using a microlandscape. SCIENCE CHINA Life Sciences, 2013, 56(8): 758-766. http://life.scichina.com:8082/sciCe/EN/abstract/abstract511458.shtml

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