May 6, 2013

It's Complicated: Landscape Diversity for Pest Control

By Wei Zhang, Research Fellow, and Mark Rosegrant, Director, Environment and Production Technology Division, International Food Policy Research Institute (IFPRI), Washington DC

On the Landscapes Blog we’ve seen how diversity on the farm level to diversity at the scale of a whole landscape can provide the natural benefits from healthy ecosystems. Two researchers from the International Food Policy Research Institute explain how a recent study, conducted in collaboration with the Chinese Academy of Sciences and Chinese Academy of Agricultural Sciences, shows the importance of different habitat functions within a landscape for supporting natural enemies to crop pests. But they also share how complex these ecological interactions are, and how one size does not fit all.
Land use diversity can support ecosystem services such as biological pest control and reduced need for insecticides. Empirical evidence in support of this principle has been collected in North America, Europe and Australia. But very limited information is available from developing countries, where smallholder farming is predominant and local people likely need safe food and environmentally benign pest control by natural enemies more than in the developed countries.

As the world’s largest pesticide producer and consumer, China uses 1.3 million tons of pesticides annually – 2.5 times the global average usage per unit area. Chinese farmers have relied on chemical insecticides as their primary pest control method, with insecticides often applied in an indiscriminant fashion. China is the world’s largest cotton producer, a crop that accounts for a large share of insecticides used, both in aggregate and per hectare use, even after the extensive adoption of insect-resistant Bt cotton.

Chinese agriculture is characterized by highly disturbed agro-ecosystems, but also very diverse land use at the small plot level. At the same time, globalization and rural-to-urban migration is shifting land use toward monoculture systems. A pervasive concern is that such landscape simplification may result in an increase in insect pest pressure, and thus an increased need for insecticides.

A study funded by the Ecosystem Services for Poverty Alleviation (ESPA) programme is providing the first empirical evidence on effects of land use diversity on the abundance of natural enemies for pests in smallholder field crops in a developing country context, measured with remote sensing and ground-truthing. Results indicate that high land use diversity itself, as measured by Shannon or Simpson index, is not associated with high density of natural enemies in cotton. Rather, the effects of land use on natural enemies are explained by the functions provided by a diversity of crop and non-crop habitats that may support the insect species that provide biological control services in these landscapes.

In this study, a high land use proportion under maize was associated with greater densities of ladybeetles in cotton. This result is explained by the low use of pesticides in maize on the one hand, and the previous crop of maize (within the same season) and wheat, which supports high densities of natural enemies that may spill over to other crops, both in space and time. However, these landscape effects contrast with those reported previously in the USA, where maize area is negatively associated with the density and diversity of natural enemies in soybean fields, and with soybean aphid biological control. Thus, we show that landscape effects exhibit a degree of idiosyncrasy that cannot be captured by simple indices like Shannon and Simpson index.

Furthermore, we documented the intensive use of insecticides by farmers in China and its detrimental consequences for natural enemies in cotton. When farmers decide to use high levels of insecticides, they do not account for the biological control services provided by the natural enemies harmed by these chemicals. A lack of farmer awareness of the value of natural enemies for crop health indicates that better transfer of information to farmers could support the health of crops, the environment, and the farmers.

While results on the economic value of ecosystem services to smallholders are still being finalized, the findings so far demonstrate that promoting land use diversity has the potential to become an effective means to support biological control services in agricultural landscapes. However, at present, enhancing biological control services by promoting land use diversity is unlikely to be cost-effective for smallholder producers given the high return rate for insecticide use. On the other hand, coordinated habitat management at the landscape scale can potentially be economically viable. Policies that encourage farmers to account for the human health and environmental costs of insecticides would help incentivize the adoption of better habitat management.

This work ‘Landscape Diversity and Ecosystem Services in Agricultural Ecosystems: Implications for Sustainable Growth and Rural Poverty in China (NE/I004335/1)’, was partly funded with support from the Ecosystem Services for Poverty Alleviation (ESPA) programme. The ESPA programme is funded by the Department for International Development (DFID), the Economic and Social Research Council (ESRC) and the Natural Environment Research Council (NERC) of the UK.

Photo credit: Wopke van der Werf