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Arthropod pest outbreaks are becoming more common in the grasslands of Inner Mongolia, China, most likely due to the expansion of agriculture. The area of cropland has increased from 43,300 km 2 in 1949 to 76,300 km 2 in 2005. To understand the effects of agricultural activities on arthropod distribution, sweep net sampling was conducted in a natural grassland. We collected 1287 individuals belonging to 23 families and 9 orders of arthropods from 41 sites. We divided these samples into two guild types (predator and herbivore) and analyzed six groups (grasshoppers, herbivorous Coleoptera, herbivorous Hemiptera, Cicaddidae, Araneae, predatory Coleoptera) of them. Using a negative binomial regression analysis we analyzed the relationships between each group's sampled population size and measured environmental factors including pesticide application and mean temperature of the previous year, agricultural site, and vegetation condition. The best model for each group was determined using Akaike's information criterion. Grasshoppers showed a significant positive response to pesticide application and degraded vegetation, whereas spider (natural enemies of some insect pests) showed a negative response to these factors. The population of grasshoppers increased at sites where pesticides were applied, vegetation was degraded, and spider numbers were reduced. Other groups were not significantly correlated with pesticide application and degraded vegetation. Some of the environmental factors we studied promoted pests the following year. Our results also suggest that the current use of pesticides may not be effective for pest control and that alternative options should be considered in the grasslands of Inner Mongolia.

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