Agricultural development in lower-income countries and resulting increases in agricultural productivity are generally accompanied by a shift from extensive to more intensive types of land use. The objective of this paper is to analyze how pest and plant disease management among smallholder farmers has changed along with the process of land use intensification, the aim being to identify constraints as well as possible approaches to the use of more sustainable pest and plant disease control practices. Using survey data from 240 smallholder farms located in the upland areas of northern Thailand, we show that land use intensification is accompanied by a reduction in the use of traditional methods of pest management and an increase in the use of synthetic pesticides. While farms with a low level of land use intensity sprayed on average twice a year and used a total of 1.4 kg of active ingredients per ha, farms with a high level of land use intensity sprayed on average 16 times and used 22.0 kg/ha. They also used a greater number of different products and tended to mix them together. The intensity of pesticide use was particularly high for cash crops such as tomatoes, chilies and strawberries. Many farmers experienced health problems related to pesticide use because pesticides were not correctly handled. Greater investment is needed in the development of integrated pest management in the long-term, and health problems may be reduced in the short-term by raising awareness among farmers regarding the risks they are exposing themselves to, as well as by promoting good agricultural practices.