"Pesticide use has been profitable for many farmers and economies. One estimate is that, in the United States in 1997, each US$1 invested in pesticides returned US$4, so that the US$6.5 billion invested in pesticides saved US$26 billion in crop losses (Pimentel 1997). All other things being equal, pesticides have been effective in reducing crop losses.
However, despite the substantial increases in the volume and value of pesticide use since the 1950s, there appears to have been very little, if any, decline in the proportion of agricultural output being lost to pests. Some analyses indicate that there have actually been increases in the proportion of crop being lost to pests. According to Pimentel, data from the U.S. Department of Agriculture (USDA) show a 10-fold increase in both the amount and toxicity of insecticide use in the United States from the early 1940s to the 1990s.
During the same period, though, crop losses from pests rose from 30 to 37 percent, losses from insects increased from 6 to 13 percent, and losses to plant pathogens from 10 to 12 percent, while losses from weeds decreased from about 14 per cent to 12 percent...
The trend of an increasing proportion of crop output being lost to pests, despite a multi-billion-dollar investment in pesticides, appears to be a global phenomenon. Oerke et al. (1994) compared the estimates of global pest-induced losses between 1965 and 1990 for the eight major crops they studied. The comparison between Cramer 1965 and their own 1990 estimates showed that losses increased during the 25-year period for all crops except coffee, with wheat, potatoes, and barley suffering the largest increases in percentage lost.
A partial explanation for the paradox is that the industrialization of agriculture and the reliance on agrochemicals has led to changed farming systems that have produced higher yields, but have also led to an increased vulnerability of crops to pests."
Yudelman, M., Ratta, A. and D. Nygaard (1998). "Pest Management and Food Production: Looking to the Future." International Food Policy Research Institute.