agricultural pilots attempt to fly just above the crops being treated. Fields are often surrounded by obstacles such as trees, telephone lines, and farm buildings. Purpose-built agricultural airplanes have strengthened cockpits to protect the pilot if an accident occurs. Conflicting views on aerial spraying Aerial spraying has been controversial since the 1960s, due to environmental concerns about pesticide drift (raised for example by Rachel Carson's book Silent Spring). It is now often subject to restrictions, for example spraying pesticide is generally banned in Sweden, although exceptions can be made such as for an area plagued by mosquitoes during summer. The aerial spread of fertilizer has also raised concerns, for example in New Zealand fertilizer entering streams has been found to disproportionately promote growth of species that are more able to exploit the nutrients, in a process known as eutrophication, which has led to restrictions on topdressing near waterways. See also Aerial topdressing Aerial firefighting Aerial application Pesticide drift Notable cropduster pilots Alan Prichard Manufacturers Air Tractor Embraer Pacific Aerospace PZL Mielec Thrush Aircraft External links Farming Takes to the Air September 1940 Popular Mechanics Nazarov, V. (1971). Helicopters and the yield. Pesticides. Agricultural machinery