In the next few weeks Saginaw County, Michigan will begin their mosquito spraying season. The red chemical sprayed on rivers and streams is called a larvicide.
Larvicides are products used to kill immature mosquitoes. They
can be either biological (such as toxin from specific bacteria that is
lethal to mosquito larvae but not to other organisms) or chemical
products, such as insect growth regulators, surface films, or
organophosphates. Larvicides are applied directly to water sources
that hold mosquito eggs or larvae. When used well, larvicides can
help to reduce the overall mosquito population by limiting the
number of new mosquitoes that are produced.
After crossing the river the pilot will make a hammerhead turn and cross in the opposite direction taking advantage of the wind. He goes width-wise because the wind, being fickle, may wipe out a lengthwise coverage.