Rice culture can provide a suitable environment for mosquitos to breed. In cases where rice fields interface with urbanized or public areas, mosquitoes can be a public nuisance, and certain mosquito species can create health problems for humans and livestock. Culex tarsalis and several other species can transmit the viruses that cause encephalitis, including West Nile virus. Anopheles freeborni can transmit the pathogens that cause malaria.
Mosquito control in rice fields is often carried out by mosquito abatement or vector control personnel who are authorized to visit rice fields and treat for mosquito infestations. Mosquito abatement or vector control districts employ a variety of methods to manage mosquitoes in rice fields including insecticide applications and stocking fields with mosquitofish, Gambusia affinis that eat mosquito larva. Some mosquito control agencies use methoprene alone or in combination with the bacteria Bacillus thuringiensis spp. israelensis (Bti) or Lysinibacillus (=Bacillus) sphaericus (Ls), which are effective in killing mosquito larvae, yet have low toxicity to other organisms. Agencies also use ultra-low volume pesticide fogs to control flying adult mosquitoes in rice-growing areas (usually using pyrethroids or malathion). These fogs do not kill fish or insects and aren’t lethal for some of the invertebrates in the water. The fogs may, however, kill some terrestrial insects, especially smaller ones. To find out the specific control measures being used in your district, contact your local agency. This is especially important with certified organic fields.
Coordinate with mosquito abatement personnel so they can access roads to monitor fields. In addition to the control measures taken by mosquito abatement districts, there are numerous cultural practices growers can employ to help with mosquito control.
- Drain and eliminate borrow pits and seepage areas outside the field.
- Seek assistance from your local mosquito abatement district staff to develop the best possible abatement program for a field.
- Check with abatement district personnel to find out how to minimize unwanted effects of pesticides used in a pest management program on mosquitofish. Notify them if disruptive pesticides are used after the end of May so that they can monitor the fish populations.
Preserving Mosquito Predators
Many naturally-occurring insects in rice fields are predators of mosquitoes. These include backswimmers, scavenger beetle larvae, giant water bugs, predaceous diving beetles and their larvae, damselfly nymphs, and dragonfly nymphs. While these predators consume the majority of mosquito larvae, supplemental mosquito control is usually necessary because the mosquito larvae that survive can still mature into problematic numbers of adults. It is very important, however, to conserve the natural predators that accomplish most of the mosquito control. Whenever possible, follow integrated pest management practices for invertebrate pests that limit the use of broad-spectrum pesticides that are toxic to fish. This helps the survival of mosquitofish and other natural enemies of mosquitoes which prevents the increase of mosquito numbers.