How to Manage Pests

Mosquitoes

Managing Mosquitoes on the Farm Section 9: Rice Published 2005 Sections of this publication:

Why You Should Care About Mosquito Control West Nile Virus You May Be Raising the Mosquito That Is Biting You! Mosquito Prevention Three Basic Principles of Mosquito Prevention Natural Waters Associated With Farms Managing Stagnant Waters Created by Agricultural Activities Irrigated Fields for Upland Crops Mosquito-Free Irrigated Pastures Rice Dairy Operations Biological and Chemical Mosquito Control Common Mosquitoes and Their Life Cycles Acknowledgments For More Information

Over one-half million acres (200,000 ha) of rice are currently grown in California. The standing water of rice fields and associated irrigation structures produces a great challenge for mosquito control. The majority of mosquitoes in rice fields are Culex tarsalis in the summer and Anopheles freeborni from August through when the fields are drained. Culex tarsalis is the vector of encephalitis viruses, including West Nile virus, and Anopheles freeborni can carry malaria, although this disease is now rare in the state. Some fields may also produce floodwater mosquitoes (Ochlerotatus melanimon) within the first 1 to 3 weeks of flooding. Some MVCDs set up regular inspection and treatment visits for rice fields. They often add mosquitofish to fields and may control mosquitoes with biological or chemical agents as well (see the section Biological and Chemical Mosquito Control).

Grower practices can affect the numbers of mosquitoes produced in rice fields. The following sections describe how you can manage various aspects of rice cultivation to minimize mosquito production and help MVCDs work most efficiently.

Basin Shape

Mosquito larvae find refuge from predators in shallow waters and grassy areas. Reduce mosquito breeding areas by configuring your fields, field edges, levees, and irrigation ditches so that they drop rapidly into deep water. Eliminate shallow edges and any small seepage puddles near levees and fix leaky water control structures to prevent isolated puddles.

Fertilizing

Overfertilizing can cause algal blooms. When algal mats rise to the surface they create isolated pockets of surface water where mosquito larvae are protected from predators. Incorporate fertilizers several inches into the soil to reduce algal blooms. Reducing algae also helps improve yield by reducing competition between rice and algae.

Flood-up

Floodwater mosquitoes (Ochlerotatus melanimon) can breed in rice fields. These mosquitoes appear within a day or two of flood-up and can occur in very large numbers. Check with your local MVCD to see whether these mosquitoes are a problem in rice in your area; if so, alert the MVCD when you begin flooding. Rapid flooding of the fields encourages mosquito eggs to hatch in synchrony, allowing MVCDs to eliminate most larvae with one treatment.

Weeds

Weedy fields and levee edges can provide shelter for mosquito larvae. Predators hunt less efficiently in dense weeds, so good weed control practices help control mosquitoes. However, many herbicides are oily and will suffocate air-breathing predaceous insects that provide some natural mosquito control, and some herbicides have direct toxicity to insects as well.

Herbicides are best used early in the season so that the predators are not reduced when the water is warmer and mosquitoes develop faster. Predator-friendly weed control methods include deeper flooding early in the growing season, weed removal along levees, and crop rotation to upland crops every few years.

Pesticides

Some pesticides kill both mosquitoes and their predators, but since mosquitoes have shorter generation times, they typically recover faster than the predators. This may lead to an increase in mosquitoes. Some pesticides can kill mosquitofish, especially pyrethroids (e.g., lambda-cyhalothrin, or Warrior and Karate). Advise MVCDs if you use pesticides after fish have been introduced to the fields so they can add more fish if needed. MVCDs typically introduce fish within several weeks after flooding.

Draw-downs and Draining Rice Fields

Water may be drawn down or drained from rice fields during the growing season for weed control. While mosquitoes can't breed in a dry field, reflooding can lead to a mosquito problem for several reasons. Draining removes most of the predators from the field, increasing the chance of mosquito resurgence after reflooding. Culex spp. mosquito females find newly flooded fields attractive for egg laying. Reflooded fields may also produce the floodwater mosquito.

Advise MVCDs if you need to drain and reflood a field midseason so they don¡t waste inspection trips when fields are dry and so they can resume inspections in time to prevent mosquito emergence. They may also need to restock mosquitofish.

Draining fields at the end of the growing season does not usually generate a mosquito problem because remaining predators tend to concentrate in residual water. Nevertheless, well-graded and even-draining fields, borrow pits, and ditches help reduce mosquito numbers because some areas may lack predators. Rice fields can also produce mosquitoes in the fall if they are reflooded for straw decomposition and to create wildlife habitat. Fields should be reflooded as late as possible because mosquito reproduction decreases in cold weather.

Managing Mosquitoes on the Farm, UC ANR Publication 8158 Sharon P. Lawler and Gregory C. Lanzaro, Department of Entomology, University of California, Davis

Copyright © 2005 The Regents of the University of California, Division of Agriculture and Natural Resources. All rights reserved.