Rice fields provide excellent habitat for some bird and rodent species.The Norway rat is probably the most serious vertebrate pest of rice, feed-ing on newly planted seed, seedlings, or ripening grain. Muskrats bur-row in rice levees, damaging drainage systems and irrigation structures.Waterfowl and blackbirds may also cause yield losses in some localizedareas.

The most successful vertebrate pest management program is one thatmanages pest populations at levels at which significant damage neveroccurs. This requires knowledge of the biology and behavior of thepotential pests and regular monitoring for them in and around fields.Historical records of pest population levels, control measures imple-mented, economics of control procedures and the success of methodsused, can be used to help determine the best management approach.Consideration also should be given to the presence of non-pest speciesand the potential risks of a control method.

The methods and materials available for vertebrate control are constant-ly changing. Check with your County Agricultural Commissioner onlaws and regulations concerning the status of wildlife species and themethods and materials available to control them.

WaterfowlAnnually, large numbers of waterfowl (ducks,geese, coots) migrate along the Pacific flywayto and from their northern breeding groundsand may spend from a few weeks to months infall and winter in California's Central Valley.Flooded rice fields provide an ideal habitat forwaterfowl and have an important role in theconservation of these birds. Because most ricein California is harvested prior to the arrival ofmigrating waterfowl, and planted after theirdeparture, damage to rice is usually kept to aminimum. Most problems occur where water-fowl become 'resident'. Ducks and geese causethe most serious losses in rice by feeding onmaturing grain and sometimes causing lodg-ing. Coots may sometimes damage newlyplanted fields.

Management guidelines

All species of waterfowl are migratory gamebirds that are protected by federal and statelaws. Waterfowl cannot be lethally controlled

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Vertebrate Pests of Rice

Section author:

Desley Whisson,Extension Wildlife

Specialist Department ofWildlife, Fish and

Conservation Biology,UC Davis

without a depredation permit. Damage can usually be alleviated usingfrightening (hazing) techniques. Propane cannons, electronic noise mak-ers, pyrotechnics, mylar tape and other sound and visual scare devicesmay be used to frighten waterfowl from areas where they are causingdamage. Waterfowl may habituate to these techniques so they mightonly have short-term effectiveness. Persistence in applying the methodsand alternating frightening devices are important in achieving success.

BlackbirdsBlackbirds may damage ripening rice, especiallyduring the milk and dough stages. Losses may bequite high in some fields that are close to impor-tant roosting areas.

Management guidelines

Frightening techniques are mostcommonly used to manage black-birds. Unfortunately, these techniques are even lesseffective for blackbirds than waterfowl. To be effective,you must instigate these controls as soon as birdsappear in the field. A permit is not needed to lethallyremove blackbirds that are causing or threatening todamage rice crops. To date, repellents have not provento be effective in reducing blackbird damage to rice inCalifornia.

Norway ratsRat damage to growing rice isusually most serious shortlyafter planting when the wateris temporarily lowered forseed germination and stand

e s t a b l i s h -ment. Thiscan be espe-cially severewhere fields are not level and high spots are exposedto air. The rats pull up the sprouting plants and eatthe seeds. Rats may also consume ripening grain asthe cereal heads come into the milk stage but lossesare generally not as serious.

The Norway rat (Rattus norvegicus) is responsible formost rat damage in rice fields. Norway rats have abulky appearance and a tail that is shorter than thelength of their head and body combined. Rats aremainly active at night but if their numbers are high,

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activity may be observed during the day. They are omnivorous and feedon a wide variety of plant and animal materials. Norway rats dig bur-rows and burrow systems are frequently found along levees beside ricefields. These burrows can weaken levees systems and affect irrigation.

Norway rats are prolific breeders. They are capable of breeding year-round under optimal conditions but most breeding is in spring and fall.Females produce about 4 litters per year. Average litter size is about 6,and the young are weaned at 3 weeks and become sexually mature at 2to 3 months of age. These reproductive characteristics enable rat popu-lations to rapidly increase and become widespread in response to onsetof optimal environmental conditions. Populations typically undergocycles of abundance. Problems are most likely to occur following mildwinters and when food supply is abundant.

Management guidelines

Norway rats are non-native mammals and may be taken at any time andin any manner when they are causing damage to crops or other proper-ty. Where possible, non-crop habitats should be managed year round toreduce shelter and food supply for rats. Good weed control on levees isessential. Ground vegetation in areas adjacent to rice fields should bekept to a minimum by grazing or mowing.

When rat populations continue to be high despite habitatmodification, rodenticides may be used. Currently (2003)registered rodenticides for Norway rat control on levees andadjacent non-cropped areas include the acute toxicant zincphosphide and the anticoagulants diphacinone andchlorophacinone. The rodenticide and application methodshould be chosen with regard for potential non-target haz-ards. Consult your Agricultural Commissioner for specificinformation.

Zinc phosphide is an acute toxicant that is metabolizedquickly within the target animal, and has minimal (if any)secondary poisoning risks. However, because of its fastaction, rodents might only ingest a sublethal dose of baitbefore becoming sick. This may result in rodents becoming'bait shy'. Consequently, it is best to wait at least 3 months,and preferably 6 months between applications. Zinc phos-phide bait is placed according to label instructions in active burrows orin places frequented by rats but inaccessible to livestock, poultry, non-target wildlife, pets and children. When possible, prebait with cleangrain several days before bait application to determine if rats are takingthe bait and to overcome any bait shyness. Prebaiting is especiallyimportant where other foods are abundant.

Anticoagulant baits act by reducing the clotting ability of the blood. Thetarget animal must consume a number of doses of bait over a period of

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several days to obtain a lethal dose. Because a residue of the anticoagu-lant bait may remain in the target animal (primarily in the liver), preda-tors or scavengers may also be at risk of consuming a lethal dose of bait.Risk increases where treated rat populations levels are extremely high(i.e., high availability of carcasses containing anticoagulant) and in areaswhere and at times when predator and scavenger populations are espe-cially abundant. Risk also increases when too much bait is applied. Aswith all rodenticides, follow the label carefully and use as little bait aspossible to bring the population under control.

Anticoagulants may be applied in bait stations, by spot application, or inparaffinized bait blocks. Bait stations protect bait from rain and preventnon-target species feeding on the bait. To achieve control, keep the sta-tions well-supplied until feeding ceases. Bait blocks made from paraffinmay be placed in areas of rat activity. These blocks aren't as readilyaccepted as loose baits but are relatively waterproof and eliminate theneed for bait boxes. Replace blocks as necessary and discard of uneatenbait when the control program is completed.

MuskratsM u s k r a t s( O n d a t r azibethicus) ares e m i - a q u a t i crodents namedfor their conspic-uous odorresulting fromsecretions frommusk glands atthe ventral baseof the tail. Theysometimes inhabit water supply canals and drainage ditches near ricefields. Their burrowing, especially around headgates can cause breaksin levees and dikes. Significant yield loss may occur before repairs canbe completed. Muskrats also occasionally cut and eat rice plants.

Muskrats have dense fur, a long, laterally flattened tail and partiallywebbed feet. Adults are about 18 inches long and weigh from 1.5 to 2.5pounds. Their native range in California was along the Colorado Riverby the Arizona border and in scattered locations on the eastern side ofthe Sierra Nevada from Mono to Lassen County. Construction of irriga-tion canals in the early 1900's enabled them to expand their range intosouthern California. A high demand for muskrat fur during the 1920'sresulted in the release of muskrats elsewhere. Muskrats now occupycanals, ponds, and irrigation ditches throughout most of California.

Muskrats are very prolific. Most females have two or three litters per

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year, with an average of about 6 or 7 kits per litter. Most births occur inspring. The gestation period is between 25 and 30 days. The youngbecome active and able to swim within 14 days, and are weaned at 28days. Most become sexually active the spring after their birth.

Depending on the environment and season, muskrats construct eitherhouses or dig burrows into banks. Unless the population is very dense,muskrats prefer to burrow into banks rather than build houses. Soil typeand slope of the bank determine the permanence and complexity of aburrow system. Burrows often begin in the water, from 6- to 8-inchesbelow the surface, and penetrate the embankment on an upward slant.Burrows are not typically found when banks are less than 0.2 metershigh, slope is less than 10 degrees, or when combined sand/gravel con-tent is less than 90 percent. Burrows may extend 20 feet or more intobanks. Houses are usually constructed from the dominant emergentplants in the area. They are built at water level with several underwatertunnels or "leads" for entrances.

Muskrats are primarily herbivorous although animal matter like crayfishmay occasionally be consumed. Muskrats feed on aquatic vegetationgrowing in the vicinity of their dwellings. Characteristic signs ofmuskrat feeding activity include food platforms and feeding houses.Most activity occurs at night, with peaks at dusk and dawn.

Management guidelines

Muskrats are classified as furbearers but can be taken at any time whenthey are causing damage to crops or other property. Management ofvegetation to reduce muskrat food sources on levees and on ditch bankscan help minimize muskrat problems. In some situations however,lethal control of muskrats is necessary. Trapping can be very effective inreducing muskrat populations and damage. Conibear traps are proba-bly the most effective but it is important to note that while they can beused to remove muskrats causing damage, they cannot be used to trapmuskrats for fur (see Fish and Game Code).

Anticoagulant baits (diphacinone and chlorophacinone) may also beused. Bait can be placed in floating bait boxes or bait blocks (also usedfor Norway rat control) may be placed in muskrat feeding areas on lev-ees and banks.

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Information SourcesClark, J. P., editor. 1994. Vertebrate Pest Control Handbook. FourthEdition. Division of Plant Industry Integrated Pest Control Branch,California Department of Food and Agriculture, Sacramento, California,USA. (For sale at the Agriculture Commissioner's office.)

Hygnstrom, S. E., R. M. Timm, and G. E. Larson, editors. 1994.Prevention and control of wildlife damage. Volumes 1 and 2. Universityof Nebraska Cooperative Extension, U. S. Department of AgricultureAnimal Damage Control, Great Plains Agricultural Council. (Availableas a 2-volume set or on CD-ROM in UCCE offices. May also be orderedfrom the Internet Center for Wildlife Damage Management:http://wildlifedamage.unl.edu/)

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