Agriculture: Rice Pest Management Guidelines

Rice Water Weevil

  • Lissorhoptrus oryzophilus
  • Description of the Pest

    The rice water weevil adult (about 0.125 inch or 4 mm long) has a prominent beak and is gray with a dark marking on its back from the base of its head to the middle of its wing covers. It is distributed throughout the Sacramento Valley, the upper San Joaquin Valley, and central San Joaquin Valley (Merced County). It overwinters as an adult at the base of grass clumps, in weedy debris, on levees, ditch banks, and in soil crevices.

    When daytime temperatures rise above 70°F (21°C) in late winter or early spring, rice water weevil adults begin feeding on grasses to build up their wing muscles. On calm, warm evenings, from sunset to midnight, they fly in search of plant hosts growing in water. They are attracted to flooded rice fields and begin feeding on emerged rice or grasses in water along the levee banks. Longitudinal scars on the upper leaf surface indicate their presence in the field, if they are not directly observed.

    No males occur in California, and females reproduce without mating. After arriving in a field and feeding for a few days, they lay their eggs singly under water in the leaf sheath tissue above the plant crown. One female may lay over 200 elongated eggs (0.03 inch or less than 1 mm in length) over a period of several weeks. The adults that initially infest a field prefer to feed and lay their eggs in areas of the field adjacent to the levee margins and headlands.

    About 7 days after being laid, the eggs hatch. Small larvae mine the leaf sheath for about a day and then move to the soil to feed on the roots, where they stay through four larval instars. The legless larvae are milky white with light-brown heads and have spinelike projections on their backs to pierce the roots and obtain oxygen. When their growth is completed, larvae pupate in mud-coated cocoons that they attach to the roots of rice, sedges, or various grasses.

    Adults emerge from the pupal cells from early July until late September. They feed on rice leaves, but by this time most plants are growing vigorously and are not harmed by this late adult feeding. A few of these adults will lay eggs in July or August but most enter a resting stage called diapause. They fly to overwintering sites and remain at the base of plants, particularly perennial grasses, or in debris through the winter.

    The life cycle from egg to adult takes about 78 days in the laboratory at 73°F (23°C) with approximately 7 days in the egg stage, 50 days as larvae, and 21 days for pupation. The minimum time for development in the field is about 60 to 65 days.

    Damage

    Root pruning by larvae causes reduced yields. Plants with damaged roots may become stunted and lose yield through reduced production of tillers and panicles, or because maturity is delayed. Reduced tillering and slower growth also allow weeds to establish more readily. The heaviest infestations and most serious damage can be expected to occur between late May and July within 15 to 20 feet of the margins of the fields and levees, where weevils are concentrated; moderate damage can occur in areas 20 to 35 feet from levees.

    Adult feeding appears as linear slits of varying length on the upper surface of the leaves but generally does not cause economic losses. High numbers of adults feeding on young rice seedlings just as they emerge through shallow water may kill some seedlings, but such injury is uncommon.

    Management

    Management of rice water weevil may be improved by weed control in areas around the rice field, dry seeding, and winter flooding, depending upon current management strategies for other pests. In fields with chronic, damaging populations of rice water weevil, a preventive insecticide application may be advisable with application limited to the field edges.

    Cultural Control

    Rice water weevil adults have shown a preference, in California, for the areas adjacent to levees and field edges during the critical period of infestation at the beginning of the season. Large, laser-leveled fields generally have less land per acre associated with levees and field edges and, therefore, large fields will have less area per acre subject to infestation.

    Eliminating weedy vegetation on levees in spring, near the time of seeding, can reduce rice water weevil infestations in fields and subsequent larvae.

    Dry and drill seeding involves seeding the dry rice field, irrigating the soil to germinate the seed, and keeping the soil moist for 4 to 6 weeks, at which time the field is flooded. By the time the field is flooded, rice plants are more tolerant to rice water weevil injury. These methods of control reduce or eliminate exposure of susceptible plants to weevil populations but would require different weed management strategies.

    Winter flooding of the field, to improve straw decomposition and provide waterfowl habitat, can help with weevil control and reduce numbers by about 50%.

    Aquatic weeds are strong competitors of rice for nutrients and space, particularly during the tillering stage. Because the rice water weevil larvae prune roots and reduce tillering, it is particularly important to have early and effective weed control to maximize recovery from water weevil injury.

    Organically Acceptable Methods

    Use cultural control methods, including weed control and increasing field size, in an organically certified crop.

    Monitoring and Treatment Thresholds

    Treatment decisions for rice water weevil are primarily based on history of a particular field, proximity to weevil overwintering sites (ditch banks, riparian areas, weedy canal banks, etc.), and economics. Feeding scars can be used to confirm weevil presence. However, since egg laying can occur before plants have emerged from the water, significant injury can occur even if only a few adult feeding scars are found.

    Lambda-cyhalothrin, zeta-cypermethrin, and diflubenzuron are foliar treatments that are applied after flooding. These insecticides control rice water weevil adults by disrupting their life cycle but are not toxic to weevil larvae, which is the most damaging stage to rice. Lambda-cyhalothrin and zeta-cypermethrin work by killing the adults, therefore reducing the number of eggs deposited and the subsequent larval population. Rice plants are protected by the reduction of larval numbers. Diflubenzuron functions by sterilizing adult females, causing them to lay nonviable eggs, thus reducing the number of larvae. It also is toxic to newly laid eggs that are less than 4 days old.

    Because there is only one generation per year, timing is critical with these pesticides. If they are applied too late, the eggs and larvae may already be present and the pesticide will have little effect. They are fairly short-lived pesticides applied to the flood water and if they are applied too early, they will dissipate before the adults are present in the field and will have minimal effect.

    Applications of these pesticides to the 30 to 50 feet adjacent to the levees in a rice field can provide acceptable rice water weevil control in most conditions. This is the area with the most significant rice water weevil infestation, and the insecticide applied only to this area persists in an adequate concentration long enough to affect the adult weevils if timed correctly.

    Lambda-cyhalothrin is also registered for preflood application. In areas that have a history of rice water weevil infestations, this application may be considered. Applications can be made within 5 days of flooding and light incorporation into the soil improves efficacy.

    If significant injury unexpectedly occurs, clothianidin can be applied foliarly as a rescue treatment. This insecticide will kill some of the larvae feeding on roots, but its effectiveness is limited.

    Common name Amount per acre REI‡ PHI‡
    (Example trade name)   (hours) (days)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.
     
    A. CLOTHIANIDIN
      (Belay) 4.5 fl oz 12
      MODE-OF-ACTION GROUP NUMBER1: 4A
      COMMENTS: May be applied preflood or postflood at 1- to 3-leaf stage with majority of plants in the 2-leaf stage. Best control results of rice water weevil larvae have been seen with the postflood application. A rescue treatment may be applied at the 5-leaf stage if an infestation isn’t caught in time. This treatment only provides partial control but can prevent the majority of the crop from getting damaged.
     
    B. LAMBDA-CYHALOTHRIN
      (Warrior II with Zeon) 1.6–2.56 fl oz 24 21
      MODE-OF-ACTION GROUP NUMBER1: 3A
      COMMENTS: A pyrethroid that kills adult weevils for about 7 days and kills larvae as they hatch from eggs; can also be applied preflood under a 24(c) Special Local Need label, which expires May 4, 2025. Apply at 1- to 3-leaf stage with majority of plants in the second leaf stage. Can be used safely when propanil products are being used for weed control. Do not release floodwater within 7 days of an application. See label for other restrictions. Notify the mosquito vector control district personnel so that they can monitor populations of mosquitofish after use of this insecticide.
     
    C. ZETA-CYPERMETHRIN
      (Mustang) 3.4–4.3 fl oz 12 14
      MODE-OF-ACTION GROUP NUMBER1: 3A
      COMMENTS: A pyrethroid that kills adult weevils for about 7 days and kills larvae as they hatch from eggs. Make applications at least 7 days apart and do not release flood water within 7 days of application. Do not apply more than 1.1 pt (0.2 lb a.i.)/acre per season. Notify the mosquito vector control district personnel so that they can monitor populations of mosquitofish after use of this insecticide.
     
    D. DIFLUBENZURON
      (Dimilin 2L) 8–16 fl oz 12 80
      MODE-OF-ACTION GROUP NUMBER1: 15
      COMMENTS: An insect growth regulator; sterilizes adult females for about 7 days and kills eggs up to 3 days old already in the plant. Apply 2 to 5 days after rice emerges above water (i.e., 2- to 3-leaf stage). Can be used safely when propanil products are being used for weed control. Do not apply if flooding is in progress; activity will be reduced. This material is water active so the entire field must be treated. Do not disturb flooded area after application for at least 7 days. Treated water should be held for at least 14 days. Do not use on wild rice or near crayfish aquaculture. See label for other restrictions.
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without personal protective equipment. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of the two intervals is the minimum time that must elapse before harvest.
    1 Rotate pesticides with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; insecticides with a 1B group number should be alternated with insecticides that have a group number other than 1B. Mode-of-action group numbers for insecticides and miticides (un=unknown or uncertain mode of action) are assigned by IRAC (Insecticide Resistance Action Committee).
    Text Updated: 04/24
    Treatment Table Updated: 04/24
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