Rice Water Weevil

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.

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 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.

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.

Use cultural control methods, including weed control and increasing field size, in a certified organic crop.

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.