How to Manage Pests

UC Pest Management Guidelines

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 and ditch banks, or 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 (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 long) 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.

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 July until September. They feed on rice leaves, but by this time most plants are growing vigorously and are not adversely affected 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 remain at the base of plants, particularly perennial grasses, or 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

Rice water weevil is the most economically damaging of the invertebrate pests found in California. Root pruning by larvae is the major cause of reduced yields. Plants with damaged roots may become stunted and lose yield through reduction of tillers and panicles or because maturity is delayed. Reduced tillering and slower growth also allow weeds to become better established. 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 populations of adults feeding on young rice seedlings just as they emerge through shallow water may kill some seedlings, but such damage is uncommon.

MANAGEMENT

Management of rice water weevil may be enhanced by weed control in areas around the rice field, drill seeding, and winter flooding, depending upon current management strategies for other pests. In fields with chronic, damaging populations of rice water weevil, a preventive treatment 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.

Removing weedy vegetation on the levees in spring near the time of seeding can reduce rice water weevil infestations in fields and subsequent larval populations.

Drill seeding involves seeding the rice field, irrigating the soil to germinate the seed, and keeping the soil moist for 6 to 8 weeks, at which time the field is then flooded. By the time the field is flooded, the infestation period for rice water weevil is over. This method of control reduces or eliminates weevil populations but is not compatible with current weed management strategies.

Winter flooding of the field to enhance straw breakdown and provide waterfowl habitat can help with weevil control and reduce populations 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

Cultural control methods including weed control and increasing field size are acceptable for use on organically certified rice.

Monitoring and Treatment Decisions

Treatment decisions for rice water weevil are currently based on past history of a particular field, proximity to weevil overwintering sites (ditch banks, riparian areas, weedy canal banks, etc.), and economics. Research is currently under way to develop a new sampling method using a floating barrier trap to determine the need to treat using the presently registered post-flood insecticides.

Lambda-cyhalothrin (Warrior), zeta-cypermethring (Mustang), and diflubenzuron (Dimilin) are foliar treatments applied after flooding; these products control rice water weevil adults by disrupting their life cycle but have no toxicity against the weevil larvae, which is the primary damaging stage. Warrior and Mustang work by killing the adults, therefore reducing the number of eggs deposited and the resulting larval population. Rice plants are protected by the reduction of larval populations. Dimilin functions by sterilizing adult females, causing them to lay nonviable eggs, thus reducing the larval populations. It also is toxic to newly laid eggs, i.e., those that are less than 4 days old.

Because there is only one generation a year, timing is critical with these products. If they are applied too late, the eggs and/or larvae may already be present and the product will have little effect. Also, they are fairly short-lived products in water; if they are applied too early, they will dissipate before the adults are present in the field and have minimal effect.

Research has shown that applications of these products to only the 30 to 50 feet adjacent to the levees 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.

PUBLICATION

UC IPM Pest Management Guidelines: Rice
UC ANR Publication 3465
Invertebrates
L. D. Godfrey, Entomology, UC Davis

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