How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

The rice leafminer adult is a small, olive-green fly (0.018 inch long) commonly found in the early season walking on the water surface or on rice leaves lying on the water surface. The females lay their elongate, white eggs singly on the upper surface of these leaves. They prefer leaves floating on the water, and high humidity (80-100% relative humidity) is required for hatching. In 3 to 5 days, eggs hatch into cream-colored, maggot-shaped larvae. The larvae burrow between the outside leaf layers and mine the leaf. Larvae may pupate in an existing mine or migrate to a different leaf to form a new mine. Total development time from egg to adult is about 2 weeks at 85° to 90°F. Rice leafminers generally overwinter as adult flies, and they may begin to lay eggs on leaves of a wide range of grasses associated with aquatic habitats as early as February.

DAMAGE

Injury is caused by leafminer larvae feeding in mines between the two epidermal layers of a leaf. The mines usually contain a swelling, which is the body of the feeding or pupating leafminer. The mined area on the leaf fades to a light green color at first, then turns yellow and may appear white with time if it dries. Because high humidity is required for hatching, leafminer infestations are usually confined to leaves lying on the water surface. The larvae are mobile and move on to new leaves after old ones are completely mined. In severe infestations, they may also mine the leaf sheath.

Plant vigor and weather conditions govern the extent and seriousness of the injury. Any factor affecting plant growth, especially deep water culture or cool weather, that increases the number of leaves remaining prostrate on the water or the length of time they are fully in contact with water will extend the period of injury. Leaf loss and the resulting reduction in photosynthesis, is critical at this time as food reserves from the seed have already been depleted to get the plant through the water. The plant is usually able to put forth additional leaves, but continued mining can result in reduced tillering, greater susceptibility to later pest attack, delayed maturity, or death of the plant. Once leaves start growing upright above the water, the rice leafminer does not cause economic damage.

MANAGEMENT

Leafminers can be found in every field every year, but the seriousness of their attack will be closely related to the speed with which the plant grows erect and out of the water. The temperature of the air and water, plant vitality, and water depth all play a role. Manage water levels in the field to encourage the rice to emerge quickly and grow erect. Monitor for rice leafminers to determine the need to treat. Although populations of leafminer parasites can be high in rice fields, they generally do not build up to adequate levels early enough to prevent economic damage from the first generation.

Biological Control

Several parasitic wasps attack the rice leafminer. The most effective are Chorebus aquaticus and Opius hydrelliae. Parasites control up to 50% of the generations of leafminers that feed on grasses before rice fields are flooded. In rice, parasitism of the first generation of leafminers is low, but increases to 70 to 80% on the second and third generations. Normally a combination of parasites, predators, and high temperatures cause leafminer populations to drop rapidly by June.

Cultural Control

To reduce the potential for damage, level the field as accurately as possible and start the crop in 3 to 4 inches (7-10 cm) of water. Under these conditions, the rice will more likely emerge quickly and develop stout stems and erect leaves. Increase the water depth slowly after the leaves begin to grow upright. Similarly, where the crop is growing slowly in a cool season, lower the water to encourage more rapid growth. (See restrictions on water release in section on TREATMENT DECISIONS.) If the source of water is cold, such as some wells, you may want to establish a warming basin before the first seeded check.

Organically Acceptable Methods

Biological and cultural controls are organically acceptable methods.

Monitoring

Begin monitoring 2 to 4 weeks after planting, just after most of the rice plants have emerged from beneath the water and the leaves are lying on the water surface. Use a monitoring ring or floating plastic tubing 1 square foot in diameter. Take ten, 1-square-foot samples in a transect of the field or field. Check the plants within the ring for unhatched leafminer eggs on upper leaf surfaces. Unhatched eggs are opaque, while hatched eggs are clear and flat. Also check for larvae of all instars; second and third instar larvae are the easiest to detect because you can feel the swelling they cause in the leaf.

Treatment Decisions

The following treatment guidelines are for fields seeded at 150 pounds per acre:

If the average plant stand is less than 25 emerged plants per square foot and the infestation level is 10% or higher, lower the water level and use a recommended insecticide. If the infestation level is less than 10%, lower the water level and sample again in 4 days. If the infestation level stays this low, treatment will not be required. When most of the leaves of the rice plants are upright, the rice leafminer will no longer cause economic losses. Consider prevailing weather conditions in cases where the need to treat is not clear cut. Cool growing conditions may favor leafminer damage.

Any release of the water is dependent upon what pesticide residues may be in the water and the time restrictions for holding the water in the paddies. Stopping the water flow into the paddies may allow an adequate reduction in the water level as a result of perculation and evaporation.

No treatments are required after leaves start growing upright above the water.

PUBLICATION

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

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