Rice Leafminer

The rice leafminer adult is a small, 0.018 inch (0.5 mm) long, olive-green fly, commonly found walking on the water surface or on rice leaves lying on the water surface, in the early-season. The females lay their elongate, white eggs singly on the upper surface of these leaves. They prefer leaves floating on the water, and 80 to 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. The 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.

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 onto new leaves after older 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 that increases the number of leaves lying flat on the water or the length of time they are fully in contact with water will extend the period of susceptibility, especially deep-water culture or cool weather.

Seedling leaf loss, and the resulting reduction in photosynthesis, is critical at this time as energy reserves from the seed have already been depleted to get the plant through the water. The plant is usually able to grow 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.

Leafminers can be commonly found in rice fields, but because modern rice varieties grow above the water quickly, economic injury is rare. Air temperature, water temperature, plant vitality, and water depth all affect the time it takes the plant to emerge above the water. 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 numbers of leafminer parasites can be high in rice fields, they generally do not reach adequate levels early enough to prevent economic damage from the first generation.

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

To reduce the potential for damage, level the field as accurately as possible, and start the crop in 3 to 4 inches of water. Under these conditions, the rice plants 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 Action Thresholds.) If the source of water is cold, such as water from some wells, you may want to establish a warming basin before the first seeded check.

Use biological and cultural controls in an organically certified crop.

Begin monitoring 2 to 4 weeks after planting, just after most of the rice plants have emerged from beneath the water and their leaves are lying on the water surface. Use a monitoring ring or floating plastic tubing covering 1 square foot. Take ten, 1-square-foot samples in a transect of the 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, second and third instar larvae are the easiest to detect because they create a detectable swelling in the leaf that can be felt.

If the average stand consists of 25 or more plants per square foot and an average of 20% or more of the plants have eggs or larva present, then lower the water level to a point the rice can stand upright.

If the average stand consists of fewer than 25 emerged plants per square foot and the infestation level is 10% or higher, lower the water level to a point the rice can stand upright.

When most of the leaves of the rice plants are upright, the rice leafminer will no longer cause economic losses. Consider the 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 field. Stopping the water flow into the fields may allow an adequate reduction in the water level as a result of evapotranspiration.