How to Manage Pests
UC Pest Management Guidelines
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.
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 susceptibility. Seedling 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.
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.
Organically Acceptable Methods
If the average plant stand is 25 or more plants per square foot and an average of 20% or more of the plants are infested, lower the water level and apply an insecticide. If the infestation level is 5 to 19%, lower the water and sample again in 4 days. If infestation levels do not increase and the plant stand remains at 25 or more, no treatment is needed.
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 percolation and evaporation.
No treatments are required after leaves start growing upright above the water.
UC IPM Pest Management Guidelines: Rice