Rice Seed Midges

Midges are the most common group of insects in rice fields. Over 30 species have been found in California rice fields, but comparatively few species cause seed and seedling injury. Adult midges swarm in small clouds over rice fields and other bodies of water in spring. They resemble very small mosquitoes but do not bite.

Females deposit their eggs in masses or strings, generally on the water surface. Eggs hatch in 1 or 2 days and larvae form silken tubes on vegetation or the soil. The tubes are brown and have bits of debris, diatoms, and algae stuck to them. The larvae may be white, green, or reddish. Larvae feed on the material adhering to their tubes and forage from the tubes, which serve as their retreat. In the spring, the larvae go through 4 instars in 7 to 10 days. The third and fourth instars (0.16 to 0.24 inch or 4 to 6 mm long) are the most damaging to rice. Rice seed midges pupate in the tubes, complete their development, and come to the water surface where the adults emerge. Three to four generations can occur each growing season, but only the first two are of economic concern to rice growers.

Injury to rice is limited to germinating seeds and very young seedlings. Midge larvae feed on the emerging shoot, leaves, roots, or may hollow out the embryo and kill the germinating seed. Once the seedling is several inches tall, it can usually outgrow the feeding of midge larvae, which causes irregularly-shaped holes in leaves. Midge larvae may also feed on floating leaves, causing small holes that extend completely through the leaves. Again, injury to these older leaves is not of economic concern if other leaves are upright.

Seed midge and tadpole shrimp injury to leaves and roots may look similar, but the chewed areas caused by tadpole shrimp will be larger and more irregular because of the larger size of the shrimp’s mandibles. Midge larvae often eat the inside of the seed, leaving it hollow; tadpole shrimp never cause this kind of injury. If the injury is caused by midges, the midge larva and tube are often still on the seed or plant at the time of examination. However, if the injury is several days old, secondary organisms may invade the plant tissue, and the pest that caused the injury may be difficult to associate with the injury.

The primary management strategy for controlling rice seed midges is draining fields.

Seeding should be done as soon as possible after flooding, preferably within 2 days of initial flooding. Any delay in seeding will expose germinating seed to older and larger numbers of midge larvae. In large fields that take longer than a few days to flood, seed parts of the field in sequence as they fill with water. Rapid root and shoot growth will reduce the period of time that the rice is susceptible to damage by midge larvae.

If monitoring during the first sample period (5 to 7 days after flooding) indicates action is needed, drain the field and reflood after a 3- to 4-day drying period. The length of the drying period depends on weather conditions and the time it takes to reflood. If the number of plants in a stand is unacceptably low, consider reseeding. Although reseeding fields with serious stand losses has had mixed results, if done soon after seeding, it may be successful. As the time between original planting and reseeding increases, chances of reseeding success decrease.

If monitoring during the second sample period (8 to 14 days after flooding) indicates action is needed, the field can be drained or the water lowered until it is barely covering the soil and reflooded after 3 to 4 days. This will discourage feeding and encourage rapid rice growth.

Draining a field has factors to consider. The potential increase in weed problems, pesticide residue problems associated with drainage water, loss of fertilizer, and irrigation costs may outweigh the benefits of midge reduction.

Drain fields, as described above, in a certified organic crop.

When monitoring for rice seed midge, determine the number of healthy seedlings. For example, use a metal or plastic cylinder or square with open ends that encloses 1 square foot and with sides higher than the water depth, to count the plant stand. Place it in the water gently so the area to be observed remains clear. If the water is murky because of wind or wave action, allow a day or two for the water to clear. Examine injured seedlings or seeds with a hand lens for chew marks, the presence of midge larvae, and the presence of tubes. Because the distribution of midge larvae and their injury may be irregular throughout a field, examine each field’s basin.

Take the first sample 5 to 7 days after flooding begins. If there are fewer than 25 healthy seedlings per square foot and midge injury is evident, action should be taken. If more than 25 healthy seedlings per square foot are found, take a second sample 8 to 14 days after flooding. Check roots and shoots for damage. Generally, injury by midges at this time does not kill rooted plants. If the plant stand is below 25 plants per square foot and midge injury is still present, cultural action can be taken.