How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

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 are associated with 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 open water. Eggs hatch in 1 or 2 days and larvae form silken tubes on vegetation or the soil. The tubes are brown in color and have bits of debris, diatoms, and algae adhering to them. The larvae may be white, green, or reddish in color. Larvae feed on the material adhering to their tubes and forage from the tubes, which serve as their retreat. The larvae go through 4 instars in 7 to 10 days in spring; the 3rd and 4th 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 development, and come to the water surface where the adult emerges. Three to four generations can occur each growing season, but only the first two are of economic concern to rice growers.

DAMAGE

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 plant. Once the seedling is several inches long, it can usually outgrow the feeding of midge larvae, even though irregular holes may be eaten in the leaf. 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.

While midge larvae often eat the inside out of the seed, leaving it hollow, tadpole shrimp never cause this kind of injury. Tadpole shrimp and seed midge injury to the leaves and roots may be similar but the chewed areas caused by tadpole shrimp will be larger and more irregular because of the larger size of the shrimp mandibles. If the injury is caused by midges, the midge larva and tube are often still on the plant at the time of examination. 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.

MANAGEMENT

The primary management strategy for controlling rice seed midges is draining fields. No chemicals are available for controlling this pest in rice.

Cultural Control
Seeding should be done as soon as possible after flooding, preferably within 2 days of initial flooding: any delay 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 brief 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 stand is unacceptably low, consider reseeding. Although reseeding fields with serious stand losses has had mixed results, if done early enough, 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. This will discourage feeding and encourage rapid rice growth.

The decision to drain must be made carefully because the potential increase in weed problems, herbicide and insecticide residue problems relating to drainage water, loss of fertilizer, and irrigation costs may outweigh the benefits for midge reduction.

Organically Acceptable Methods
Draining fields, as described above, is an organically acceptable management method.

Monitoring
Monitor for midge injury at the same time and with the same sampling devise used for tadpole shrimp. Examine each injured seedling or seed with a hand lens for chewing marks and the presence of midge larvae and 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 you find less than 30 healthy seedlings per square foot and midge injury is evident, action should be taken. If more than 30 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.

PUBLICATION

UC IPM Pest Management Guidelines: Rice
UC ANR Publication 3465
Invertebrates
L. D. Godfrey, Entomology, UC Davis
L. A. Espino, UC Cooperative Extension, Colusa County

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