Invertebrate pests
Rice water weevil Light Trap Counts 2000.
Field pests
The rice water weevil (Lissoroptus oryzophilus) and the tadpole shrimp (Triops longicaudatus) are the two most important invertebrate pests in California rice culture. Other pests include the rice leafminer (Hydrellia griseola), rice seed midges (family Chironomide), armyworm (Pseudaletia unipuncta), western yellowstriped armyworm (Spodoptera praefica), and the crayfish (Procambarus clarki).

Figure 25a. Rice water weevil larva feeding on rice roots.	b. Adult rice water weevil. Photos by Jack Kelly Clark

Rice water weevil (fig. 25) is the most economically damaging of the invertebrate pests found in California. The adult weevil overwinters in weedy areas near rice fields or in soil crevices. In the early spring when night temperatures become warm, adults fly to the fields and lay eggs in the young rice plant. Observing feeding scars on the leaves of young plants can determine the presence of adults. The damage, however, is done by the larvae, which hatch and feed on the roots. Heaviest infestations usually occur around the field borders and adjacent to levees. Symptoms include leaf-feeding scars and plants that are stunted and chlorotic as a result of root pruning. The rice water weevil is difficult to control with cultural practices and is currently controlled with approved insecticides.

More information on Rice Water Weevil and the Stewardship Program for Granular Carbofuran.
Tadpole shrimp are crustaceans that feed on young roots and shoots and thereby uproot rice seedlings. Turbid water following rice seeding and before stand emergence is often indicative of tadpole shrimp. Muddy water caused by their digging reduces photosynthetic activity and growth of the young rice plant. Immediate seeding after flooding is the best prevention for tadpole shrimp. Their eggs hatch within two days of flooding. The longer the time between flooding and seeding, the greater the population and size of the tadpole shrimp at times when the rice is most susceptible to damage. Approved pesticides control this crustacean.
Rice seed midges resemble small mosquitoes, and are found in most rice fields. Midge larvae injure rice seedlings by feeding on seeds and the emerging shoot, young leaves, and newly developing root tips of rice seedlings. Symptoms include small holes in the seeds and floating, dislodged seedlings. Although rice seed midges are commonly found in the rice field, economic damage is most often associated with weather and water conditions that result in slow seedling growth, or when the time between flooding and seeding is long enough for large numbers and older midge larvae to develop. Rice seed midges are controlled only by field drainage.
Rice leafminer is a small, olive-green fly commonly found walking on the water surface or on rice leaves in the early season. Economic injury is usually confined to stressed plants where eggs are laid singly on leaves floating horizontally on the water. The larvae cause damage by mining tunnels within the leaves. These mines appear yellow at first, and become transparent with time. Lowering the water level to encourage more rapid leaf emergence or by applying approved insecticides controls rice leafminer. The introduction of more erect-leaved semidwarf varieties and improved water management have greatly reduced injury from this pest.
Armyworms are occasionally found in rice in midsummer. After other food sources are depleted the adult moths may fly into rice fields and lay eggs. Armyworm damage is most serious during rice stem elongation and grain formation. Larvae defoliate the plants, typically by chewing angular pieces off the leaves. They may also feed on the panicle however yield losses depend primarily on the amount of defoliation. Armyworms are partially controlled by natural enemies, including predators, pathogenic microorganisms, and parasites. If necessary, they may be controlled by approved pesticides.

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