|Invertebrate Pest Control 83
Project Leader and Principal UC Investigators
A. A. Grigarick,Dept. of Entomology, UC Davis
The long-range objectives of this project are to determine the physical and biological factors that allow the buildup and movement of invertebrate pest populations that injure rice, to determine the most effective control of rice invertebrate pests, and to maintain the quality of the environment.
Rice Seed Midge
Midge larvae feed on emerging leaves or root shoots or may hollow out the seed contents, killing the plants. Studies were conducted to determine the effect of draining the fields as a control measure. Fields were drained for five days at five and at ten days after the initial flooding. These treatments were compared to continuously flooded paddies. The continuously flooded treatment showed a high peak in larvae populations about the ninth day after flooding. This buildup dropped to low levels by the fifteenth day.
The five-day drain at five days after flooding showed that larvae in the drying mud decreased daily to insignificant numbers by the fourth day. A moderate resurgence of larvae was observed five days after reflooding. Draining for five days at ten days after initial flooding also decreased larvae to insignificant numbers by the fourth day. There was a high resurgence of larvae 10 days after reflooding. The water management treatments did not significantly promote weed growth or reduce rice yields.
Studies on the early growth of rice from unsoaked coated and presoaked uncoated seed were made to determine susceptibility of the seeds to seed midges. Significant reductions in mean shoot length of seedlings resulted from coated seed. Feeding activity of midge larvae was not a stand-limiting factor at two locations, but there was evidence of some larval feeding in all plots.
At one location in which planting coincided with flooding, feeding was insignificant. At a second location, seeding five days after flooding resulted in larval feeding on 2.6 to 11.9 percent of the seedlings, indicating increased susceptibility the longer seeding is delayed after flooding. Two of three varieties tested in the second location showed significantly more larval feeding on seedlings from coated seed. Further testing is required to establish the relationships of midge injury to seed coatings.
Rice Water Weevil
Previous studies have shown rice water weevil infestations to be most critical during the first two weeks after rice emerges from the water. Greenhouse studies this year showed that lowering the water below the leaf sheaths when eggs are laid had the following effects on the eggs and first instar larvae of the weevil: lowering water during the first or last three days of egg development reduced the population 56 to 63 percent; lowering the water during the full six days of egg development caused a 98 percent reduction; lowering the water did not prevent first instar larvae from leaving the leaf sheath and becoming established on roots. A well-timed drain period of 6 to 7 days during the early period of rice emergence may be a possible control and should be field tested.
Studies this year continued to show that insect growth regulators are promising chemicals for postflood spray treatments of rice foliage for rice water weevil control. Greenhouse studies showed that the chemicals diflubenzuron and Bay Sir 8514 act primarily on the egg stage. Chemical contact can be by adult females eating the treated rice foliage or by treated water contacting the eggs.
A standardized test to determine rice plant tolerance to the water weevil was conducted among 36 promising selections from previous years. A heavier infestation of weevils in 1983 caused greater yield reductions than in 1982. The nine most tolerant selections produced an average of 57 percent more grain than the susceptible M9, but they also showed an average yield reduction of 24.7 percent compared with lines treated to prevent injury. The newer lines do not appear to be developing more tolerance to the water weevil, but they are showing improvements in agronomic qualities.
A shrimp population from San Joaquin County known to be resistant to (ethyl) parathion was exposed to varying concentrations of parathion and methyl parathion. Resistance of the shrimp to (ethyl) parathion was again confirmed for that area. The highest allowed rate of methyl paration, 0.75 pound, produced a maximum of 65 percent mortality. It is questionable if the 65 percent level of mortality is adequate for seedling protection. The test also indicates tadpole shrimp resistance to methyl parathion.