Invertebrate Pest Control 86

Invertebrate Pest Control-86
Project Leader and Principal UC Investigators A.A. Grigarick, Department of Entomology, University of California, Davis	Objectives Better control the rice water weevil through knowledge of adult flight habits and response to chemical pesticides, and by selecting rice plants more tolerant to larvae infestations. Determine flight habits of the adult armyworm. Entomolgists are studying the life histories of invertebrate pests of rice and of beneficial invertebrates that prey on the pests. This knowledge is used to develop management practices that are economically sound and environmentally safe. Adult rice water weevils emerge from their overwintering sites in the spring, arrive in rice fields soon after flooding and lay their eggs under water in leaf sheaths of young emerging rice plants. In both 1985 and 1986, the maximum flight periods of the adults were the second and third weeks of April, but flights continued until late May in both years. Therefore, to completely avoid infestations in rice fields, flooding would have had to been delayed until the fourth week of May. However, 90 percent of the flight was completed by May 13 in 1985 and by May 2 in 1986. If a method of predicting peak flights can be developed, it might be possible to delay flooding and planting in some years and with some varieties until after the peak Work continued in 1986 to develop more effective chemical control of the rice water weevil. Although primary damage is from larvae feeding on the roots, chemical control measures focused on the adult stage to prevent egg hatching. Field experiments compared the chemicals Alsystin and diflubenzuron sprayed five days after rice seedlings emerged from the water. These chemicals act primarily on adult weevils, feeding on the sprayed rice foliage and preventing oviposited eggs from hatching. Alsystin application rates were 0.25 and 0.375 pounds ai/acre, and diflubenzuron was applied at 0.25 pounds ai/acre. Both chemicals significantly reduced larvae numbers and were similar in effectiveness. Rice yields were improved by the spray treatments, but only Alsystin sprayed at 0.375 ai/acre gave yield increases statistically significant at the 5 percent level. In greenhouse experiments, Alsystin provided complete control of the weevil (through unhatched eggs) for five days from the day of application. Natural breakdown of the chemical after 5 days made it progressively less effective. Field and greenhouse experiments with two new experimental chemicals from Japan, cycloprothrin and ethofenprox, were highly effective for control of adult weevils, but they had little or no effect on the eggs. Only ethofenprox was successful for larvae control. Both chemicals need further testing for effectiveness and safety to wildlife. Progress is being made in breeding for improved tolerance to the weevil. Yields of chemically protected and unprotected rice lines were compared. Unprotected M9 had a 31 percent yield reduction. The five best selected lines showed yield decreases when unprotected of 4.3, 8.9, 10.5, 12.1, and 12.1 percent. Studies to determine the reasons for improved tolerance were inconclusive. Duck mortality occurred in some rice fields in the spring where carbofuran, applied as granules, was used to control the rice water weevil. It has been recommended that the granules be lightly incorporated into the soil to prevent ducks from ingesting the chemical, but it has not been shown that this management system is effective. An experiment that compared plots with surface applied granules and lightly incorporated granules produced no duck mortality or evidence of injury from either treatment. Carbofuran was used at the rate of 1 pound per acre for each treatment. A possible reason why the ducks did not contact significant amounts of the carbofuran is that duck activity unintentionally incorporated the granules. Another reason for no duck mortality may be difference in behavior of caged, penreared ducks and wild ducks. It is doubtful if further studies with caged ducks would be able to separate these factors. Armyworm larvae populations are sometimes large enough to injure rice. Collections of adults in light traps over the past three years show that peak flights occur in late June, corresponding with infestations on rice. However, the number of adults collected in light traps did not necessarily reflect the amount of injury reported in the fields. Yields were increased by spraying Alsystin. and diflubenzuron on emerging rice seedlings to keep weevil eggs from hatching. Progress continues in selecting rice lines tolerant to the rice water weevil, and weevil flight studies indicate that there is potential for delayed flooding and planting to avoid heavy infestations during some years.

Invertebrate Pest Control-86

Project Leader and Principal UC Investigators

A.A. Grigarick, Department of Entomology, University of California, Davis

Objectives

Better control the rice water weevil through knowledge of adult flight habits and response to chemical pesticides, and by selecting rice plants more tolerant to larvae infestations.
Determine flight habits of the adult armyworm.

Entomolgists are studying the life histories of invertebrate pests of rice and of beneficial invertebrates that prey on the pests. This knowledge is used to develop management practices that are economically sound and environmentally safe.

Adult rice water weevils emerge from their overwintering sites in the spring, arrive in rice fields soon after flooding and lay their eggs under water in leaf sheaths of young emerging rice plants. In both 1985 and 1986, the maximum flight periods of the adults were the second and third weeks of April, but flights continued until late May in both years. Therefore, to completely avoid infestations in rice fields, flooding would have had to been delayed until the fourth week of May. However, 90 percent of the flight was completed by May 13 in 1985 and by May 2 in 1986. If a method of predicting peak flights can be developed, it might be possible to delay flooding and planting in some years and with some varieties until after the peak

Work continued in 1986 to develop more effective chemical control of the rice water weevil. Although primary damage is from larvae feeding on the roots, chemical control measures focused on the adult stage to prevent egg hatching. Field experiments compared the chemicals Alsystin and diflubenzuron sprayed five days after rice seedlings emerged from the water. These chemicals act primarily on adult weevils, feeding on the sprayed rice foliage and preventing oviposited eggs from hatching. Alsystin application rates were 0.25 and 0.375 pounds ai/acre, and diflubenzuron was applied at 0.25 pounds ai/acre. Both chemicals significantly reduced larvae numbers and were similar in effectiveness. Rice yields were improved by the spray treatments, but only Alsystin sprayed at 0.375 ai/acre gave yield increases statistically significant at the 5 percent level.

In greenhouse experiments, Alsystin provided complete control of the weevil (through unhatched eggs) for five days from the day of application. Natural breakdown of the chemical after 5 days made it progressively less effective.

Field and greenhouse experiments with two new experimental chemicals from Japan, cycloprothrin and ethofenprox, were highly effective for control of adult weevils, but they had little or no effect on the eggs. Only ethofenprox was successful for larvae control. Both chemicals need further testing for effectiveness and safety to wildlife.

Progress is being made in breeding for improved tolerance to the weevil. Yields of chemically protected and unprotected rice lines were compared. Unprotected M9 had a 31 percent yield reduction. The five best selected lines showed yield decreases when unprotected of 4.3, 8.9, 10.5, 12.1, and 12.1 percent. Studies to determine the reasons for improved tolerance were inconclusive.

Duck mortality occurred in some rice fields in the spring where carbofuran, applied as granules, was used to control the rice water weevil. It has been recommended that the granules be lightly incorporated into the soil to prevent ducks from ingesting the chemical, but it has not been shown that this management system is effective.

An experiment that compared plots with surface applied granules and lightly incorporated granules produced no duck mortality or evidence of injury from either treatment. Carbofuran was used at the rate of 1 pound per acre for each treatment. A possible reason why the ducks did not contact significant amounts of the carbofuran is that duck activity unintentionally incorporated the granules. Another reason for no duck mortality may be difference in behavior of caged, penreared ducks and wild ducks. It is doubtful if further studies with caged ducks would be able to separate these factors.

Armyworm larvae populations are sometimes large enough to injure rice. Collections of adults in light traps over the past three years show that peak flights occur in late June, corresponding with infestations on rice. However, the number of adults collected in light traps did not necessarily reflect the amount of injury reported in the fields.

Yields were increased by spraying Alsystin. and diflubenzuron on emerging rice seedlings to keep weevil eggs from hatching. Progress continues in selecting rice lines tolerant to the rice water weevil, and weevil flight studies indicate that there is potential for delayed flooding and planting to avoid heavy infestations during some years.