Studies on ecology and control of the rice gall midge in Thailand

Country
Thailand
Technical bulletin of the Tropical Agriculture Research Center
ISSN 03889394
NII recode ID (NCID) AA00857848
Full text
To clarify on ecology and control of the rice gall midge, the present studies have been carried out mainly at the Pan Rice Experiment Station in northern Thailand from 1968 to 1972.
It was revealed that the rice gall midge is distributed in north, northeastern, and east Thailand, and recently the insect was found at some places in Central plain. There is a missgiving that the insect can extend her distribution rather easily when high yielding but susceptible varieties of rice such as RD 1, RD 2, and RD 3 will be used widely by farmers.
The highest damage was observed in Chiengrai Province in 1969, where the percentage of infestation was more than 60%, The rice gall midge showed a trend to reduce her population from 1970 to 1972. It is said that outbreak of the midge has occurred every 5 to 6 years in the past in Thailand.
The insect adult began to appear from March to May and disappeared usually in the middle of November in every year. The peak of occurrence was observed at the end of September. Number of generations was 9 to 12, and peak generation was at the 4th generation counted after transplanting. The insect which appeared from March to May on rice seedling were those migrated from wild host plants. Information in the insect occurrence in the early season gives an important key point for forecasting the insect occurrence in that season.
The most important factors among climatic condition affecting the insect incidence was relative humidity due to rainfall. Rainfall in May was very important for adult emergence and larval growth on the weeds. High humidity in paddy field after transplanting favoured egg laying, longevity of adult, emergence, egg hatching, and larval entrance to growing points of rice plants.
The insect population increase was proportional to the increase of tillers and hence growing points of the plants. In northern Thailand, the midge increased in number from July to September, reaching a peak at the maximum tillers stage. The insect population decreased sharply after the panicle primordium initiation stage, because no larvae could survive on the panicle primordia.
A peak of emergence was seen between 2200 and 2300 hours. Pupae climbing up inside the gall showed a negative phototaxis. Sex ratio (male to female) was 1 to 2.5. Parthenogenesis was not recognized at all. Ususally a male could mate to 2.5 females in an average. The adult longevity was 2.3 days in male and 2.6 days in female. A peak of the insect flight to light was 2100 to 2200 hours. Average number of egg laid per female, ovarian eggs, and eggs remained in ovary after egg laying were 221.3, 276.1, and 54.9 respectively. Time of peak of egg laying was 100 and 200 hours. More eggs were laid on leaf blade than on leaf sheath. Average incubation period was 3.2 days. A peak of egg hatching was seen between 300 and 400 hours. The larvae have 3 instars, with a larval period of 3.4 days in the first instar, 5.5 days in the second, 7.0 days in the third. Length of gall was less than 1 cm during the first instar stage and less than 4 cm at the second and third instar stages. Galls could not be observed from outside during these stage. Prepupal and pupal periods were 5.6 and 5.3 days, respectively. The gall rapidly grew up to 9.1 cm long in the prepupal stage and 27 cm long in the pupal stage. Pupation was seen at the basal part of the gall. It is suggested that the gall formation was induced by a particular substance secreted by the larvae.
Three species of the Hymenopterous parasites such as Platygaster oryzae, Platygaster sp.,and Neanastatus gracillius and one species of predator such as Ophionia indica were found as natural enemies of the rice gall midge. The natural enemies began to appear in the field in August, and their control effect on the midge was recognized from the end of September to November. A remarkable decrease of population density of the midge which occurs during the period from September to November is caused not only by shifting of plant stage from vegetative growth period to the reproductive stage but also by the attack of natural enemies. Platygaster oryzae was predominant in number than the other parasites. The parasite rapidly increased her population density during a rice season and reached more than 70% of the total number of natural enemies. Parasitism of Platygaster oryzae was significantly higher with later transplanting of rice than early transplanting, with a greater reduction of the midge population in the former case.
The wild host plants found in northern Thailand are wild rice, Ischaenum aristatum,Paspalum distichum, Leersia hexandra, and Echinochlora colonum. Habitat of I. aristatumand P. distichum is paddy fields but seeds of the plants remain dormant in soils during the rice season (June to October). Usually they appeared from November to May during the dry season. The midge appeared on wild rice throughout a year. The midge had a peak of occurrence during March to May and November. The insect maintained low population from July to October and December to February. The insect on I. aristatum began to appear on May 2nd, and disappeared on July 11th, with a peak of its density at the first of June, The insect on L. hexandra appeared in the middle of May and disappeared at the end of July, with a peak in the middle or at the end of June. During the dry season, the midge on the wild host plants, growing in dry area, stayed at larval stage mostly at the first and second instar. Number of the insect larvae on wild host plants was in order of wild rice,P. distichum, I. aristatum, and L. hexandra. The midge bred on the wild host plants was significantly smaller in body size. Size of gall on the wild host plants was also remarkably small. Female bred on rice plants laid eggs on wild host plants and the larvae developed normally, although the adult became small in body size and dark in colour. On the other hand, when adults bred ori wild host plants were transferred to rice plants, they laid eggs and larvae grew normally on the rice plants.
Number of larvae entered into the main culm was more abundant and larval instar was more advanced than that entered into tillers. Galls produced on main culm were longest as compared with galls on primary and secondary tillers. Seedlings infested by the midge can be easily distinguished from the healthy one by such characters as short plant height, younger leaf age, more tillers, inhibited growth of uppermost leaf, dull green color, and round-shaped basal part of stem.
When same number of eggs was applied experimentally to rice plants as different growth stages. Larval development differed with different stages. Damaged tillers began to increase after transplanting, reached a peak at the maximum tillers stage and then decreased sharply during the reproductive growth period of rice plants. It was suggested that the tillering stage during which the insect population pumps up is the best time for insecticidal application and release of natural enemies for the control.
From the screening test for resistant varieties of rice EK 1263 and EK 1252, Indian strains were found highly resistant. Hydridization between Thai varieties and EK strains was done to develop resistant lines with good type and grain quality. The hydrid lines were out of more than 2000 hydrid lines, five resistant lines were selected in 1972 and distributed to farmers for field trials. One of them was named RD 4, and officially registered by the Government as a recommended new variety. An important difference was that the larval development was inhibited in resistant varieties, in which the larvae remained only at the first or second instar stage. Indian variety EK 1263 developed panicle prirnordiurn one month earlier than other varieties. It was considered that this fact may also be related to high mortality of the larvae.
Less infestation of the midge and high yield of rice were obtained when transplanted at the end of August in northern Thailand. This late planting caused a short tillering period, which permitted a cycle of only three generations of the insect. In case of late planting, the midge proceeded only one generation in the tillering stage and two generations after panicle primordium formation stage.
Effective control of the insect was achieved by the application at 14 and 28 days after planting. Insecticides were applied at a rate of 2 kg (a.i.) per hectare. Diazinon granule was very effective. Diazinon coated on the tape which is named Diazinon tape was also very effective. Therefore, the application at early time of tillering period is most effective to the midge and also can avoid the killing of natural enemies which occur mostly in the later period of rice season.
Date of issued
Creator Terunobu Hidaka Precha Vungsilabutr Sawang Kadkao
Publisher Tropical Agriculture Research Center
Volume 6
spage 1
epage 119
Language eng

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