Research in Entomology Division is aimed at studying the ecology of rice pests with emphasis on the relationship between weather factors and pest incidence with a view to develop methods for forecasting pest incidence, assessing losses caused by insect damage and to evolve economically feasible, operationally viable and environmentally safe management strategies against field and storage pests.
Identification of problems related to insects and study of bionomics of major pests were the major activities prior to the introduction of high-yielding rice varieties. Other pests which received some attention apart from rice stem borers and the rice gall midge, were rice hispa, mites, green leafhopper, mealy bugs, root aphids, leaf folder etc. Varietal reaction to stem borer and gall midge and relationship of pest incidence with plant characters were studied in detail. The work on host resistance to stem borers, gall midge, green leafhopper and hispa was started in 1950. The world germplasm maintained at CRRI was screened for isolating donors with resistance to major pests.
Effect of agronomic practices like manuring, spacing, time of sowing or planting on the incidence of stem borers and rice gall midge, alternate and collateral hosts of these pests, estimation of losses due to stem borers and rice gall midge, control of these pests by spray formulations, seedling dip and assessment of new insecticides to determine their efficacy against rice pests, time, dose and method of application were the major areas that received attention.
Forecasting pest incidence
The influence of weather factors, particularly the quantity of monsoon rains (100-300 mm), on the period of gall midge activity was determined. Early monsoon rains in the months of May or June, favoured the multiplication of gall midge multiplication in ratoons of the previously harvested dry season crop resulting in maximum activity of the pest during July-August. During normal monsoon periods, gall midge incidence fluctuated between the end of September to the end of November. Based on the pre-monsoon rains, the period of infestation by rice gall midge can now be precisely forecasted for endemic areas.
Stem borer occurs in both the wet and dry seasons. In the wet season, stem borer incidence is intense during October-November resulting in white ear head damage at the flowering stage, while in the dry season, the pest incidence occurs from February to April infesting the crop both at vegetative and heading stages. A simple method of forecasting appearance of first brood of yellow stem borer, Tryporyza incertulas, by either recording soil temperature at 5 cm depth in field (appearance of first brood of stem borer is correlated to soil temperature reaching 19oC at 5 cm depth) or by regular examination of the stubbles of the harvested crop for over-wintering borer larvae during December-January was evolved.
Hot and humid conditions with stagnant water in the fields with a maximum temperature of 28 to 29oC and humidity range of 85 to 90% were favourable for the incidence of leafhoppers and planthoppers. Rice hispa was abundant when temperature was from 28 to 33oC with a relative humidity of 75 to 98%. During flowering season in October-November, prevalence of a temperature range between 27 and 28oC with a relative humidity of 80-82% favoured gundhi bug incidence.
Spraying of 0.2% mixture of BHC and DDT or 0.08% ethyl parathion, or 0.04% endrin or 0.03% diazinon proved effective in reducing stem borer incidence. Foliar spray with ethyl parathion at 0.08% and endrin 0.04% four times at tillering stage was effective against gall midge. Subsequently, when ethyl parathion and endrin were banned, many other available insecticides were tested. With the arrival of organo-phosphates in the market during the next few years, these were all tested for the control of stem borers and gall midge. A significant step in the control of rice pests was the identification of ethyl parathion for use in irrigation water during 1953 and also its use for seedling treatment.
Army worms, stink bugs, hispa, grasshoppers and jassids were effectively controlled by dusting BHC or DDT (5%) or Aldrin (5%) or endrin (1%). Mealy bugs, root aphids and thrips were controlled by spraying ethyl parathion (.5%) or endrin (0.4%). The rice case worm and leaf rollers were controlled by spraying ethyl parathion (0.8%) or endrin (0.4%). In low volume spraying, normal requirement of 360 litres of spray was reduced to 112.5 litres.
Surveys were conducted on natural enemies of stem borers and gall midge and other pests of rice, birds in rice fields and investigations undertaken on how to avoid damage by them. Studies were also initiated on paddy-cum-fish culture. In early sixties studies were intensified on nematodes parasitic on rice and their control by nematicides. Pests of storage were studied with respect to incidence, chemical control, development of storage structures and effect of botanicals to control storage pests.
The introduction of high-yielding semi-dwarf varieties during mid sixties ushered in a new era in the entomological work. The intensification of developing insect resistant varieties with the high yielding base has led to strong collaboration with the breeders for screening their segregating populations and to search for new resistant donors. Work on genetics of resistance was also initiated during this time. The outbreak of tungro virus complex transmitted by green leafhoppers in some parts of the country stimulated the initiation of work on breeding for resistance to green leafhoppers and soon work on new pests like brown planthopper and white-backed planthopper were included in the programme. With the identification of variation in virulence of rice gall midge, the division participated in national and international study for characterizing biotypes of gall midge.
The newer pest problems which arose as a consequence of intensified cultivation of the high-yielding varieties led to the further strengthening of studies on natural enemies of stem borers, gall midge, leaffolder, leaf and planthoppers, gundhi bug etc., plant parasitic nematodes and storage pests. Chemical control studies were extended to granular and dust products and different modes of application techniques like seedling root-dip, seed-soak, mud ball and paper packet application and subsoil application in root zone were developed. Toxicological studies were extended to examine persistence of chemicals and to identification of insecticides causing resurgence.
The relationship between the fluctuations in stem borer and gall midge populations and various abiotic and biotic factors and different agronomic practices aided in developing forecasting techniques. Alternate hosts of stem borers and gall midge, the nature of resistance to brown planthopper and gall midge and physiological aspects of insect behaviour particularly of brown planthopper and gundhi bug were studied and third generation pesticides like juvenile hormone and chitin inhibitors were examined. Work on plant products for control of pests in field and storage was also intensified. Some of the important findings are given below :
Sources of resistance
Thirty-four ARC, 3 AC, 1 JBS and 6 NCS accessions resistant to brown planthopper, 5 ARC accessions resistant to white-backed planthopper were identified. Five ARC accessions and 26 NCS accessions were found resistant to yellow stem borer. Thirty-two ARC accessions were resistant to gall midge and 14 AC accessions to the green leafhopper.
Development of resistant varieties
Entomologists were associated in the development of varieties/cultures suitable for different land situations with resistance to one or more insect pests. Prominent among these varieties are Ratna with field tolerance to yellow stem borer, Shakti with resistance to gall midge and green leafhopper, Saket 4 and CR 138-928 with tolerance to yellow stem borer suitable for irrigated lands, Neela and Sarasa with resistance to gall midge, and Udaya with resistance to brown planthopper, gall midge, green leafhopper and tungro virus complex suitable for medium lands, RR 50-5 and RR 2-6 with resistance to brown planthopper suitable for upland situations and Samalei with resistance to gall midge and Panidhan with field resistance to yellow stem borer suitable for lowlands (semi-deep situations).
Factors affecting resistance to major pests of rice
Laboratory studies showed that resistance of the varieties is due to "antibiosis". Resistant varieties were least preferred for ovipositon while susceptible varieties had highest egg deposition. The resistant varieties were characterized by high phenolic compounds while susceptible varieties had higher amounts of soluble sugars.
Dense leaf, pubescence, closer and more silicated cells in leaves and extra bands of selerenchyma over phloem tissues in leaves were found to be associated with resistance.
Yellow stem borer
Resistant variety TKM 6 was characterized by compact and tight leaf sheath, more number of vascular bundles, compact sclerenchymatous hypodermis, less pith area and closer silica chains in leaf sheath and leaf blades. The yellow borer resistant variety, TKM 6, exhibited highest antibiosis as far as the larval survival and growth are concerned. While Fe and Si contents are more in resistant TKM 6, the susceptible variety Jaya contained more amount of N, P, K, Ca, Mg and Mn. In addition, cultivar Jaya possessed higher amounts of total and reducing sugars, amino acids and starch whereas the resistant TKM 6 and Ptb 18 had more amount of total phenols.
The mechanism of resistance in gall midge was of antibiosis in nature. In resistant plants, the larvae responsible for gall formation were killed before they reached third instar stage. The resistant varieties possessed higher level of phenolic compounds and its concentration further increased after gall midge infestation.
Genetics of insect resistance
One dominant and one recessive gene (supplementary in interaction) in varieties like Pundia and Mohia Bankoi, single recessive gene in variety Sunabhai and Sarakanti, two recessive genes (duplicating in interaction), in variety Ladi were reported to govern resistance.
Gall midge resistance
In the F1 population of crosses (i) CR 129-118 (S) x W 1263 (R), (ii) IR 20 (S) x Shakti (R), (iii) Ratna (S) x Shakti (S), the dominant nature of gene controlling resistance was identified. The monogenic ratio confirmed the influence of one dominant gene.
From crosses Vijaya (S) x W 1263 (R), IR 20 (S) x W 1263 (R) and Ratna (S) x W 12708 (R), three dominant genes were found to control susceptibility and resistance to be recessive. In further studies with 18 crosses involving 10 resistant and seven susceptible parents, susceptibility was found to be dominant. Genes controlling resistance appeared to show duplicate and complementary types of gene interaction indicating two sources of resistance.
Biotypes of gall midge
Based on preliminary variations in reaction at different places within Orissa and outside, systematic studies were undertaken at CRRI independently and as part of a coordinated effort. The Cuttack (Orissa) populations were found to be distinctly different from those of Warangal and Raipur.
For molecular mapping of gall midge resistance gene(s) in rice against gall midge biotypes 1,2,3, and 4, crosses were made between Ptb 10, resistant and Taichung Native 1, susceptible to gall midge. F2 segregation analysis showed that a single dominant gene in Ptb 10 confered resistance against biotypes 1,2, and 4. Another cross, involving Taichung Native 1 and W 1263 was also made for tagging and mapping resistance gene against biotype 5.
In order to define the biotypes of rice gall midge at molecular level, attempts were made to develop diagnostic markers for each biotype. Adult DNA of biotypes 1,2 and 4 were amplified by 300 random 10 mer oligonucleotides. Forty-seven random primers could amplify polymorphic DNA between biotypes. Preliminary similarity coefficient analysis showed that biotype 1 is gneially different from biotype 2 and 4 which are related closely. These polymorphic bands were confirmed in single insert DNA amplified products.
Many organo-phosphates and carbamate insecticides were identified and doses were fixed for the management of important insect pests. About 76 insecticides in different formulations were screened against different insect pests and many of them were recommended for use. The emphasis of a pesticide umbrella to check insect incidence was dropped as the new information generated on effective dosages, method and time of application led to the development of the need-based chemical application approach which aid in minimizing economic loss caused by pest damage without resulting environmental pollution.
A large number of granular formulations of insecticides were tested in soil for the management of stem borer, gall midge and leaf and planthoppers. Lindane, diazinon, dursban, sevidol, endrin and carbofuran were effective against stem borers. Cytrolane, phorate and dursban were effective in reducing gall midge incidence. Both groups of insecticides were also effective against leaf and planthoppers. Two applications of any effective insecticide, first between 20 and 25 days and the second between 55 and 60 days after transplanting were effective in checking borer damage while applications at 10 and 25 days after planting were critical for the gall midge.
During eighties and nineties, newer insecticides like Eviseet, isofenphos, oncol, endosulfan and ethioprop as granules at 1 kg a.i./ ha were found effective against stem borers in the early stage of the crop. Ficam, monocrotophos or quinalphos as foliar sprays at 0.5 kg a.i./ha were effective against stem borers at heading stage of the crop. Application of oncol and ethioprop at 1 kg a.i./ha was promising against the gall midge. Phorate and BPMC granules were effective against the white-backed planthopper. MIPC, monocrotophos or quinalphos at 0.5 kg a.i./ha as spray were effective against brown planthopper.
Sub-soil application of mephospholan, isofenphos or carbofuran granules at 1 kg a.i./ha was effective than broadcasting of the same. Dry seed or sprouted seed treatment with 0.02% carbofuran, carbosulfan, chlorpyriphos or isofenphos provided effective control of both stem borers and gall midge for 30 days.
Triphenyltin hydroxide, an organotin compound was found to inhibit the feeding activity of first instar larva of yellow rice stem borer to the extent of 25-55% when used at 0.02 to 0.1% concentration over control.
Ploughing down or burning of stubbles just after harvest of the crop was found to destroy substantial stem borer larval population. Keeping the fields and bunds clean and destruction of weeds such as Echinochloa colonum reduced gundhi bug population. Mnesethia laevis, E. crusgalli and Panicum species were identified as alternate hosts for gall midge.
Insect juvenile hormone, ZR-777 was found to inhibit metamorphosis of green leafhopper. ZR-515 at 0.5% concentration impaired embryonic development in Angoumois grain moth, a stored grain pest.
Rice yellow borer (Scirpophaga insertulas) and rice leaffolder (Cnapholocrocis medinalis) were monitored by using dry funnel sleelve traps baited with 9 Z-hexdecenol and 11 Z-hexadecenol in 1:3 ration and 13 Z-octadecenyl acetate and 11 Z-hexadecenyl acetate in 10:1 ratio, respectively. Sex pheromone used for C. medinalis also attracted male Marasmia exigma population.
Resurgence of pests
Adverse effects of pesticide use such as resurgence of the pest was noticed with Evisact at 1 kg a.i./ha on gall midge. Sprays of endosulfan, methyl parathion, quinalphos and permethrin at 0.2 to 0.5 kg a.i./ha or BHC and phorate at 1 kg a.i./ha caused resurgence of brown planthoppers. Synthetic pyrethroids, decamethrin alone or with bufrofezin, cypermethrin, and tralomethrin as foliar spray showed an increase in the incidence of stem borers.
The role of indigenous parasitoids in the control of stem borer and gall midge and other pests of rice was studied and the period of activity and the extent of parasitization by important egg, larval and pupal parasitoids were determined.
For stem borers, egg parasitoids were found to be more important than the larval and pupal parasitoids in reducing pest populations. More egg-wise parasitism controled the pest in comparison to egg mass-wise parasitism. Release of laboratory reared egg parasitoids increased field parasitism of stem borers and consequently reduction in pest intensity. Trichogramma alone was not effective in controlling yellow stem borer. A combination of Trichogramma, Tetrastichus and Telenomus reduced the pest hatch.
The relationship between abundance of egg parasitoids, extent of parasitism and damage caused by the yellow stem borer was examined in a field experiment. A definite relationship between the three components. Abundance of parasitoids and the number of eggs prarasitized were positively correlated where both of the above mentioned components were negatively correlated with the damage caused by the yellow stem borer. During a study on third week of March and first week of April 1980, the egg parasitism was low, but the damage was high and vice versa, respectively.
Egg mass-wise and egg-wise, maximum number were parasitized by T. dignoides in both the dry and wet seasons in nature. Conservation and augmentation of this egg parasitoid would be more useful. This significant observation enumerated from studies on survey, incidence and extent of parasitism of S. incertulas may change the entire concept and scenario of inundative field releases of T. japonicum.
Egg parasitoid populations of Scirpophaga insertulas in rice fields.
|Year||Season||T. japonicum||T. dignoides||T. schoenobii||Total|
Leaffolders have a large parasitoid complex acting against it. The most dominant parasitoids were Trichogramma japonicum, Cardiochiles nigricollis, Macrocentrus sp., Apanteles cypris, Goniozus sp. and Brachymeria sp.
The spider eggs in the rice environment serve as alternate hosts for rice pest parasitoids. Paraphylax sp., an egg parasitoid was reared from eggs of spiders and Oxya sp. The dominant egg parasitoids of leafhoppers and planthoppers were Oligosita sp. and Anagrus sp. An insect parasitic nematode, Neoaplectana carpocapsae with an association of the bacterium Acronobacter nematophilus (DD 136) was found to effectively parasitize larvae of stem borer, cut worms, leaffolder and rice skipper in laboratory. A cheap method of mass multiplication of DD 136 on synthetic diet was perfected.
The fungi Beauveria bassiana, B. brongniartii and Entomophthora spp. bacteria Bacillus-thuringiensis and B. subtilis, HPV virus and the nematode, Parasitorhabditis spp. were found to kill rice pests. The white muscardine disease, B. bassiana, was found to kill Nilaparvata lugens (brown planthopper), Nephotetix virescens (green leafhopper), Scirpophaga incertulas, Chilo-auricilius, Melanitis leda ismene, Oxya velox and Atractomorpha crenulata for the first time. Entomophthora fumosa from N. lugens and Cicadella spectra was isolated. Beauveria brongniartii was found killing stem borer larvae and Parnara mathias for the first time. Parnara and Sesamia NPV viruses were isolated and characterized. The Parasitorhabditis sp. nematode was reared on an artificial diet developed for the purpose.
Extensive survey on the natural occurrence of microbial diseases of insect pests of rice resulted in the identification of three nuclear polyhedroses, two nematodes, two bacterioses and several mycoses of rice stem borers, leaffolders, leaf rollers, horned caterpillars, brown planthopers, green leafhoppers and grass hoppers. The technique of mass production of the fungus Beauveria bassiana and the entmophilic nematode Parasitorhabditis sp. was standardized. Among these microbes, the bacterium Bacillus subtilis was highly effective, resulting in total mortality of lepidopteran rice pests within 48 hours. Among several commercial formulations of Bacillus thuringiensis, Thuricide and Dipel were highly effective in suppressing the leaf roller as well as yellow stem borer population when sprayed in coincidence with the time of hatching of the larvae from the egg mass. These biocides persisted up to 15 days after spray and have a greater applicability, since they attack the target pest only, thereby helping in maintaining the ecological balance in the rice environment.
Integrated pest management
An integrated pest management programme for rice, including use of pest resistant/tolerant variety, minimum one application of granular insecticide in water in paddy fields at the vulnerable stage of the crop, non-application of pesticides during the period when prasites were active and use of cultural practices has been evolved and has been practiced under an operational research project in the project areas. This has helped in considerably reducing cost on crop protection and minimizing pesticidal pollution.
Pests in storage
Storage insects infest grains right on the standing mature crop in the fields, and their infestation migrates through transportation, drying and threshing of the harvested crop. Lateral infestation during these post-harvest operations is also possible. Quantitative losses caused by 18 species of insects infesting rice or paddy in storage ranged from 2 to 18% depending on the type of storage structure, period of storage, grain type and local climatic conditions.
Leaves of Vitex negundo, Lippia germinata, Aegle marmelos, Pongamia glabra, root powder of Acorus calamus and seed oil of neem Azadirachta indica significantly reduced the grain damage due to pests in stored paddy.
A promising grain protectant "2-heptatriacontanone" was isolated and identified from begunia, Vitex negundo, leaves and evaluated against grain boring insects in stored rice.
Deltamethrin (K-Othrine) 2.5% WP was found to be most effective paddy seed protectant at 8 g/1 of water for 100 kg seed treatment.
Pests deteriorating seed quality in the paddy fields
Eleven species of rice bugs, a rice thrip, Haplotrhips ganglbauri and two species of rice tarsonemid mite were reported to deteriorate paddy seed quality not only by direct feeding the developing gains but also by carrying infections of rice pathogens and subsequent grain discolouration. Seed deterioration (ill-filled and discoloured grains) to a tune of 15 to 29% during the wet season and of 9 to 23% during the dry season (1992095) was recorded. Leptococorisa acuta, Eysarcosis ventrabis and Mendia histrio were found to be the major bugs to deteriorate paddy seed quality.
Investigation on rice nematodes commenced in 1965. Sampling methods for survey and assessment of nematode fauna in rice and rice soil were devised. The nematode parasitic in rice were isolated and identified. Four new nematodes viz. Heterodera oryzae, Meloidogyne graminicola, Caloosia paxi and Trichodorus sp. were reported.
The symptom of nematode injury in rice and losses due to their infestation were determined. The losses ranged from 20 to 23% in young crop. The principal nematode problems were identified. Population dynamics of rice nematodes in relation to soil factors were studied. Alternate hosts of rice nematodes were identified. Nematode management through crop rotation was studied and found that in crop rotation with jute the soil population of H. mucronata were considerably reduced below threshold levels.
It was established that flooding the field does not control nematode problem as believed earlier. Root extracts of Tagetes patula and Aspargus were found toxic to the root knot and a few other nematodes. Second stage larvae of rice root knot nematode were killed in 5% root extract of Eclypta alba within 24 hours.
Pre-soaking seeds in water for 12 hr and treating with hot water at 52-55oC, for 10 min or soaking seeds in 500 ppm carbofuran or fenthion, for 24 hr proved effective in controlling seed-borne nematodes like the white tip nematode. Dipping roots in 500 ppm carbofuran, fensulfothion or phorate for 12 hr was found to be a good prophylactic measure against invasion by root, root lesion, lance and root knot nematodes.
Application of fensulfothion, diazinon, phorate or carbofuran at 1 to 1.5 kg or DBPC at 20 liters/ha to soil controlled soil-borne nematode parasites of rice. Spraying the standing crop sprayed with fenitrothion or trichlorofen at 0.02% prevented white tip nematode infestation and with 0.05% diazinon controlled the infestation by stem nematodes.