LATEST AND CONTROLLING
EFFECTIVE INSECT
METHODS OF PESTS ON RICE
BY P. ISRAEL (Central Rice Research Institute, Cuttack)
ABSTRACT With increased rice production under improved methods, the problem of plant protection is more accentuated. Insect infestation is found to be more,in rice under high fertility. The usual indirect methods advocated till now, have not achieved the expected results. Chemical method of control appears to be the most effective at present. Insecticidal control of pests that feed on the foliage of the plant is comparatively easier. In the ease of pests that remain concealed during their destructive phase, the choice of a suitable insecticide, the method and time of application of the insecticide seem to be the deciding factors in achieving effzctive control. At the Central Rice Research Institute, stem borers and gallfIy have been effectively controlled by timing the application of insecticides to synchronise with the pest emergence. In the case of stem borers, a set of two sprayings is necessary at the time of brood emergence. The first spray kills the moths and checks oviposition while the second one, given 8-10 days later, kills the newly hatched larvae before they bore into the stem. In the ease of gallfly, spraying four times during the vegetative phase of the crop to synchronise with brood emergence reduced the incidence considerably. Yet insecticidal method can only be an adjunct or complementary to other m~thods of pest control. Biological control of these pests needs to be fully explored for a practical solution of the problem. Establishment of a pest warning organization is an immediate necessity to enable cultivators to take suitable pest control measures in time on a co-operative effort. INSECT pests cause such colossal damage to flee, both in the field and store, that protecting the crop and preserving the produce are no less important than stepping up rice production by improved seed or improved methods of cultivation. A b o u t 100 species of insects, with varying feeding habits, have been recorded as damaging rice in field and store. N o t all pests are prevalent in one locality at the same time, as the abundance of a pest and the extent of damage caused, depend on the meteorological factors and cultural practices. Under favourable conditions, a minor pest in a certain 363
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region may become a serious pest in the same region or in another region. Though, the actual loss caused by insects has not been assessed by statistical methods, it is estimated that insects cause 10~ loss to the crop and stored grain. Saving this enormous loss depends upon devising and adopting effective and economic methods of control. Methods of control consist of the adoption of simple prophylactic measures to avoid loss normally caused by pests and emergency measures airr~d at fighting pest outbreaks. Hand-picking of egg masses, removal of dead hearts and destruction of hibernating pupae in stubbles for stem borers; uprooting of infested plants for gallflies and mealybugs; systematic handnetting for grasshoppers, leaf-hoppers and bugs; introducing insectivorous raptors for army-worms and cutworms are some of the methods of control which are not completely effective. Breeding varieties resistant to insects is a complex problem involving an exhaustive study of insect-plant interaction factors and determining the exact factor responsible for resistance. Investigations at the Central Rice Research Institute indicate that pigment and scent in plants are associated with comparative resistance to gallfly. Breeding for evolving varieties bearing heritable characters for insect resistance is underway at the Central Rice Research Institute. With the introduction of modern chlorinated hydrocarbon and organophosphorous insecticides, spectacular success has been achieved in the chemical warfare against pests. These insecticides are highly toxic at eco. nomic doses over a wide range of insects and possess greater residual toxicity. Yet, certain pests elude the efforts at control and limit the efficacy of insecticides by remaining concealed inside the central core of the stem. In order to control any pest effectively, a thorough knowledge of the biology of the pest is necessary to determine the vulnerable stage and time for control. Then, a toxic insecticide which at the same time is least hazardous has to be carefully selected. Some of the pests, like the swarming caterpillars, grabhoppers, stink bugs, leaf-hoppers, etc., which damage and feed on the exposed parts of the rice plant are comparatively easy for control. But, the internal feeders present a difficult problem for controlling even by the most toxic insecticide. Stem borers, gall midges, mealy bugs and root aphids are some of the pests which come under this category. Some of the major pests of rice and their effective control are briefly, described below:--
Latest and Effective Methods of Controlling Insect Pests on Rice
365
Stem borers.--.The following five species of rice stern borers have been reported to occur in India causing ans~here from 3 per cent to 95 per cent loss in yield: Schoenobius incertulas, Wlk. Scirpophaga innotata, Wlk. Sesamia inferens, Wlk. Chilo simplex, Butler Chilotr~ea (Proceras) polychrysa, Meyr. The relative importance of the species of the stem borer varies with different regions but Schoenobius is more predominant throughout the country, particularly in areas under double cropping. The characteristic symptoms of damage by stem borers are the presence of dead hearts at the early seedling stage and white earheads at the heading stage of the crop. It is usually presumed that the damage at the early seedling stage does not directly result so much in decreasing the yield, as the incidence at this stage has been observed to result in more tillers. It has been found that stem borer attack does result in profuse tillering, but many of the tillers thus formed do not contribute towards the yield as they remain green at the time of harvest. The loss due to stem borers at the early seedling stage has been estimated in an experiment to be 21.4 per cent. Stem borer attack at the earhead stage of the crop is more apparent as many attacked earheads are rendered chaffy. The loss due to stem borers at the earhead stage of the crop has been estimated in the same experiment to be 26.9 per cent. Insecticidal trials for the control of this pest have been in progress in the various States. BHC or D D T when sprayed or dusted was found to be effective in Bihar, Madras, Punjab and West Bengal. In Andhra, Bihar, Mysore, Madras and West Bengal, the use of modern insecticides like Parathions and Endrin has contributed largely to the reduction in the borer incidence. However the dosage, time and the number of sprayings were different in each State. The chief problem in the control of stem borers is that the egg masses of Scheenobius incertulas and Scirpophaga innotata remain covered by dense buff coloured hairs making insecticidal penetration difficult. The other factor in the biology of the stem borers is that the hatched out larvm soon bore into the sheaths and stems and remain concealed while feeding. The larvm of Chilo however nibble leaf-sheaths and flowers for sometime befor~
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they would bore into the leaf-sheaths. On account of these habits of the pests, insecticidal treatment should be directed against the ovipositing moths and the newly hatched out larvae. Experimental data collected at the Central Rice Research Institute on the efficacy of different insecticides, their dosage and time of application, indicate the following method to be the most effective in .controlling stem borers :--Spraying in the nursery or transplanted or broadcast crop should synchronise with the prevalence of moths which can be determined by the qatches at light-traps. This spray kills the adults and checks oviposition and should always be followed by another spray within a period of 8-10 days which is intended to kill the newly hatched larvm. This schedule of spraying should be repeated according to the broods. At the Central Rice Research Institute, where there are two distinct broods, spraying Folidol 0-08 per cent according to the above schedule resulted in decreasing the stem borer incidence by 84 per cent and increasing the yield by 54 per cent over check. This schedule was also adopted by spraying. 0.04 per cent Endrin in the cultivators' fields in a block of 100 acres with similar results. Spraying BHC or DDT at 0-2 per cent according to this schedule is also effective but as their residual toxicity is very low, the number of sprayings has to be increased. It was seen that infectation by stem borers, leaf-rollers, leaf-hoppers, etc., was higher in the crop receiving high doses of nitrogen as ammonium sulphate. Therefore, there is a greater need to protect the crop receiving high doses of fertilisers.
Gallfly--Pachydiplosis oryzce (W.M.) Mani. This pest has been recorded from most of the rice-growing tracts in India and has been reported to cause a loss ranging from 15 per cent to even total destruction. It is prevalent throughout the monsoon and at Cuttack the peak of infestation fluctuates between the end of August to the end of November. It has, on an average, 5-8 broods in the cropping season with considerable overlapping of generations. It has been found that the stage of the crop is an important factor in the incidence by gallfly as 90 days old crop showed higher infestation than the 60 days old crop. Weeding out of alternate hosts and setting up light-traps to capture adults are commonly adopted in all the states. Growing early varieties which come to flower before the peak period of the pest has been recommended but such early varieties are generally poor in yield. Dusting BHC !0 per cent at the oviposition stage and spraying Folidol 0.04 per cent
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367
or Endrin 0.02 per cent. in Andhra and Bihar respectively have been found to check the spread of this pest. The use of Parathion has proved promising in Bombay, Mysore and Madras. The newly hatched grub does not wander much or feed outside but soon enters the primordium of the tiller and develops inside. The entire larval and pupal stages are spent in a single tiller. On account of these habits of this pest, insecticidal treatment by timing of the spray operations, as in the case of stem borers, is essential.' It was found at lhe Central Rice Research Institute that spraying Folidol 0.08 per cent or Endrin 0.04 per cent four times during the vegetative phase of the crop, to s~,nchronise with the brood emergence of the pest, reduced the incidence by 48.0 per cent resulting in an increase of 15 per cent in yield. Dusting BHC or D D T is reported to control case-worms but often the cases containing the larva~ get dislodged from plants and remain afloat on water, as the larva~ are semi-aquatic. At the Central Rice Research Institute effective control has been obtained by spraying Folidol 0.08 per cent or Endrin 0.04 per cent and keeping the water impounded for 24 hours after the operations. Similarly, spraying Folidol 0.08 per cent or Endrin 0.04 per cent effectively controlled the caterpillars of leaf-rollers inside the webbed folds of leaf-blades. Grubs of hispa inside the leaf tissue and mealybugs inside the leaf-sheaths are effectively controlled by spraying Folidol 0.10 per cent. Army-worms and cutworms are controlled by dusting 5 per cent BHC or DDT. A more effective method was found in spraying Folidol 0.08 per cent or Endrin 0.04 per cent as the larvae died within four hours after spraying. The above method was also effective in controlling the other leaf-eating caterpillars. Grasshoppers, leaf-hoppers and rice bugs could be effectively controlled by dusting 5 per cent BHC or D D T or Aldrin as both the adults and nymphs could be brought under check. The success of the above methods of control depends on taking measures in proper time. It will be possible to forecast the onset of a pest from a knowledge of the environmental and climatic factors in relation to pest prevalence in different rice tracts and to issue a timely warning for control. This system of forewarning enables cultivators to take up control measures in proper time. Such pest warning i s b e i n g adopted successfully in other countries, notably, Japan.
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The above methods of control are only adjunct or complementary to other methods such as cultural, biological, etc. By extensive chemical control measures beneficial insects, parasites and predators of pests, are likely to be exterminated, thus presenting a fresh problem of increased pest incidence. In spite of 55 per cent egg parasitisation of Schoenobius and 89 per cent grub parasitisation of Pachydiplosis existing in nature, the ravages of the pests remain unabated. Biological control of these pests by breeding and releasing suitable parasites merits a serious all-out attempt. But until these methods are available, we have to say in the words of Wigglesworth " By all means let us use insecticides when we can do no better but we should regard them as admission of failure to be replaced by more subtle and more remunerative methods of biological control as soon as these can be worked out."
343-59 Printed a~ The ]3m~galore-Pres-s,B~galore City, by C. Vasudeva Rao, Superintendent, and Published by The Indian Academy of Science~, B~mgalore.
