Ent. exp. & appl. 12 (1969): North-Holland Publishing Co., Amsterdam

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1 Ent. exp. & appl. 12 (1969): North-Holland Publishing Co., Amsterdam STEM BORER AND LEAFHOPPER-PLANTHOPPER RESISTANCE IN RICE VARIETIES BY M. D. PATHAK International Rice Research Institute (IRRI) Los Bafios, Laguna, The Philippines A total of 10,000 varieties, representing the world rice germ plasm collection at the International Rice Research Institute, were evaluated for their resistance to the striped borer, Chilo suppressalis Walker, in field and greenhouse experiments. From these, 20 varieties were recorded as highly resistant. The resistance appears to be non-preference and/or antibiosis in nature. Several plant morphological and anatomical characters have been recorded to be correlated with stem borer resistance. On resistant varieties, the borer larvae suffer high mortality, have a slower rate of growth, and their body-size is smaller than those of the larvae reared on susceptible hosts. The selected resistant varieties have been also used in a hybridization program which sought to combine stem borer resistance with improved plant type and other desirable characters. Several progenies of one such cross between TKM-6 (resistant to borers) and Petaz X Taichung (Native) 1 (improved plant type) are particularly promising. They are of improved plant type, resistant to borers, tolerant to the green leafhopper, Nephotettix impicticeps and the brown planthopper, Nilaparvata lugens, and possess some resistance to the tungro virus, bacterial leaf blight, bacterial leaf streak and the local races of the blast disease. They have high yield potentials and fine-quality grains. The screening of about 1,000 rice varieties led to the identification of some varieties with natural resistance to the brown planthopper and to the rice green leafhopper. Resistance to the brown planthopper appears to be due to the presence of a strong repellent or a lack of feeding stimulus in the plants, while resistance to the green leafhopper is due either to the presence of toxic materials or nutritional deficiencies in the plant for the insect. Several species of stem borers, leafhoppers and planthoppers are serious pests of rice and cause extensive losses (Pathak, 1968). Infestations of these insects are particularly intense in places where 2 to 3 rice crops are grown in a year because, in such areas, the pests occur in multiple overlapping generations. The screening of about 10,000 rice varieties representing the rice germ plasm collection of the International Rice Research Institute has revealed that several varieties possess a high degree of resistance to one or more species of these pests. The selected resistant and susceptible varieties have been used in investigations on insect-host interrelations and in hybridization programs to develop insect-resistant rice varieties. Some of this work is reviewed in this paper.

2 790 M.D. PATHAK RESISTANCE TO STEM BORERS Varieties were screened in field experiments for their resistance to the striped rice borer, Chilo suppressalis (Walker) under natural insect infestations. These were tested over five crop seasons in batches of 1,000 to 2,000 each season. The planting of the test varieties at a time when the neighboring crop was nearing maturity generally brought about heavy borer infestations. The varieties which showed low borer incidence in these tests were evaluated further in field and greenhouse experiments. In the later tests, the plants were infested artificially with uniform borer population. These tests showed distinct differences in varietal susceptibility to the borers. The borer larvae caged on resistant varieties suffered high mortality and had a slower rate of growth, smaller body-size, and lower percentage of pupation than those caged on susceptible varieties (Figs 1 and 2). Similar results were also obtained by rearing the larvae either on stem pieces or on the seedlings of resistant and susceptible varieties. CAUSES OF BORER RESISTANCE Preference o] moths for oviposition Field experiments indicated the strong preference of the moths for ovipositing on certain varieties. In these experiments, even under low borer population, certain varieties received a much larger number of eggs and were heavily infested while others were almost insect-free. Many resistant varieties, however, had only a few egg masses even under heavy infestations. The results of one test are presented in Fig. 1. As is evident from this figure, the number of egg masses on variety TKM-6 was about ten times lower than the number laid on the susceptible variety Rexoro. Also, the varieties which contained a larger number of eggs generally had a higher percentage of deadhearts, indicating the important role that oviposition preference plays in determining the borer damage on these varieties. The varieties Chianan 2, Yabami Montakhab and Taitung 16 had comparatively more eggs but lower percentages of deadhearts. In subsequent experiments these varieties were observed to exert adverse effects on the borer larvae caged on them. Also, Chianan 2 and Taitung 16 exhibited a rather interesting phenomenon; they were highly resistant during the vegetative phase but became susceptible after flowering when they developed higher percentages of whiteheads. It appears that, after flowering, these varieties lose their resistance to larval survival and development on them. Differences in larval survival on resistant and susceptible varieties The adverse effects of resistant varieties on larval survival and development appears to be another major factor of varietal resistance (Fig. 2). On the resistant varieties Chianan 2 and Taitung 16 the borer larvae had only about half as much survival as on the susceptible varieties Rexoro and Sapan Kwai. Also, the larvae pupated earlier, and in greater numbers on the latter varieties than on the resistant ones. Furthermore, the larvae caged on the susceptible varieties weighed about twice as much as those caged on the resistant varieties (Fig. 3).

3 RESISTANCE IN RICE VARIETIES TO INSECTS E C.hilo. suppresalis wlk. Tryporyzo incerfulos wlk. Y : X r:o o---o Dead heart ( % ) TO o i"1 1',- IL') (1) -1-0 o, w "6 -~ 30-- t::. z O ~P ~6 2 t- Q_ 20 J 10 w I0 0J ~ td 5 o E.4,.Q r >- I'-- g: 0 Fig. 1. Ovipositional preference of stemborer moths and percentage deadhearts on selected resistant and susceptible varieties.

4 792 M.D. PATHAK Correlations between plant morphological and anatomical characters and stem borer susceptibility A general association between several plant morphological and anatomical characters and stem borer resistance has been recorded (Table I). Each of these characters appears to contribute to borer resistance but none by itself appears to be the real cause of such resistance. This was evident in several varieties having one of these characters positively correlated with resistance but still reacting as susceptible to the borers. TABLE I Correlations between pelventages o/ in/ested tillers and plant characters Plant Characters Height of culm Number of elongated internodes Length of the 3rd elongated internode Length of the flag leaf Width of the flag leaf External diameter of culm at half of its length 88 to the base Internal diameter of culm at half of its culm at 88 to the base Number of tillers per plant Number of borers per plant vs. percentages of infested tillers Percentage of stem area occupied by vascular bundle sheaths Correlation Coefficient.796**.632**.715"*.798**.836**.672**.785**.671"*.790** **.863"* ** Tall varieties, because of their height, might have been more attractive to ovipositing moths. The number of internodes and the elongation of the third internode contribute to the height of the plants. The length and width of the flagleaf blade were positively correlated with borer susceptibility. In separately conducted ovipositional preference tests these characters were positively correlated (r = and 0.924, respectively) with the number of egg masses laid. Also, it appears that the hairy leaf blade surface acts as a physical repellent for the female moths during oviposition. Most of the eggs were laid either on the smooth lower leaf surface or along the smooth midrib area or the upper leaf surface. The role of hairiness of the leaf blade surface was further investigated by shaving off the hairs from the leaf blade of the resistant variety TKM-6 and then comparing the number of eggs laid on it with the number deposited on the susceptible variety Rexoro. The results showed that even after shaving, the number of eggs laid on TKM-6 was significantly lower than that laid on Rexoro. This, therefore, indicates that hairiness by itself may not be the major factor deterring the moths from ovipositing on a resistant variety.

5 RESISTANCE IN RICE VARIETIES TO INSECTS CODE " '--] CHIANAN 2 TAITUNG 16 ~ REXORO I SAPAN KWAI SURVIVAL OF THE INSECT: M R /o MOTH EMERGENCE % PUPATION ~ LARVAL SURVIVAL EACH BAR REPRESENTS PERCENTAGES OF SURVIVAL OF 600 LARVAE CAGED 60 o~ o.>_ > 50 co 40 o.._j I0 0 s i] I 2o 1 t s5 1 t 45 ] Number of Days from Infestation Fig. 2. Survival and development of Chile suppressalis on resistant and susceptible varieties of rice. IRRI

6 794 M. i). PATI~*,I,: The borer larvae which hatch from the egg masses migrate within the leaf sheaths and the rice stem generally within 48 hours after hatching and feed on the leaf sheath tissues for about 6 days, after which they bore inside the rice stem. Thus, the varieties whose internodes are completely covered by tight leaf sheaths offer more resistance to the first-instar larvae than those whose internodes are only partially covered by loose leaf sheaths. 120 o o Sapan Kwai = ~ Rexoro o -o Taitun3 16 o~e Chianan 2 t v./ 30 m is zo 2~ 3o ~ 4o Age o[ Larvae (days) Fig. 3. Average weight of individual Chilo suppressalis larvae reared on resistant and susceptible varieties of rice. Several plant anatomical characters such as heavily sclerotized stem tissues, closely spaced vascular bundle sheaths, rigid stem surface, and high silica content have shown positive correlations with stem borer resistance. (Van & Guan, 1959; Israel, Vedarmurthy & Rao, 1961; Djamin & Pathak, 1967 and Patanakamjorn & Pathak, 1967). Each of these characters has been observed to interfere with larval feeding. It has been reported that, on rice varieties with high silica content, stem borer larvae suffer high mortality and their mandibles tend to wear off (Sasamoto, 1961; Djamin & Pathak, 1967). Also, many of the larvae die without being able to bore inside the stems, which is not the case with larvae on varieties with low silica content.

7 RESISTANCE IN RICE VARIETIES TO INSECTS 795 DEVELOPMENT OF STEM BORER POPULATION ON RESISTANT AND SUSCEPTIBLE VARIETIES The cumulative effects of varietal resistance on stem borer population were investigated by confining an identical number of borers for several generations separately on a resistant variety (Chianan 2) and on a susceptible one (Sapan Kwai). The number and emergence of moths and the number of eggs laid on these varieties were observed periodically and recorded. The plants in each cage were replaced with uninfested healthy plants of the same variety at 40-day intervals. At 120 days after infestation, 91 larvae and 2 egg masses were recovered from the resistant variety Chianan 2, in contrast to 1,583 larvae and 83 egg masses recovered from the susceptible variety Sapan Kwai. At this stage, 56.30/0 of the total number of tillers of Sapan Kwai showed deadheart damage while only less than 10/0 of the tillers of Chianan 2 were similarly damaged. Besides the slow rate of growth and low survival of stem borer larvae on resistant varieties, the uneven emergence of moths appeared to be a major cause of the low number of egg masses deposited. BREEDING FOR STEM BORER RESISTANCE About 20 varieties have been identified through field and greenhouse screening as resistant to stem borers. Although most of these varieties showed consistently lower borer infestations in repeated field experiments, many were damaged either by other insect pests or by some diseases. An exception was variety TKM-6 which besides being resistant to the striped borer, also possesses tolerance to the green leafhoppers and the brown planthoppers and some resistance to the bacterial leaf blight, bacterial leaf streak and the tungro virus. It has good quality grains but its plant type is rather poor and it frequently lodges even before flowering. Thus, it has a low yield potential. To improve its plant type but retain its resistance to insects and diseases, TKM-6 was crossed with [Peta/3 x Taichung (Native) 1] in cooperation with the IRRI plant breeders. The progenies, now in the F s generation, possess the desirable qualities of TKM-6 and also have a better plant type. A promising selection, IR532E576, is resistant to borers, tolerant to the green leafhoppers and brown planthoppers, and possesses some resistance to the tungro virus, bacterial leaf blight, bacterial leaf streak and the local races of the blast disease. Furthermore, it has fine-quality grains. When grown without any insecticidal protection, it outyielded IR8 in several field experiments, and when protected from insect damage, it produced yields similar to those of IR8. Its major weakness appears to be its weak stems which cause it to lodge at high fertility levels. Several other crosses involving TKM-6 and other stem borer resistant varieties are also under investigation.

8 796 M.D. I~ATHAI,: RESISTANCE TO THE BROWN PLANTHOPPER, Nilaparvata lugens STAL, AND THE RICE GREEN LEAFHOPPER, Nephotettix impicticeps (ISHIHARA) About 1,000 varieties selected from the preliminary screening of the 10,000 lines for stem borer resistance were also screened for their resistance to the green leafhopper and brown planthopper. The screening was done by exposing 15- to 20-day old seedlings growing in cm wooden flats to heavy insect populations inside a cm cage made of fine mesh screen. Separate cages were used for each insect species and the population in each cage was maintained at such a high level as to kill more than 90% of the plants of the susceptible check variety in 7 to 10 days after infestation. The varieties which survived the infestation were evaluated further for their true resistance by caging a known number of insects on individual plants. Resistance to Nilaparvata lugens The variety Mudgo exhibited a very high degree of resistance to N. lugens. It showed no apparent damage in the screening tests long after most of the other test varieties had been killed. When a uniform number of N. lugens nymphs were caged on individual Mudgo plants, the nymphs died generally within 10 days without becoming adults, whereas those on susceptible varieties had normal survival and growth (Fig. 4). As evident from this figure, at 5 days after caging, only 2.0~ of the nymphs were living on Mudgo plants while more than 900/0 were still alive on Pankhari 203 and Taichung (Native) 1. AU the nymphs on Mudgo died during the next five days, while about 90~ of those on Pankhari 203 and Taichung (Native) 1 survived to become adults. When adult insects were caged on these varieties, those on Mudgo lived for only a short period and produced very few nymphs while those on Pankhari 203 had normal survival and fecundity. Resistance to Nephotettix impicticeps In similar studies, the variety Mudgo reacted as susceptible to N. impicticeps but Panldaari 203 was highly resistant. Taichung (Native) 1 was susceptible to both N. lugens and N. impicticeps. Also, IR8 exhibited high resistance to N. impicticeps. In the mass-screening tests, both Pankhari 203 and IR8 suffered little damage while plants of most of the other test varieties were killed. In repeated experiments, leafhoppers caged on these two varieties suffered higher mortality than those caged on susceptible hosts (Fig. 4). Furthermore, those caged on Mudgo and Taichung (Native) 1 grew faster and hence reached the adult stage earlier. Also, the adults were larger. It is interesting that a variety highly resistant to one species was susceptible to the other and vice versa, indicating that the resistance is highly specific. Also, the level of resistance of these varieties was so high that infestations of individual 15-day old seedlings with 100 or 200 nymphs caused very little plant damage (Fig. 5). The levels of insect infestations used in these tests were much higher

9 *Similar insect survival was also recorded in experiments using 15, 45 and 90-day old plants. t~ ioo~~ 0 [..., r,.9 z ~9 Z 0 5 I , r v-- I 5 Days after Caging I Fig. 4. Survival and development of first instar NilaParvata lugens (A) and Nephotetti x impicticeps (B) nymphs on 60-day old plants of resistant and susceptible varieties. IRRI 1968.

10 798 M.D. PATHAK than natural infestations on young plants. Thus, these varieties provide valuable materials for developing leafhopper- and planthopper-resistant rice varieties and excellent differentials for basic studies on the nature of insect resistance in host plants. 6 i I Nephotettix impicticeps/ r o 0 5 I lo Days after Infestation Fig. 5. Damage caused by caging 100 first instar nymphs on resistant and susceptible varieties, IRRI Larger numbers indicate more plant damage and 0 represents no visible plant damage while 6 indicate that all plants were killed. TABLE H Sites o/feeding punctures and termination oj brown planthopper salivary sheaths on susceptible and resistant rice varieties. IRRI, 1968 Rice Variety Salivary Sheaths Taichung (Native) 1 Mudgo (susceptible) (resistant Total no. of salivary sheaths observed Stylets penetration (%) through Fibers Just beside fibers Parenchyma % salivary sheaths with different no. of branches Termination of salivary sheaths (%) A. Considering each branch as a separate probe Phloem Xylem 10 6 Parenchyma Air space 26 5 Others 3 2 B. Termination of at least one branch of the salivary sheath in Phloem Xylem 17 4 Phloem and xylem (at least 1 branch) 11 7 All ending in non-vascular tissues 4 21

11 RESISTANCE IN RICE VARIETIES TO INSECTS 799 Causes of resistance to Nilaparvata lugens and Nephotettix impicticeps and possibility of incorporating resistance in one plant The high mortality of N. lugens and N. impicticeps insects within a few days after caging on resistant plants suggests that either the insects are not capable of feeding on these plants, or the plants contain substances deleterious to these species. No mechanical barrier to the feeding of these insects has been observed in the plants (Table II). However, N. lugens adults have been recorded as doing very little feeding on Mudgo as compared to Pankhari 203 or Taichung (Native) 1. Although the N. impicticeps adults did more feeding on a susceptible host [Taichung (Native) 1] than on resistant varieties (Pankhari 203 and IR8), the differences in the feeding rates were not as distinct as with N. lugens. These indicate that the Mudgo plants either lack feeding stimulus or contain strong repellents for N. lugens, but the varieties resistant to N. impicticeps contain some toxic materials or lack vital nutrients for the insect. Current experiments have' shown that resistance to these two insects can be combined in a single plant and is dominant over susceptibility. Several selections from the cross Mudgo IR8 possess a high level of resistance and have desirable plant types similar to that of IR8. Thus, it seems possible to develop plants with high yield potentials and possessing resistance to both N. impicticeps and N. lugens. ZUSAMMENFASSUNG STENGELBOHRER- UND ZIKADEN-RESISTENZ BEI REISSORTEN Eine Gesamtheit yon Varietiiten, die beim Internafiona!en Reis-Forschungs-Institut das Welt-Reis-Sortiment darstellen, wurde auf ihre Resistenz gegeniiber dem Gestreiften Stengelbohrer, Chilo suppressalls, in Feld- und Gew~ichshausversuchen gepriift. Unter diesen werden 20 als hochresistent bewertet. Die Resistenz scheint Nonpriiferenz- und/ode r Antibiosis-Charakter zu haben. Verschiedene morphologische und anatomische Eigenschaflen der Pflanzen sollen in Beziehung zur Stengelbohrer-Resistenz stehen. Auf resistenten Pflanzen erleiden die Stengelbohrer-Larven hohe Moctalitiit, haben eine geringere Wachstumsrate, und ihre Ktrpergrtl-Se ist geringer als die yon Larven, welche auf anfiilligen Wirten aufgezogen wurden. Die selektierten resistenten Varietiiten sind auch in einem Hybridisierungs-Programm benutzt worden, das die Stengelbohrer-Resistenz mit einem verbesserten Pflanzentyp und anderen wiinschenswerten Eigenschaften zu verbinden sucht. Verschiedene Nachkommenschaften einer solchen Kreuzung zwischen TKM-6 (resistent gegen Stengelbohrer) und Peta3 X Taichung (wild) i (verbesserter Pflanzentyp) sind besonders vielversprechend. Sie haben einen verbesserten Pflanzentyp, sind widerstandsfiihig gegen Stengelbohrer, tolerant gegen die Gfiine Zwergzikade, Nephotettix cincticeps und die Braune Spornzikade, Nilaparvata lugens, und besitzen eine gewisse Resistenz gegen das Tungro-Virus, bakteriellen Blattmehltau, bakterielle Blattstreifen und lokale Rassen der Pestkrankheit. Sic haben hohe Ertragspotentiale und gute Kornqualit~it. Die Priifung yon etwa 1000 Reissorten ffihrte zur Feststellung einiger Varietiiten mit natiiflicher Resistenz gegen die Braune Spornzikade und die Griine Zwergzikade. Die Resistenz gegen die Braune Spornzikade scheint auf der Anwesenheit eines starken Repellents oder auf dem Fehlen eines" Fraf~stimulants in den Pflanzen zu beruhen, w~ihrend die Resistenz gegen die Griine Zwergzikade entweder eine Folge der Anwesenheit toxischer Stoffe oder minderer Nahrungsqualitiit der Pflanze fijr das!nsekt ist,

12 800 M.D. PATHAK REFERENCES DjAMIN, A. & PATHAK, M. D. (1967). The role of silica in resistance to Asiatic rice borer, Chilo suppressalis (Walker) in rice varieties. J. econ. Entomol. 60: ISRAEL, P., VEDARMtrRTnY, G. & RAO, S. (1961). Varietal resistance to gall midge, Pachydiplosis oryzae Mani and other insect pests of paddy. Conf. Int. Rice Comm. 5 p. PATANAKAMJORN, S. & PATHAIr M. D. (1967). Varietal resistance to Asiatic rice borer Chilo suppressalis (Walker) in rice and its association with various plant characters. Ann. entomol. Soc. Am. 60: PATnAK, M.D. (1968). Ecology of common insect pests of rice. Annu. Rev. Entom. 13: SASAMOTO, K. (1961). Resistance of the rice plant applied with silicate and nitrogenous fertilizers to the rice stem borer, ChiIo suppressaiis (Walker). Proc. Fac. Liberal Arts & Educ., Yamanasaki Univ. Japan, 3. VAN, T. K. & Gt~, G.K. (1959). The resistance of Oryzae ridleyi Hook to paddy stem borer (ChiIo suppressalis Walk.) attack. Malayan agr. ]. 42: