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Integration of varietal resistance and predation for the management of Nephotettix virescens (Homoptera: Cicadellidae) populations on rice
C R O P P R O T E C T I O N (1986) 5 (4), 259-265
Integration of varietal resistance and predation for the management of Nephotettix virescens (Homoptera: Cicadellidae) populations on rice M. M. MYINT*, H. R. RAPUSASAND E. A. HEINRICHSt
Department of Entomology, International Rice Research Institute, PO Box 933, Manila, Philippines
ABSTRACT. Levels of resistance of nine rice varieties to Nephotettix virescens (Distant) were determined. As judged by the seedbox screening, survival and oviposition, feeding and population growth tests, IR29 and IR56 were resistant, IR8, IR36, and IR42 moderately resistant, and IR22, IR46, Utri Rajapan and TN1 susceptible. Degree of predation by the spider, Lycosa pseudoannulata Boes. et Str. and the mirid bug, Cyrtorhinus lividipennis Reuter was the same on all varieties, regardless of the level of varietal resistance to N. virescens. N. virescens mortality was highest and populations lowest in treatments where resistant varieties IR29 and IR56 were combined with predators.
Introduction The green leafhopper, Nephotettix virescens (Distant) is distributed throughout South and South-East Asia. Nymphs and adults suck plant sap from the leaves but seldom reach populations which cause feeding damage. N. virescens are primarily common pests because they are efficient vectors of tungro virus and tungro-like diseases (Rivera et al., 1972; John et al., 1979). Twenty-six rice varieties, IR5 to IR62, have been developed by the International Rice Research Institute. As judged by the standard seedling screening of 7-day-old seedlings, most of the IR varieties are moderately resistant (MR) or resistant (R) to N. virescens (Heinrichs et al., 1985). This study was conducted to determine more accurately the levels of resistance of selected IR varieties to N. virescens using several different tests. These varieties were then evaluated to determine their effect on N. virescens populations when integrated with the feeding of predators.
Native 1 (TN1) to N. virescens was evaluated in the greenhouse and in the insectary at a temperature of 28°C_5°C. IR29 was used as the resistant check (control); TN1 and Utri Rajapan served as susceptible checks. Utri Rajapan is susceptible to N. virescens but is resistant to the tungro virus (Panda, Heinrichs and Hibino, 1984). A greenhouse culture of N. virescens reared on TN1 served as the test insects.
Determination of the levels of resistance of test varieties
Standard seedbox screening test (SSST). Twenty pregerminated seeds of each variety were sown in a 5 cm row in a 5cm depth of soil in wooden seedboxes (45x60x 10cm). The boxes were placed in a galvanized iron tray filled with water. Seven replications were made (one seedbox represented one replication) and they were arranged in a randomized complete block design (RCBD). Seven days after sowing (DAS), each seedbox was enclosed with nylon mesh cage and the plants were infested with second-instar nymphs of N. virescens at the rate of four nymphs per plant. Plant damage ratings were taken when 90% of the susceptible check (TN1) plants were dead and every 2 days thereafter for four additional ratings. Ratings were based on the 0-9 scale of the Standard Evaluation System (SES) for rice (IRRI, 1980).
The resistance of rice varieties, IRS, IR22, IR29, IR36, IR42, IR46, IR56, Utri Rajapan and Taichung
Modified seedbox screening test (MSST). The SSST was not sufficiently sensitive to detect moderately
Materials and methods
* Present address: Agricultural Research Institute, Yezin, Pyinmana, Burma t Present address: Department of Entomology, 402 Life Sciences Building, Louisiana State University, Baton Rouge, Louisiana 70803-1710, USA 0261-2194/86/04/0259-07 $03.00 © 1986 Butterworth & Co (Publishers) Ltd
Integrated management of Nephotettix virescens on rice
260
resistant varieties, because they were usually rated as susceptible: the SSST was therefore modified. All M S S T procedures were the same as SSST, except that the infestation rate was lower (two second-instar nymphs per plant). Damage ratings were recorded when the progeny of the initially infested test insects had killed 90% of the susceptible check (TN1) plants, and were repeated every 2 days thereafter for four additional ratings. Treatments were replicated seven times and arranged in a RCBD. Each seedbox represented one replication. N. virescens adult survival, oviposition rate and egg hatchabil#y. Seven-day-old plants of the test varieties were transplanted in clay pots (diameter 12cm) with three seedlings per pot. The pots were arranged randomly in a RCBD in a galvanized iron tray with water. Five replications were made, with one pot representing one replication. Fourteen days after transplanting (DT), the plants were cleaned and enclosed with Mylar film cages (90cm high, 10cm in diameter). One week after caging, the plants were examined and any arthropods present inside the cages were removed. T h e n five pairs (male and female) of three-day-old N. virescens adults were introduced into each cage. Dead insects were counted daily from one day after infestation (DAI) to 5 DAI and the sex of the dead insects was noted at each counting. All insects were removed from the cages at 6 DAI. When nymphs started to hatch, they were counted daily and removed from the cage. When no more hatching was observed, the plants were dissected under a binocular microscope to count the unhatched eggs. The total number of eggs laid (nymphs+ unhatched eggs) was determined and percentage egg hatchability was computed thus: % egg hatchability =
No. of nymphs x 100 No. of nymphs + unhatched eggs
richs, 1982), before installation at the base of the chamber. Treatments were replicated five times, one chamber representing one replication, and were arranged in RCBD. Two newly emerged N. virescens female adults starved for 4 h were placed in each feeding chamber. After 24 h, the filter papers were removed, the honeydew spots traced on tracing paper and the area measured over millimetre squared graph paper. Blue spots indicating phloem feeding (basic reaction) were measured separately from the white spots (acidic reaction) from xylem feeding.
Predator efficiency on N. virescens feeding on rice varieties with different levels of resistance The effect of the integration of selected predators with host plant resistance for N. virescens was determined in the greenhouse and in the insectary. Test varieties were the same as in the previous experiments. The spider Lycosa pseudoannulata Boes. et Str. (Araneae: Lycosidae) and the mirid bug Cyrtorhinus lividipennis Reuter (Hemiptera:Miridae) were evaluated for their efficiency on N. virescens. A split-plot design was used with four replications.
Efficiency of L. pseudoannulata. Seven-day-old seedlings of the test varieties were transplanted in clay pots (diameter 12 cm) with three seedlings per pot. The potted plants were placed in a galvanized iron tray filled with water. At 14 D T the plants were cleaned and enclosed with Mylar film cages (one pot per cage). At 20 D T the plants were checked and any insects, parasites or predators were removed from the cage. Then, at 21 DT, one L. pseudoannulata and 20 N. virescens adults were simultaneously released into each cage. Observations on numbers of prey were recorded daily and missing or dead prey and predators were replaced to keep a constant 20:1 prey:predator ratio. Data were taken for 10 days.
Population growth of N. virescens. The ability of N. virescens to develop a population for two generations on rice varieties IRS, IR29, IR36, IR42 and IR56 was determined. The plants were prepared as in the adult survival and oviposition test. When the plants were at 30 DAS, they were infested by introducing into each cage four pairs (male and female) ofN. virescens adults. The first-generation progeny were counted at 24 DAI and the second-generation progeny at 60 DAI.
Efficiency of C. lividipennis predation on N. virescens eggs. The procedures for the preparation of plants were the same as that for the predation efficiency of L. pseudoannulata. At 21 DT, the plants were infested with four pairs ofN. virescens adults, 5-6 days old. At 2 DAI of N. virescens, five C. lividipennis adults were released into each cage. Five days after releasing C. lividipennis, all N. virescens and C. lividipennis were removed from the cage. Nymphs that hatched were counted daily until hatching ceased.
Honeydew excretion. To measure the feeding activity of N. virescens, seven-day-old seedlings of the test varieties were transplanted in clay pots (6 cm diameter) with two plants per pot. At 30 DAS, a feeding chamber (Paguia, Pathak and Heinrichs, 1980) was attached to the middle portion of the leaves of one plant per pot. Filter papers were treated with a pH indicator, bromocresol green dye (2 mg/ml ethanol) (Pathak and Hein-
Efficiency of C. lividipennis predation on N. virescens nymphs. The procedure for plant preparation was the same as that described for predation efficiency of L. pseudoannulata. At 21 DT, 25 first-instar N. virescens nymphs and five newly emerged C. lividipennis adults were simultaneously released into each cage. Seven days later, the N. virescens in the cages were counted.
M . M . MYINT, H . R. RAPUSAS AND E. A. HEINRICHS
Results
Determination of the levels of varietal resistance of test varieties Standard seedbox screening test (SSST).
Ratings of the seedlings for plant damage started at 5 DAI when TN1 had an average rating of 6" 9, IR29, IR56 and IR36 had resistant (R) ratings of 1.7-3.1 and the other varieties had moderately resistant (MR) ratings of 4-6 (Figure 1). At 7 DAI there was an increase in the ratings of all the varieties and the increase in the severity of damage progressed with time. At 9 DAI, the susceptible (S) varieties could be identified as IR46, IR22, Utri Rajapan and TN1, with ratings of 6 . 9 8" 9; IR8, IR42 and IR36 showed MR reactions, while IR29 and IR56 had R reactions. At 13 DAI, however, IR8 and IR42 also showed S reactions while IR36 remained MR and IR56 and IR29 were still R.
rating of 8. At this time IR22 and IR46 had a rating of 6.4 and Utri Rajapan 6.1 showing MR reactions (Figure 1); the other varieties had ratings of 0-0-2.6. At 26 and 28 DAI there was an increase in the ratings of the varieties although they remained in the same category of S, MR and R. At 30-32 DAI, IR8 and IR24 were MR; IR36, IR29 and IR56 remained as R, and IR22, IR46, Utri Rajapan, and TN1 were S. N. virescens adult survival, oviposition and egg hatchability. There were no significant differences in the survival of the females among the different varieties at 1, 3, and 4 DAI (Figure 2). At 2 DAI, survival was significantly lower on IR29 and IR56 than on the other varieties and at 5 DAI, survival on these two varieties (52%) differed from that of the other varieties except for IR36 (76%). All insects survived on TNI and Utri Rajapan until 5 DAI.
Rating the plants for damage started at 24 DAI when TN1 had a
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Days after infestation FIGURE 1. Damage ratings in the standard (SSST) and modified (MSST) seedbox screening of rice varieties against IV,. virescens. Average of seven replications.
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I 1
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Days after infestation FIGURE2. Survival of newly emerged N. virescens adult females (upper) and males (lower) on rice varieties with different levels of resistance. Average of five replications.
Integrated management of Nephotettix virescens on rice
262
Survival of males was generally lower than that of females and the decrease in survival over the 5-day period was more evident on the R varieties, IR29 (20%) and IR56 (28%) (Figure 2). Survival on IR36 was intermediate and was significantly more than that on IR29 and IR56 but significantly less than that on IR42 and IR8. The results indicated that survival of both sexes was least on IR29 and IR56, followed by IR36, IR8 and IR42. Survival was highest on TN1, Utri Rajapan, IR22 and IR46. The numer of eggs laid by N. virescens over a 5-day period on the different varieties is shown in Table 1. Significantly less eggs were laid on IR29, IR56, and IR36 (41-84) than on the other varieties. Eggs laid on IR42 and IR8 were significantly more than those on IR29, IR36 and IR56 but significantly less than those on IR46, IR22, Utri Rajapan and TN1. The number of eggs laid on TN1 was about seven- to eightfold that on the R IR29 and IR56, fourfold that on IR36 and double that on IR42 and IR8. Although the number of eggs laid differed significantly among the varieties, egg hatchability was high (95-98%) in all varieties (Table 1).
30o r~ 25o .~ 200 ~ 15o o ~, 100
E z=
50
IR29
IR56
IR36
IR42
IR8
Varieties FIGURE 3. F 1 and F 2 N. virescens populations at 24 and 60 days after rice varieties were infested w i t h four pairs (males and females) o f adults. Average of four replications. W i t h i n each generation, bars with the same letter are not significantly different at the 5% level by D u n c a n ' s multiple range test. Analysis based on values transformed to
-[~ Xylem (acidic)] Phloem (basie)~
700" ~E 600 ~ n 500 = o o ~ 400 ~o_= = ~ o 300 ,, '~ < ~ 200 100
Population growth of N. virescens. The first- and second-generation progeny (F1 and F2) differed significantly among the varieties (Figure 3). IR42 and IR8 supported the highest population of the insect, followed by IR36, IR56 and IR29 in descending order. At 60 DAI, however, when the second-generation progeny (F2) were counted, no insect had survived on IR29 and only six on IR56. An increase in the number of insects from the F1 to F2 was observed on IR36 (from 50 to 70). On IR42 the population was doubled and on IR8 the second generation was three times greater than the first generation.
Honeydew excretion. Although numerical differences were observed in the total area of honeydew spots on the filter papers, the differences were not statistically significant except on IR8 where it was significantly less than the rest of the varieties (Figure 4). Honeydew TABLE 1. N. virescens oviposition and egg hatchability on rice varieties w i t h different levels of resistance. * I R R I , 1984
Varietyt IR29 IR56 IR36 IR42 IR8 IR46 IR22 U t r i Rajapan TN1
N u m b e r of eggs~ 41 46 84 166 177 258 322 313 338
c c c b b a a a a
Percentage o f eggs laid as based on TN1 12 17 25 49 52 76 95 92 100
Egg hatchability (%) 99 98 95 95 97 98 97 95 96
a a a a a a a a a
*In a column, means followed by a common letter are not significantlydifferent at the 5% level by Duncan's multiple range test. The analysis is based on values transformed to ,f'~g-0~. ?Thirty-day-old plants. :l:Means of five replications with five pairs of adults per replication for 5 days. Insects were previously reared on TN1.
IR29 IR56
IR36 IR42 IR8
IR46 IR22 Utri TN1 Rajapan Rice varieties
FIGURE 4. Area of honeydew excretion spots by two N. virescens females on rice varieties with different levels o f N . virescens resistance. Average o f five replications.
from phloem feeding caused a basic reaction with the bromocresol green solution, indicated by the blue colour of the honeydew spot. As judged by the colour reaction of the honeydew on the bromocresol greentreated filter paper, more phloem feeding was observed on IR46, IR22, Utri Rajapan and TN1. Phloem feeding on the S varieties was more than that on the R and M R varieties. There was relatively very little phloem feeding on IR29 and IR56 ( 1 - 3 m m 2) and on IR36 (10ram2). Xylem feeding was highest on R IR56 and least on IR8.
Predator efficiency on N. virescens feeding on rice varieties with different levels of resistance Efficiency
of L. pseudoannulata. The predation efficiency ofL. pseudoannulata on N. virescens feeding on rice varieties with different levels of resistance, as indicated by the differences in mortality of the prey, is shown in Table 2. N. virescens caged on R varieties (IR29, IR56) had significantly higher mortality than on the other varieties. On M R varieties (IR36, IR42 and IR8), mortality of N. virescens without the predator was intermediate between that on the R varieties and S varieties (IR46, IR22, Utri Rajapan and TN1). Comparing the mortality of N. virescens on varieties with and without L. pseudoannulata, significant differences were observed in all varieties tested
M. M.
MYINT,
H. R. RAPUSASANDE. A. HEINRICHS
(Table 2) indicating that L. pseudoannulata predation contributed to the higher mortality ofN. virescens. The increase over the no-predator treatment was about 10-15%.
Efficiency of C. lividipennis predation on N. virescens eggs. Predation of N. virescens eggs by C. lividipennis was based on the number of nymphs hatching from the eggs laid on the different varieties. Because of the different levels of resistance, the number of eggs laid varied among the varieties tested, as indicated by the number of nymphs observed without the predator (Table 3). Significantly fewer nymphs were counted on the R varieties IR29 and IR56 (31-38) and the highest number of nymphs was observed on S varieties (247 on TN1); the number of nymphs on the MR varieties was intermediate (79-170). When caged with the predator, however, fewer N. virescens nymphs were observed than were seen without the predator on all the varieties, because of egg predation (Table 3). The predators decreased the N. virescens population by more than 50% on most varieties. The rate of predation for the period of 5 days was significantly different among varieties with R, MR, TABLE 2. Nephotettix virescens mortality due to the combined effect of predation by Lycosa pseudoannulata and varietal resistance. I R R I , 1984 M o r t a l i t y (%) for a period o f 10 days*
Varietyt IR29 IR56 IR36 IR42 IR8 IR46 IR22 U t r i Rajapan TN1
With
Without
L. pseudoannulata
L. pseudoannulata
Difference
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5"As Table 1. #Average of four replications. N. virescenswere 5-6-day-old adults. In a column, means followed by a common letter are not significantlydifferent at the 5°70 level by Duncan's multiple range test. Analysis is based on values transformed to arcsin ~ . I n s e c t s were previously reared on TN1 plants. * *Significant at the 1% level.
263
and S reactions to N. virescens (Table 3). The predation rate was related to the number of prey: it was highest on the S varieties IR22, Utri Rajapan and TN1, where the N. virescens population was the highest, and lowest on the R varieties IR29 and IR56, where the N. virescens population was the lowest. C. lividipennis predation on N. virescens nymphs. Mortality of N. virescens nymphs on the varieties without the predator was significantly higher (64-66%) on the R IR29 and IR56 than on the MR and S varieties (Table 4), and mortality on the MR was higher than on the S varieties. With the predator, no significant difference was observed among the MR and S varieties but mortality was significantly higher on the R varieties. The lowest mortality of 32% was obtained on S IR46, with the highest on the R varieties IR29 and IR56 (92 and 87%, respectively). The differences in mortality between treatments with and without the predator among the varieties were not significant.
Discussion The SSST is a rapid means of screening large volumes of material for qualitative resistance. This method is used in the greenhouse for screening for N. virescens resistance throughout Asia. At IRRI, more than 60 000 entries are evaluated each year for N. virescens using the SSST (Heinrichs, Medrano and Rapusas, 1985). With this method, however, MR varieties are often not detected and are rated as S. Hence, the MSST was used to identify varieties with moderate levels of resistance which may be due to tolerance or due to the low levels of antibiosis or non-preference. Results showed that the reactions of the R and S varieties were similar in both tests but that the MR varieties had higher ratings in the SSST than in the MSST. The resistance levels of IR8, IR36, and IR42 increased with plant age as indicated by the MR-S reactions at the seedling stage in the SSST but R - M R reactions of older plants in the MSST.
TABLE 3. Population o f N . virescens as affected by varietal resistance and predation by C. lividipennis on N. virescens eggs on rice varieties w i t h different levels o f N . virescens resistance. I R R I , 1984 N u m b e r of N. virescens*
VarietyS IR29 IR56 IR36 IR42 IR8 !R46
IR22 U t r i Rajapan TN1
W i t h predator 6 10 18 50 64 109 91 68 116
d cd c b b a ab b a
W i t h o u t predator
Varietal means o f with and without predator
31 g 38 g 79 ef 135 cd 170 bc 213ab 220 ab 188 b 247 a
19 f 24 f 49 e 93 d 117 cd 161 ab 156ab 128 bc 182 a
Predation rate per predator/day S 0"99 1.12 2.43 3-42 4.24 4.14 5.12 4.83 5.26
e e d c b b a ab a
TAs Table 1. =l:Meansof four replications with four pairs ofN. virescensand five of C. lividipennis. In a column, means followed by a common letter are not significantlydifferent at the 5% level by Duncau's multiple range test. Analysis is based on values transformed to "f'~. gPredation rate/day was computed by: Nymphs in control (without predator) - nymphs in treatment (with predator) (5 predators x 5 days)
264
Integrated management of Nephotettix virescens on rice
TABLE 4. Effect of levels of varietal resistance and C. lividipennis predation on N. virescens nymphs. IRRI, 1984
Variety'~ IR29 IR56 IR36 IR42 IR8 IR46 IR22 Utri Rajapan TN1
Variety means Mortality (%) ofN. vlrescens nymphs:~ o f % mortality with and With Without without predator predator Difference~ predator 92 87 48 45 56 32 40 43 41
a a b b b b b b b
66 64 28 27 34 11 7 8 5
a a b b b c c c c
26 a 23 a 20a 18a 22a 21 a 33 a 35 a 36 a
79 76 38 36 45 22 24 26 23
a a b b b c c c c
tAs Table 1. SMeans of four replications with 25 first-instar N. virescensnymphs and five C. lividipennis for 7 days. In a column, means followed by a common letter are not significantly different at the 5% level by Duncan's multiple range test. Analysis is based on values transformed to V~- ~The difference in mortality (with-withom predator) was significant at the 1% level for all varieties.
Egg production by~N. virescens on the different varieties showed significant differences among R, M R and S varieties: more eggs were laid on the S and less on the R varieties. The lower number of eggs laid on the R varieties might be the result of the lower female adult survival on these varieties or because R varieties are not preferred by the insect for oviposition. As previously reported by Heinrichs and Rapusas (1983), levels of resistance of the varieties did not affect hatching, although the fecundity of the insect was affected. Karim (1978), however, reported that egg hatchability of N. virescens was significantly less on R than on S varieties when comparing TN1 and IR36. The number of N . virescens progeny produced differed significantly among the varieties in the F1 and F2 generations: IR42 and IR8 supported the highest population and the lowest was on IR29. The low population growth on the R varieties suggested an antibiotic effect. Because the varieties tested had different levels of resistance, the levels of antibiosis may differ. N. virescens fed on all varieties but more phloem feeding was observed on S varieties, as indicated by the basic reaction of the honeydew spot. On R varieties, however, the insect fed mainly on the xylem sap, which was acidic. Auclair, Baldos and Heinrichs (1982) investigated the biochemical evidence for the feeding sites and reported that N. virescens fed predominantly on phloem sap on S varieties and on xylem sap on R varieties. Because of the low level of nutrients in xylem sap, N. virescens reproduction and development was low on IR29 and IR56 in spite of the large amount o f xylem sap ingested. As judged by all the tests used to evaluate the levels of resistance of the varieties tested in this study, IR29 and IR56 are R; IR8, IR36, and IR42 are M R , and IR22, IR46 and Utri Rajapan are S to N. virescens. Predation was not significantly higher on R varieties than on S varieties, as was reported for L. pseudoannulata on the brown planthopper Nilaparvata lugens (St~l) (Kartohardjono and Heinrichs, 1984). They attributed the fiigher predation to the greater
movement of N. lugens on R varieties in search of suitable feeding sites. N. virescens is more active than N. lugens on both R and S varieties, and also feeds on R varieties, even though it is in the xylem; N. lugens does not feed on R varieties. When N. virescens were caged with either L. pseudoannulata or C. lividipennis, mortality was highest on the R varieties; this was due to the combined effect of the high levels of mortality caused by varietal resistance and mortality caused by predation. Thus, even a M R variety such as IR36 had populations only one-sixth that of S TN1 when caged with C. lividi-
pennis. Predation by C. lividipennis was highest on the S varieties where the N. virescens population was the highest. This suggests an increase in predation rate with an increase in host-egg density. Predation by C. lividipennis ranged from one to five eggs per day whereas Chiu (1979)reported that one C. lividipennis adult feeds on 1-2 eggs at one feeding and IRRI (1978) reported that C. lividipennis can prey on four eggs per day. Predation by C. lividipennis was reported to be one N. virescens nymph per day (Dyck and Orlido, 1977) and 0 . 4 S. furcifera per day (Salim, 1985). These tests indicated the high degree ofN. virescens control when varieties with high levels of N. virescens resistance were confined with either the spider, L. pseudoannmulata or the mirid bug, C. lividipennis. The effectiveness of L. pseudoannulata under field con. ditions, however, may be less than in these tests where the prey was confined to cages and the predator had a limited choice of food. L. pseudoannulata has been reported to feed on C. lividipennis and on members of its own species, thus limiting its value (Salim and Heinrichs, 1985). Swezey (1936) reported that C. lividipennis feeds on both parasitized and nonparasitized hopper eggs, thereby reducing the effectiveness of egg parasites under natural conditions. Field tests are currently being conducted to determine the populations ofN. virescens and its predators on the varieties used in the greenhouse study. References AUCLAIR, J. L., BALDOS, E. AND HEINRICHS, E. A. (1982). Biochemical evidence for t h e feeding sites o f t h e leathopper, Nephotettix virescens w i t h i n susceptible a n d resistant rice plants. Insect Science and its Application 3, 2 9 - 3 4 . CHIU, S. C. (1979). Biological control o f t h e b r o w n p l a n t h o p p e r . In: Brown Planthopper: Threat to Rice Production in Asia, pp. 3 3 5 - 3 5 6 . L o s Banos, L a g u n a , Philippines: I R R I . DYCK, V. A. AND ORLIDO, G. C. (1977). C o n t r o l o f the b r o w n p l a n t h o p p e r (Nilaparvata lugens) b y natural e n e m i e s a n d timely application o f n a r r o w s p e c t r u m insecticides. In: The Rice Brown Planthopper, pp. 5 5 - 7 2 . Taipei: F F T C (ASPAC). HEINRICHS, E. A. AND RAPUSAS, H . R. (1983). Correlation o f resistance to t h e g r e e n leafhopper, Nephotettix virescens ( H o m o p t e r a : Cicadellidae) w i t h t u n g r o virus infection in rice varieties h a v i n g different genes for resistance. Environmental Entomology 12, 2 0 1 - 2 0 5 . HEINRICHS, E. A., MEDRANO, F. G. AND RAPUSAS, H. R. (1985). Genetic Evaluation for Insect Resistance in Rice. L o s Bafios, L a g u n a , Philippines: I R R I .
M. M. MYINT, H. R. RAPUSASAND E. A. HEINRICHS HEINRICHS, E. A., MEDRANO, F. G., RAPUSAS,H. R., VEGA, C., MEDINA, E., ROMENA, A., VIAJANTE, V., SUNIO, L., DOMINGO,I. AND CAMANAG,E. (1985). Resistance of varieties IR5 to IR62 to rice insect species. International Rice Research Newsletter 10 (6), 12-13. IRRI (1978). International Rice Research Institute Annual Report for 1977. Los Bafios, Laguna, Philippines: IRRI. IRRI (1980). Standard Evaluation System for Rice, Second edn. Los Bafios, Laguna, Philippines: IRRI. JOHN, V. T., HEU, M. H., FREEMAN,W. H. AND MANANDHAR, D. (1979). A note on dwarf disease of rice in Nepal. Plant Disease Reporter 62 (9), 1. KARIM, A. N. M. R. (1978). Varietal Resistance of Rice to Green Leafhopper, Nephotettix virescens (Distant): Sources, Mechanisms and Genetics of Resistance. PhD thesis, University of the Philippines at Los Bafios. KARTOHARDJONO,A. AND HEINRICHS,E. A. (1984). Populations of the brown planthopper, Nilaparvata lugens (Still) (Homoptera: Delphacidae)and its predators on rice varieties with different levels of resistance. Environmental Entomology 13, 359-365. PAGUIA, P., PATHAK, M. D. AND HEINRICHS, E. A. (1980). Honeydew excretion measurement techniques for determining differential feeding activity of biotypes of Nilaparvata lugens
265
on rice varieties. Journal of Economic Entomology 73, 35-40. PANDA,N., HEINRICHS,E. A. AND HIBINO, H. (1984). Resistance of the rice variety Utri Rajapan to ragged stunt and tungro viruses. Crop Protection 3, 491-500. PATHAK, P. K. AND HEINRICHS,E, A. (1982). Bromocresol green indicator for measuring feeding activity of Nilaparvata lugens on rice varieties. Philippine Entomologist 5, 195-198. RIVERA, C. T., AGUIERO,V. M., DIMASUAY,D. F. AND LING, K. C. (1972). New vector of rice tungro and yellow dwarf. Philippine Phytopathology 8 (1/2), 10. SALIM, M. (1985). Interaction of Resistant Rice Varieties, Arthropod Predators and Insecticide on Whitebacked Planthopper, Sogatella furcifera (Horv6th) (Homoptera: Delphacidae) Populations. MSc thesis, University of the Philippines at Los Bafios. SALIM, M. AND HEINRICHS,E. A. (1985). Intra and interspecific feeding of whitebacked planthopper, Sogatella furcifera (Horvfith) (Homoptera: Delphacidae) predators. International Rice Research Newsletter (in press). SWEZEY, O. H. (1936). Biological control of the sugarcane leafhopper in Hawaii. Hawaii Plant Research 49, 57-101.
Accepted 18 November 1985
Integration of varietal resistance and predation for the management of Nephotettix virescens (Homoptera: Cicadellidae) populations on rice M. M. MYINT*, H. R. RAPUSASAND E. A. HEINRICHSt
Department of Entomology, International Rice Research Institute, PO Box 933, Manila, Philippines
ABSTRACT. Levels of resistance of nine rice varieties to Nephotettix virescens (Distant) were determined. As judged by the seedbox screening, survival and oviposition, feeding and population growth tests, IR29 and IR56 were resistant, IR8, IR36, and IR42 moderately resistant, and IR22, IR46, Utri Rajapan and TN1 susceptible. Degree of predation by the spider, Lycosa pseudoannulata Boes. et Str. and the mirid bug, Cyrtorhinus lividipennis Reuter was the same on all varieties, regardless of the level of varietal resistance to N. virescens. N. virescens mortality was highest and populations lowest in treatments where resistant varieties IR29 and IR56 were combined with predators.
Introduction The green leafhopper, Nephotettix virescens (Distant) is distributed throughout South and South-East Asia. Nymphs and adults suck plant sap from the leaves but seldom reach populations which cause feeding damage. N. virescens are primarily common pests because they are efficient vectors of tungro virus and tungro-like diseases (Rivera et al., 1972; John et al., 1979). Twenty-six rice varieties, IR5 to IR62, have been developed by the International Rice Research Institute. As judged by the standard seedling screening of 7-day-old seedlings, most of the IR varieties are moderately resistant (MR) or resistant (R) to N. virescens (Heinrichs et al., 1985). This study was conducted to determine more accurately the levels of resistance of selected IR varieties to N. virescens using several different tests. These varieties were then evaluated to determine their effect on N. virescens populations when integrated with the feeding of predators.
Native 1 (TN1) to N. virescens was evaluated in the greenhouse and in the insectary at a temperature of 28°C_5°C. IR29 was used as the resistant check (control); TN1 and Utri Rajapan served as susceptible checks. Utri Rajapan is susceptible to N. virescens but is resistant to the tungro virus (Panda, Heinrichs and Hibino, 1984). A greenhouse culture of N. virescens reared on TN1 served as the test insects.
Determination of the levels of resistance of test varieties
Standard seedbox screening test (SSST). Twenty pregerminated seeds of each variety were sown in a 5 cm row in a 5cm depth of soil in wooden seedboxes (45x60x 10cm). The boxes were placed in a galvanized iron tray filled with water. Seven replications were made (one seedbox represented one replication) and they were arranged in a randomized complete block design (RCBD). Seven days after sowing (DAS), each seedbox was enclosed with nylon mesh cage and the plants were infested with second-instar nymphs of N. virescens at the rate of four nymphs per plant. Plant damage ratings were taken when 90% of the susceptible check (TN1) plants were dead and every 2 days thereafter for four additional ratings. Ratings were based on the 0-9 scale of the Standard Evaluation System (SES) for rice (IRRI, 1980).
The resistance of rice varieties, IRS, IR22, IR29, IR36, IR42, IR46, IR56, Utri Rajapan and Taichung
Modified seedbox screening test (MSST). The SSST was not sufficiently sensitive to detect moderately
Materials and methods
* Present address: Agricultural Research Institute, Yezin, Pyinmana, Burma t Present address: Department of Entomology, 402 Life Sciences Building, Louisiana State University, Baton Rouge, Louisiana 70803-1710, USA 0261-2194/86/04/0259-07 $03.00 © 1986 Butterworth & Co (Publishers) Ltd
Integrated management of Nephotettix virescens on rice
260
resistant varieties, because they were usually rated as susceptible: the SSST was therefore modified. All M S S T procedures were the same as SSST, except that the infestation rate was lower (two second-instar nymphs per plant). Damage ratings were recorded when the progeny of the initially infested test insects had killed 90% of the susceptible check (TN1) plants, and were repeated every 2 days thereafter for four additional ratings. Treatments were replicated seven times and arranged in a RCBD. Each seedbox represented one replication. N. virescens adult survival, oviposition rate and egg hatchabil#y. Seven-day-old plants of the test varieties were transplanted in clay pots (diameter 12cm) with three seedlings per pot. The pots were arranged randomly in a RCBD in a galvanized iron tray with water. Five replications were made, with one pot representing one replication. Fourteen days after transplanting (DT), the plants were cleaned and enclosed with Mylar film cages (90cm high, 10cm in diameter). One week after caging, the plants were examined and any arthropods present inside the cages were removed. T h e n five pairs (male and female) of three-day-old N. virescens adults were introduced into each cage. Dead insects were counted daily from one day after infestation (DAI) to 5 DAI and the sex of the dead insects was noted at each counting. All insects were removed from the cages at 6 DAI. When nymphs started to hatch, they were counted daily and removed from the cage. When no more hatching was observed, the plants were dissected under a binocular microscope to count the unhatched eggs. The total number of eggs laid (nymphs+ unhatched eggs) was determined and percentage egg hatchability was computed thus: % egg hatchability =
No. of nymphs x 100 No. of nymphs + unhatched eggs
richs, 1982), before installation at the base of the chamber. Treatments were replicated five times, one chamber representing one replication, and were arranged in RCBD. Two newly emerged N. virescens female adults starved for 4 h were placed in each feeding chamber. After 24 h, the filter papers were removed, the honeydew spots traced on tracing paper and the area measured over millimetre squared graph paper. Blue spots indicating phloem feeding (basic reaction) were measured separately from the white spots (acidic reaction) from xylem feeding.
Predator efficiency on N. virescens feeding on rice varieties with different levels of resistance The effect of the integration of selected predators with host plant resistance for N. virescens was determined in the greenhouse and in the insectary. Test varieties were the same as in the previous experiments. The spider Lycosa pseudoannulata Boes. et Str. (Araneae: Lycosidae) and the mirid bug Cyrtorhinus lividipennis Reuter (Hemiptera:Miridae) were evaluated for their efficiency on N. virescens. A split-plot design was used with four replications.
Efficiency of L. pseudoannulata. Seven-day-old seedlings of the test varieties were transplanted in clay pots (diameter 12 cm) with three seedlings per pot. The potted plants were placed in a galvanized iron tray filled with water. At 14 D T the plants were cleaned and enclosed with Mylar film cages (one pot per cage). At 20 D T the plants were checked and any insects, parasites or predators were removed from the cage. Then, at 21 DT, one L. pseudoannulata and 20 N. virescens adults were simultaneously released into each cage. Observations on numbers of prey were recorded daily and missing or dead prey and predators were replaced to keep a constant 20:1 prey:predator ratio. Data were taken for 10 days.
Population growth of N. virescens. The ability of N. virescens to develop a population for two generations on rice varieties IRS, IR29, IR36, IR42 and IR56 was determined. The plants were prepared as in the adult survival and oviposition test. When the plants were at 30 DAS, they were infested by introducing into each cage four pairs (male and female) ofN. virescens adults. The first-generation progeny were counted at 24 DAI and the second-generation progeny at 60 DAI.
Efficiency of C. lividipennis predation on N. virescens eggs. The procedures for the preparation of plants were the same as that for the predation efficiency of L. pseudoannulata. At 21 DT, the plants were infested with four pairs ofN. virescens adults, 5-6 days old. At 2 DAI of N. virescens, five C. lividipennis adults were released into each cage. Five days after releasing C. lividipennis, all N. virescens and C. lividipennis were removed from the cage. Nymphs that hatched were counted daily until hatching ceased.
Honeydew excretion. To measure the feeding activity of N. virescens, seven-day-old seedlings of the test varieties were transplanted in clay pots (6 cm diameter) with two plants per pot. At 30 DAS, a feeding chamber (Paguia, Pathak and Heinrichs, 1980) was attached to the middle portion of the leaves of one plant per pot. Filter papers were treated with a pH indicator, bromocresol green dye (2 mg/ml ethanol) (Pathak and Hein-
Efficiency of C. lividipennis predation on N. virescens nymphs. The procedure for plant preparation was the same as that described for predation efficiency of L. pseudoannulata. At 21 DT, 25 first-instar N. virescens nymphs and five newly emerged C. lividipennis adults were simultaneously released into each cage. Seven days later, the N. virescens in the cages were counted.
M . M . MYINT, H . R. RAPUSAS AND E. A. HEINRICHS
Results
Determination of the levels of varietal resistance of test varieties Standard seedbox screening test (SSST).
Ratings of the seedlings for plant damage started at 5 DAI when TN1 had an average rating of 6" 9, IR29, IR56 and IR36 had resistant (R) ratings of 1.7-3.1 and the other varieties had moderately resistant (MR) ratings of 4-6 (Figure 1). At 7 DAI there was an increase in the ratings of all the varieties and the increase in the severity of damage progressed with time. At 9 DAI, the susceptible (S) varieties could be identified as IR46, IR22, Utri Rajapan and TN1, with ratings of 6 . 9 8" 9; IR8, IR42 and IR36 showed MR reactions, while IR29 and IR56 had R reactions. At 13 DAI, however, IR8 and IR42 also showed S reactions while IR36 remained MR and IR56 and IR29 were still R.
rating of 8. At this time IR22 and IR46 had a rating of 6.4 and Utri Rajapan 6.1 showing MR reactions (Figure 1); the other varieties had ratings of 0-0-2.6. At 26 and 28 DAI there was an increase in the ratings of the varieties although they remained in the same category of S, MR and R. At 30-32 DAI, IR8 and IR24 were MR; IR36, IR29 and IR56 remained as R, and IR22, IR46, Utri Rajapan, and TN1 were S. N. virescens adult survival, oviposition and egg hatchability. There were no significant differences in the survival of the females among the different varieties at 1, 3, and 4 DAI (Figure 2). At 2 DAI, survival was significantly lower on IR29 and IR56 than on the other varieties and at 5 DAI, survival on these two varieties (52%) differed from that of the other varieties except for IR36 (76%). All insects survived on TNI and Utri Rajapan until 5 DAI.
Rating the plants for damage started at 24 DAI when TN1 had a
~
..,,.. ,,-.'" S S ' ~ TN1
........
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Modified seedbox screening test (MSST). 9
261
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Days after infestation FIGURE 1. Damage ratings in the standard (SSST) and modified (MSST) seedbox screening of rice varieties against IV,. virescens. Average of seven replications.
0
I 1
I 2
I 3
I 4
I 5
Days after infestation FIGURE2. Survival of newly emerged N. virescens adult females (upper) and males (lower) on rice varieties with different levels of resistance. Average of five replications.
Integrated management of Nephotettix virescens on rice
262
Survival of males was generally lower than that of females and the decrease in survival over the 5-day period was more evident on the R varieties, IR29 (20%) and IR56 (28%) (Figure 2). Survival on IR36 was intermediate and was significantly more than that on IR29 and IR56 but significantly less than that on IR42 and IR8. The results indicated that survival of both sexes was least on IR29 and IR56, followed by IR36, IR8 and IR42. Survival was highest on TN1, Utri Rajapan, IR22 and IR46. The numer of eggs laid by N. virescens over a 5-day period on the different varieties is shown in Table 1. Significantly less eggs were laid on IR29, IR56, and IR36 (41-84) than on the other varieties. Eggs laid on IR42 and IR8 were significantly more than those on IR29, IR36 and IR56 but significantly less than those on IR46, IR22, Utri Rajapan and TN1. The number of eggs laid on TN1 was about seven- to eightfold that on the R IR29 and IR56, fourfold that on IR36 and double that on IR42 and IR8. Although the number of eggs laid differed significantly among the varieties, egg hatchability was high (95-98%) in all varieties (Table 1).
30o r~ 25o .~ 200 ~ 15o o ~, 100
E z=
50
IR29
IR56
IR36
IR42
IR8
Varieties FIGURE 3. F 1 and F 2 N. virescens populations at 24 and 60 days after rice varieties were infested w i t h four pairs (males and females) o f adults. Average of four replications. W i t h i n each generation, bars with the same letter are not significantly different at the 5% level by D u n c a n ' s multiple range test. Analysis based on values transformed to
-[~ Xylem (acidic)] Phloem (basie)~
700" ~E 600 ~ n 500 = o o ~ 400 ~o_= = ~ o 300 ,, '~ < ~ 200 100
Population growth of N. virescens. The first- and second-generation progeny (F1 and F2) differed significantly among the varieties (Figure 3). IR42 and IR8 supported the highest population of the insect, followed by IR36, IR56 and IR29 in descending order. At 60 DAI, however, when the second-generation progeny (F2) were counted, no insect had survived on IR29 and only six on IR56. An increase in the number of insects from the F1 to F2 was observed on IR36 (from 50 to 70). On IR42 the population was doubled and on IR8 the second generation was three times greater than the first generation.
Honeydew excretion. Although numerical differences were observed in the total area of honeydew spots on the filter papers, the differences were not statistically significant except on IR8 where it was significantly less than the rest of the varieties (Figure 4). Honeydew TABLE 1. N. virescens oviposition and egg hatchability on rice varieties w i t h different levels of resistance. * I R R I , 1984
Varietyt IR29 IR56 IR36 IR42 IR8 IR46 IR22 U t r i Rajapan TN1
N u m b e r of eggs~ 41 46 84 166 177 258 322 313 338
c c c b b a a a a
Percentage o f eggs laid as based on TN1 12 17 25 49 52 76 95 92 100
Egg hatchability (%) 99 98 95 95 97 98 97 95 96
a a a a a a a a a
*In a column, means followed by a common letter are not significantlydifferent at the 5% level by Duncan's multiple range test. The analysis is based on values transformed to ,f'~g-0~. ?Thirty-day-old plants. :l:Means of five replications with five pairs of adults per replication for 5 days. Insects were previously reared on TN1.
IR29 IR56
IR36 IR42 IR8
IR46 IR22 Utri TN1 Rajapan Rice varieties
FIGURE 4. Area of honeydew excretion spots by two N. virescens females on rice varieties with different levels o f N . virescens resistance. Average o f five replications.
from phloem feeding caused a basic reaction with the bromocresol green solution, indicated by the blue colour of the honeydew spot. As judged by the colour reaction of the honeydew on the bromocresol greentreated filter paper, more phloem feeding was observed on IR46, IR22, Utri Rajapan and TN1. Phloem feeding on the S varieties was more than that on the R and M R varieties. There was relatively very little phloem feeding on IR29 and IR56 ( 1 - 3 m m 2) and on IR36 (10ram2). Xylem feeding was highest on R IR56 and least on IR8.
Predator efficiency on N. virescens feeding on rice varieties with different levels of resistance Efficiency
of L. pseudoannulata. The predation efficiency ofL. pseudoannulata on N. virescens feeding on rice varieties with different levels of resistance, as indicated by the differences in mortality of the prey, is shown in Table 2. N. virescens caged on R varieties (IR29, IR56) had significantly higher mortality than on the other varieties. On M R varieties (IR36, IR42 and IR8), mortality of N. virescens without the predator was intermediate between that on the R varieties and S varieties (IR46, IR22, Utri Rajapan and TN1). Comparing the mortality of N. virescens on varieties with and without L. pseudoannulata, significant differences were observed in all varieties tested
M. M.
MYINT,
H. R. RAPUSASANDE. A. HEINRICHS
(Table 2) indicating that L. pseudoannulata predation contributed to the higher mortality ofN. virescens. The increase over the no-predator treatment was about 10-15%.
Efficiency of C. lividipennis predation on N. virescens eggs. Predation of N. virescens eggs by C. lividipennis was based on the number of nymphs hatching from the eggs laid on the different varieties. Because of the different levels of resistance, the number of eggs laid varied among the varieties tested, as indicated by the number of nymphs observed without the predator (Table 3). Significantly fewer nymphs were counted on the R varieties IR29 and IR56 (31-38) and the highest number of nymphs was observed on S varieties (247 on TN1); the number of nymphs on the MR varieties was intermediate (79-170). When caged with the predator, however, fewer N. virescens nymphs were observed than were seen without the predator on all the varieties, because of egg predation (Table 3). The predators decreased the N. virescens population by more than 50% on most varieties. The rate of predation for the period of 5 days was significantly different among varieties with R, MR, TABLE 2. Nephotettix virescens mortality due to the combined effect of predation by Lycosa pseudoannulata and varietal resistance. I R R I , 1984 M o r t a l i t y (%) for a period o f 10 days*
Varietyt IR29 IR56 IR36 IR42 IR8 IR46 IR22 U t r i Rajapan TN1
With
Without
L. pseudoannulata
L. pseudoannulata
Difference
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12-5"* 10.7"* 17.6"* 14.1"* 10.6 ** 11.9"* 14-9"* 12-7"* 11-9"*
5"As Table 1. #Average of four replications. N. virescenswere 5-6-day-old adults. In a column, means followed by a common letter are not significantlydifferent at the 5°70 level by Duncan's multiple range test. Analysis is based on values transformed to arcsin ~ . I n s e c t s were previously reared on TN1 plants. * *Significant at the 1% level.
263
and S reactions to N. virescens (Table 3). The predation rate was related to the number of prey: it was highest on the S varieties IR22, Utri Rajapan and TN1, where the N. virescens population was the highest, and lowest on the R varieties IR29 and IR56, where the N. virescens population was the lowest. C. lividipennis predation on N. virescens nymphs. Mortality of N. virescens nymphs on the varieties without the predator was significantly higher (64-66%) on the R IR29 and IR56 than on the MR and S varieties (Table 4), and mortality on the MR was higher than on the S varieties. With the predator, no significant difference was observed among the MR and S varieties but mortality was significantly higher on the R varieties. The lowest mortality of 32% was obtained on S IR46, with the highest on the R varieties IR29 and IR56 (92 and 87%, respectively). The differences in mortality between treatments with and without the predator among the varieties were not significant.
Discussion The SSST is a rapid means of screening large volumes of material for qualitative resistance. This method is used in the greenhouse for screening for N. virescens resistance throughout Asia. At IRRI, more than 60 000 entries are evaluated each year for N. virescens using the SSST (Heinrichs, Medrano and Rapusas, 1985). With this method, however, MR varieties are often not detected and are rated as S. Hence, the MSST was used to identify varieties with moderate levels of resistance which may be due to tolerance or due to the low levels of antibiosis or non-preference. Results showed that the reactions of the R and S varieties were similar in both tests but that the MR varieties had higher ratings in the SSST than in the MSST. The resistance levels of IR8, IR36, and IR42 increased with plant age as indicated by the MR-S reactions at the seedling stage in the SSST but R - M R reactions of older plants in the MSST.
TABLE 3. Population o f N . virescens as affected by varietal resistance and predation by C. lividipennis on N. virescens eggs on rice varieties w i t h different levels o f N . virescens resistance. I R R I , 1984 N u m b e r of N. virescens*
VarietyS IR29 IR56 IR36 IR42 IR8 !R46
IR22 U t r i Rajapan TN1
W i t h predator 6 10 18 50 64 109 91 68 116
d cd c b b a ab b a
W i t h o u t predator
Varietal means o f with and without predator
31 g 38 g 79 ef 135 cd 170 bc 213ab 220 ab 188 b 247 a
19 f 24 f 49 e 93 d 117 cd 161 ab 156ab 128 bc 182 a
Predation rate per predator/day S 0"99 1.12 2.43 3-42 4.24 4.14 5.12 4.83 5.26
e e d c b b a ab a
TAs Table 1. =l:Meansof four replications with four pairs ofN. virescensand five of C. lividipennis. In a column, means followed by a common letter are not significantlydifferent at the 5% level by Duncau's multiple range test. Analysis is based on values transformed to "f'~. gPredation rate/day was computed by: Nymphs in control (without predator) - nymphs in treatment (with predator) (5 predators x 5 days)
264
Integrated management of Nephotettix virescens on rice
TABLE 4. Effect of levels of varietal resistance and C. lividipennis predation on N. virescens nymphs. IRRI, 1984
Variety'~ IR29 IR56 IR36 IR42 IR8 IR46 IR22 Utri Rajapan TN1
Variety means Mortality (%) ofN. vlrescens nymphs:~ o f % mortality with and With Without without predator predator Difference~ predator 92 87 48 45 56 32 40 43 41
a a b b b b b b b
66 64 28 27 34 11 7 8 5
a a b b b c c c c
26 a 23 a 20a 18a 22a 21 a 33 a 35 a 36 a
79 76 38 36 45 22 24 26 23
a a b b b c c c c
tAs Table 1. SMeans of four replications with 25 first-instar N. virescensnymphs and five C. lividipennis for 7 days. In a column, means followed by a common letter are not significantly different at the 5% level by Duncan's multiple range test. Analysis is based on values transformed to V~- ~The difference in mortality (with-withom predator) was significant at the 1% level for all varieties.
Egg production by~N. virescens on the different varieties showed significant differences among R, M R and S varieties: more eggs were laid on the S and less on the R varieties. The lower number of eggs laid on the R varieties might be the result of the lower female adult survival on these varieties or because R varieties are not preferred by the insect for oviposition. As previously reported by Heinrichs and Rapusas (1983), levels of resistance of the varieties did not affect hatching, although the fecundity of the insect was affected. Karim (1978), however, reported that egg hatchability of N. virescens was significantly less on R than on S varieties when comparing TN1 and IR36. The number of N . virescens progeny produced differed significantly among the varieties in the F1 and F2 generations: IR42 and IR8 supported the highest population and the lowest was on IR29. The low population growth on the R varieties suggested an antibiotic effect. Because the varieties tested had different levels of resistance, the levels of antibiosis may differ. N. virescens fed on all varieties but more phloem feeding was observed on S varieties, as indicated by the basic reaction of the honeydew spot. On R varieties, however, the insect fed mainly on the xylem sap, which was acidic. Auclair, Baldos and Heinrichs (1982) investigated the biochemical evidence for the feeding sites and reported that N. virescens fed predominantly on phloem sap on S varieties and on xylem sap on R varieties. Because of the low level of nutrients in xylem sap, N. virescens reproduction and development was low on IR29 and IR56 in spite of the large amount o f xylem sap ingested. As judged by all the tests used to evaluate the levels of resistance of the varieties tested in this study, IR29 and IR56 are R; IR8, IR36, and IR42 are M R , and IR22, IR46 and Utri Rajapan are S to N. virescens. Predation was not significantly higher on R varieties than on S varieties, as was reported for L. pseudoannulata on the brown planthopper Nilaparvata lugens (St~l) (Kartohardjono and Heinrichs, 1984). They attributed the fiigher predation to the greater
movement of N. lugens on R varieties in search of suitable feeding sites. N. virescens is more active than N. lugens on both R and S varieties, and also feeds on R varieties, even though it is in the xylem; N. lugens does not feed on R varieties. When N. virescens were caged with either L. pseudoannulata or C. lividipennis, mortality was highest on the R varieties; this was due to the combined effect of the high levels of mortality caused by varietal resistance and mortality caused by predation. Thus, even a M R variety such as IR36 had populations only one-sixth that of S TN1 when caged with C. lividi-
pennis. Predation by C. lividipennis was highest on the S varieties where the N. virescens population was the highest. This suggests an increase in predation rate with an increase in host-egg density. Predation by C. lividipennis ranged from one to five eggs per day whereas Chiu (1979)reported that one C. lividipennis adult feeds on 1-2 eggs at one feeding and IRRI (1978) reported that C. lividipennis can prey on four eggs per day. Predation by C. lividipennis was reported to be one N. virescens nymph per day (Dyck and Orlido, 1977) and 0 . 4 S. furcifera per day (Salim, 1985). These tests indicated the high degree ofN. virescens control when varieties with high levels of N. virescens resistance were confined with either the spider, L. pseudoannmulata or the mirid bug, C. lividipennis. The effectiveness of L. pseudoannulata under field con. ditions, however, may be less than in these tests where the prey was confined to cages and the predator had a limited choice of food. L. pseudoannulata has been reported to feed on C. lividipennis and on members of its own species, thus limiting its value (Salim and Heinrichs, 1985). Swezey (1936) reported that C. lividipennis feeds on both parasitized and nonparasitized hopper eggs, thereby reducing the effectiveness of egg parasites under natural conditions. Field tests are currently being conducted to determine the populations ofN. virescens and its predators on the varieties used in the greenhouse study. References AUCLAIR, J. L., BALDOS, E. AND HEINRICHS, E. A. (1982). Biochemical evidence for t h e feeding sites o f t h e leathopper, Nephotettix virescens w i t h i n susceptible a n d resistant rice plants. Insect Science and its Application 3, 2 9 - 3 4 . CHIU, S. C. (1979). Biological control o f t h e b r o w n p l a n t h o p p e r . In: Brown Planthopper: Threat to Rice Production in Asia, pp. 3 3 5 - 3 5 6 . L o s Banos, L a g u n a , Philippines: I R R I . DYCK, V. A. AND ORLIDO, G. C. (1977). C o n t r o l o f the b r o w n p l a n t h o p p e r (Nilaparvata lugens) b y natural e n e m i e s a n d timely application o f n a r r o w s p e c t r u m insecticides. In: The Rice Brown Planthopper, pp. 5 5 - 7 2 . Taipei: F F T C (ASPAC). HEINRICHS, E. A. AND RAPUSAS, H . R. (1983). Correlation o f resistance to t h e g r e e n leafhopper, Nephotettix virescens ( H o m o p t e r a : Cicadellidae) w i t h t u n g r o virus infection in rice varieties h a v i n g different genes for resistance. Environmental Entomology 12, 2 0 1 - 2 0 5 . HEINRICHS, E. A., MEDRANO, F. G. AND RAPUSAS, H. R. (1985). Genetic Evaluation for Insect Resistance in Rice. L o s Bafios, L a g u n a , Philippines: I R R I .
M. M. MYINT, H. R. RAPUSASAND E. A. HEINRICHS HEINRICHS, E. A., MEDRANO, F. G., RAPUSAS,H. R., VEGA, C., MEDINA, E., ROMENA, A., VIAJANTE, V., SUNIO, L., DOMINGO,I. AND CAMANAG,E. (1985). Resistance of varieties IR5 to IR62 to rice insect species. International Rice Research Newsletter 10 (6), 12-13. IRRI (1978). International Rice Research Institute Annual Report for 1977. Los Bafios, Laguna, Philippines: IRRI. IRRI (1980). Standard Evaluation System for Rice, Second edn. Los Bafios, Laguna, Philippines: IRRI. JOHN, V. T., HEU, M. H., FREEMAN,W. H. AND MANANDHAR, D. (1979). A note on dwarf disease of rice in Nepal. Plant Disease Reporter 62 (9), 1. KARIM, A. N. M. R. (1978). Varietal Resistance of Rice to Green Leafhopper, Nephotettix virescens (Distant): Sources, Mechanisms and Genetics of Resistance. PhD thesis, University of the Philippines at Los Bafios. KARTOHARDJONO,A. AND HEINRICHS,E. A. (1984). Populations of the brown planthopper, Nilaparvata lugens (Still) (Homoptera: Delphacidae)and its predators on rice varieties with different levels of resistance. Environmental Entomology 13, 359-365. PAGUIA, P., PATHAK, M. D. AND HEINRICHS, E. A. (1980). Honeydew excretion measurement techniques for determining differential feeding activity of biotypes of Nilaparvata lugens
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on rice varieties. Journal of Economic Entomology 73, 35-40. PANDA,N., HEINRICHS,E. A. AND HIBINO, H. (1984). Resistance of the rice variety Utri Rajapan to ragged stunt and tungro viruses. Crop Protection 3, 491-500. PATHAK, P. K. AND HEINRICHS,E, A. (1982). Bromocresol green indicator for measuring feeding activity of Nilaparvata lugens on rice varieties. Philippine Entomologist 5, 195-198. RIVERA, C. T., AGUIERO,V. M., DIMASUAY,D. F. AND LING, K. C. (1972). New vector of rice tungro and yellow dwarf. Philippine Phytopathology 8 (1/2), 10. SALIM, M. (1985). Interaction of Resistant Rice Varieties, Arthropod Predators and Insecticide on Whitebacked Planthopper, Sogatella furcifera (Horv6th) (Homoptera: Delphacidae) Populations. MSc thesis, University of the Philippines at Los Bafios. SALIM, M. AND HEINRICHS,E. A. (1985). Intra and interspecific feeding of whitebacked planthopper, Sogatella furcifera (Horvfith) (Homoptera: Delphacidae) predators. International Rice Research Newsletter (in press). SWEZEY, O. H. (1936). Biological control of the sugarcane leafhopper in Hawaii. Hawaii Plant Research 49, 57-101.
Accepted 18 November 1985