Papers Mass rearing, larval behaviour and effects of plant age on the rice caseworm, Nymphula depunctalis (Guen6e) (Lepidoptera: Pyralidae) James A. Litsinger*, Jovito P. Bandong t and Narong Chantaraprapha* Entomology Division, International Rice Research Institute, PO Box 933, 1099 Manila, Philippines The mass-rearing method for efficient production of rice caseworm Nymphula depunctalis (Guen6e) was improved. Moths oviposit most in ventilated cages on the undersides of floating cut rice leaves aged 4-8 weeks after transplanting (WT). There was no effect of leaf width on oviposition. Greatest egg production occurs with a 12 h scotophase. Moths are highly tolerant of crowding in oviposition cages, as up to 40 females averaged 86-115 eggs per female in 0.14 m 3 cages. Honey as a carbohydrate energy source increases the rate of oviposition but not total oviposition. Larvae require standing water tofill their cases as respiration occurs with tracheal gills. Larval survival was equal when reared in distilled water, pesticide-free ricefield water, or chlorinated tap water, whether aerated or not. Larvae survive better, grow larger, mature more rapidly, and become more fecund adults when reared on plants 2-6 WT. Pupae are non-aquatic. The caseworm's inability to develop satisfactorily on post-vegetative rice may be attributed to poor nutrition and less favourable oviposition substrates. Females prefer to oviposit on drooping leaves floating on the water surface. As the crop matures, fewer leaves touch or float on the water surface. Eggs desiccate if laid on aerial portions of plants and are more exposed to natural enemies. Keywords: rice caseworm; mass rearing; plant age The rice caseworm, Nymphula depunctalis ( G u e n r e ) , is a pyralid defoliator of wetland rice. T h e semi-aquatic larvae are found only in flooded ricefields, p r e d o m i n antly in the vegetative growth stage. All but the first instar respire by branched filamentous tracheal gills projecting in pairs from the lateral margins of each body segment from the m e s o t h o r a x to the eighth abdominal segment (Viraktamath, Puttarudriah and C h a n n a - B a s a v a n n a , 1974) (Figure 1). By capillary action, water fills a tubular case m a d e from a leaf, permitting the larvae to respire. T h e larvae, protruding from their cases, use their legs to climb up plants to feed on foliage, making them semi-aquatic in contrast to true aquatic species, which feed and develop on s u b m e r g e d plants. A new tubular case is m a d e with each moult. The larvae are active at night and remain floating on the water surface during the day (Figure 2). Prevailing winds drive the floating caseworm larvae to the leeward side of the field, concentrating the resulting d a m a g e in patches (Reissig et al., 1986). T h e d a m a g e d area of a field appears as patches of whitish foliage. All that remains after the leaves are scraped is the p a p e r y epidermis (Heinrichs and Viajante, *To whom correspondence should be addressed, at IRRI; present address: 1365 Jacobs Place, Dixon, CA 95620, USA; tCiba-Geigy Foundation, La Fuente, Sta Rosa, Nueva Ecija, Philippines; *present address: Entomology and Zoology Division, Department of Agriculture, Bangkhen, Bangkok 10900, Thailand 0261-2194/94/07/0494--09 ~) 1994 Butterworth-Heinemann Ltd 494 Crop Protection 1994 Volume 13 Number 7 1987). T o p p i n g of the leaf blades by larvae to construct cases also adds to defoliation. Larval cases range in length from 5 to 23 m m (Sen and C h a k r a v o r t y , 1970). Severe infestation results in stunted plants and eventual plant death, creating bare spots in the field. Although larval feeding is reported to be restricted to vegetative rice ( R a m a s u b b a i a h , Sanjeeva R a o and Khalid A h m e d , 1978), no explanation has been offered. Studies on the life history of the rice caseworm have Figure 1. Fifth-instar caseworm larva protruding from its case with tracheal gills exposed Rice caseworm: J.A. Litsinger et aL Emerging adults were placed in oviposition cages. Oviposition occurred on floating leaves cut at both ends, utilizing the middle 8 cm. Cut leaf sections laden with eggs were placed in Petri dishes half-filled with water until hatching, whereupon they were placed in larval cages to resume the cycle. Moths were introduced as newly emerged pairs in copula obtained from adult emergence cages. All experiments used a randomized complete block design. Except where noted, data were analysed by analysis of variance. Unless otherwise stated, the level of significance was at the 95% confidence level. If the treatment effect was significant, means were separated by Tukey's test (Steel and Torrie, 1980). Means are expressed as (+) standard errors. Figure 2. Caseworm larva floating within its case at the base of a rice plant been limited, owing to lack of efficient techniques to maintain greenhouse colonies that continuously produce a large number of healthy individuals. Petri dishes have been used as oviposition cages by Sison (1938) and Viraktamath et al. (1974). Bunches of leaves with the cut ends wrapped in a wad of moist cotton were provided as oviposition substrate. The latter study provided diluted sugar in cotton wads as sustenance for the moths. Limited numbers of larvae have been reared in Petri dishes (Viraktamath et al., 1974) or in open glass cylinders over potted plants (Sison, 1938; Pillai and Nair, 1979). In the study reported here, we improved rearing methods and observed larval behaviour in feeding and case construction. We studied oviposition behaviour and the effects of plant age on life history parameters to explain why the caseworm is more prevalent during the vegetative stage of the rice plant. Materials and m e t h o d s General Research was conducted at the Experimental Farm of the International Rice Research Institute, Los Bafios, Laguna, Philippines. A greenhouse colony was established following the method of Heinrichs, Medrano and Rapusas (1985), using oviposition, larval, and adult emergence cages. IR36 rice, susceptible to the caseworm, was used in all studies, which were conducted either in a greenhouse, a ventilated headhouse with windows at shaded outdoor air temperatures (28 + 5°C), or in the field. Larvae collected from the field were placed in larval rearing cages with standing water and allowed to feed until pupation. Potted plants with attached pupae were removed daily from the larval rearing cages and placed in adult emergence cages without standing water. Groups of three pots were placed on a bench and enclosed with a Mylar cylinder tube with a nylon mesh (1 mm 2) top. A mesh sleeve was inserted near the top of the cage to allow removal of moths in vials. The top one-third of the plants was cut to force emerging moths to rest on the inside of the cage rather than on plants. Mass rearing Oviposition cage ventilation. Four oviposition cage designs of varying ventilation were compared for egg production and moth survivorship in a headhouse. Each cage was set over a clay pot (20 cm diameter) with three 14-day-old rice seedings equally spaced apart. Cages were tubular, 25 cm diameter and 54 cm high of nylon mesh or Mylar plastic with nylon mesh windows, set in plastic basins (36 cm long x 29 cm wide X 12 cm high) filled with water to submerge the clay pot. The first cage, made of nylon mesh draped over a tubular wire frame, was fully ventilated. The second cage was plastic with a mesh top and two mesh side windows. The side vents occupied one-third of the cylinder surface. The third cage was plastic with only the top covered with mesh. The fourth cage, entirely of plastic, was unventilated. Thirty leaf sections were spread over the water surface of each cage as oviposition substrates. The relative humidity (r.h.) in the headhouse and in the cages was measured daily by a hygrometer (+1% r.h.) at midday. Temperature was measured daily inside and outside the cages with mercury thermometers allowed to equilibrate for 15 rain. Fifteen pairs of caseworm moths were introduced per cage in each treatment. The experiment was replicated five times. The number of eggs laid was recorded daily until all moths died. Moth sustenance. Two carbohydrate sources, honey and sucrose, were compared with water-saturated cotton wicks for fecundity in mesh oviposition cages. Twenty newly emerged mated pairs were introduced per cage. There was one cage per treatment, and the experiment was replicated five times. Egg production on leaf sections was recorded daily until all moths died. Moth density. The effect of moth density on egg production in a mesh oviposition cage on a withoutcarbohydrate moth sustenance system was evaluated in the headhouse with 5, 10, 20 and 40 adult pairs per cage. Each cage was a treatment and the experiment was replicated five times. Leaf sections were removed daily to record egg numbers until oviposition ceased. Photoperiod and oviposition. The effects of three light regimes on oviposition, moth longevity and egg viability were tested in the headhouse. Small tubular oviposition cages (12 cm diameter X 26 cm high) were Crop Protection 1994 Volume 13 Number 7 495 Rice caseworm: J.A. Litsinger et al. used without air vents. The light source was ceiling fluorescent lamps (40 W). The 24 h scotophase treatment had carbon paper covering the entire cage. T h e fluorescent lamps were left on during the experiment in the room and the 24 h photophase cages were exposed continuously. In the 12 h photophase and 12 h scotophase treatments the carbon paper-covered cage was placed in position at 18:00 and replaced at 06:00 with a clear Mylar plastic cage. Five pairs of moths were placed in each cage without sustenance and were offered four leaf sections (8 cm) on which to oviposit. Moth longevity was noted daily. The n u m b e r of eggs laid was recorded per cage and the percentage hatched noted. T h e r e were five replicates. Oviposition behaviour. Oviposition preference for different plant parts was determined by a free-choice test. A clay pot (25 cm diameter) with three seedlings aged 14 days after transplanting was placed in a ventilated oviposition cage. Water was added until the pot was submerged. All drooping leaves touching water were removed. In addition, five cut leaf sections were placed on the water surface per cage. T h e r e was one caged pot per treatment with two pairs of moths each. The experiment was replicated four times. The n u m b e r of eggs observed on the leaf sheaths, leaf blades, floating leaf sections, and on the cage itself, was recorded daily. The effect of the width of floating leaves was tested in the headhouse. Leaf size as a variable for oviposition was determined by offering leaf sections (8 cm) cut to 0.2, 0.4, 0.6 and 0.8 cm widths to five mated pairs in oviposition cages (25 cm × 54 cm). One cut leaf of each width was offered per cage in a free-choice test. The leaves were aged 4 weeks after transplanting (WT) and the cage, placed in a plastic basin (46 X 53 cm), was flooded to 20 cm depth. No carbohydrate sustenance was offered to the moths. Egg production was recorded after 3 days. Each treatment consisted of a single cage. The set-up was replicated four times. Larval~pupal rearing. The larval rearing cage for mass rearing in the greenhouse was constructed from a wooden frame (1 m 3) with mesh top and sides and a hinged door. It sat over six clay pots (10 cm diameter) with eight plants/pot. A series of larval rearing cages was placed on a galvanized tray on a greenhouse bench. The metal tray was flooded 12 cm deep to cover the pots, allowing free movement of larvae. About 150-200 eggs were introduced per cage, and the larvae developed to pupation without need of replacing the rice plants. Larval survivorship from first instar to pupation was compared in four water regimes, varying from saturated soil to flooded conditions (10 cm deep), with water changed daily, weekly, or not at all. Twenty-five firstinstar larvae were placed in a larval rearing cage per treatment in a randomized complete block design of five replicates. Each cage was a tubular Mylar cage, 25 cm diameter × 27 cm high, with a mesh top. Five 2week-old seedlings were transplanted in a clay pot (10 cm diameter) set in a plastic tray (36 X 29 X 12 cm). Water quality was also investigated in four treatments with different water sources. In the first treatment, larvae were reared in larval cages as described above, with tap water containing 3.0 ppm O2 and 0.6 ppm 496 Crop Protection 1994 Volume 13 Number 7 chlorine. In a second treatment, chlorinated tap water was continuously aerated with an aquarium air pump, giving 4.4 ppm 02. The third and fourth treatments were distilled water and pesticide-free ricefield water both at 3.0 ppm Oz. One hundred first-instar larvae were introduced per treatment on potted plants and survival was recorded until pupation. The experiment was replicated four times. Larval behaviour Larvae were observed in greenhouse cultures while constructing cases and feeding. Individual larvae of each instar were noted. The parameters of feeding injury from a naturally infested field were reassessed by randomly choosing 100 damaged leaves. The length and width of feeding scars were measured to the nearest millimetre. Whether the larvae fed on the ventral or dorsal leaf surface was noted. Plant age Oviposition preference. The effect of plant age on oviposition preference was determined in four experiments - three in the headhouse and one in the field. The headhouse experiments compared oviposition on plants aged 2, 4, 6, 8 and 10 WT. The youngest fully formed leaf on a plant was selected. The first experiment involved a free choice among five leaf ages. Five leaf sections per treatment were pinned through the midrib at one end of each blade into the top of a stake pushed into a clay pot submerged in a gasoline drum (36 cm x 90 cm); the section was the middle 8 cm of a leaf blade. The blades were fanned apart in the shape of a star floating on water. The drum was large enough to hold cut leaves from all five plant ages. Ten pairs of moths were placed in the drum covered by a mesh top. Egg counts were made after the moths died. The second experiment offered leaves drooping in water (done by pinning the tips below the water surface into submerged wooden stakes in pots). A pot with three plants of each age was submerged in the same gasoline drum set-up, offering a free choice to females released to oviposit until their death. Three leaves from each pot were pinned into a submerged stake such that blades floated. The third experiment offered erect leaves. Each treatment with ten moth pairs was replicated six times in all three experiments. The occurrence of suitable oviposition sites in a rice crop was compared in a fourth experiment, in a field divided into 13 plots, each plot sequentially planted weekly. Variety IR36 matured in 13 weeks and all the growth stages were present at any one time. The n u m b e r of leaves touching the water surface per hill was recorded from 20 randomly selected hills in plots aged 2, 3, 4, 6, 8 and 10 WT. Larval~pupal development. The effect of plant age on larval weight gain, survival and development was determined by rearing 25 neonate larvae per treatment in ventilated tubular cages in a greenhouse. Each cage was pushed into the soil over plants aged 2, 4, 6, 8 and 10 WT. Plants were changed twice a week to maintain a constant age. Water was also changed twice a week. After 14 days, larvae were weighed flesh _+ 0.1 mg Rice c a s e w o r m : J.A. Litsinger et al. without their cases. Larval duration until pupation was recorded for each larva, as was survival to adult emergence. The experiment was replicated six times. Fecundity. Egg production of moths reared from rice plants 2, 4, 6, 8 and 10 WT (females emerging from the previous experiment) was determined using ventilated oviposition cages in the greenhouse without carbohydrate sustenance. Floating cut leaves from plants aged 4 WT were offered as oviposition substrates. Each treatment comprised ten adult pairs per cage. Cumulative eggs laid / day (%) Jl I00 75 a, 50 ab 25 Results I Mass rearing Oviposition cage ventilation. Egg production per female in the three partially or totally ventilated cages (105-136 eggs per female) was significantly greater than in the unventilated cage (67) (Table I). During the experiment, r.h. in the room was 76 + 4%, whereas r.h. in the unventilated oviposition cage rose to 91 + 0.8% during the day. A slightly lower r.h. (87 + 0.8%) in the partially ventilated cage with top mesh did not affect egg production. Temperatures in the cages were similar, averaging 24-29°C, in the four cage designs. Ambient room temperature was 27 +_ 4°C during the experiment. The longevity of ovipositing moths (2.03.0 days) was not affected by cage design. Moth sustenance. Neither honey (112 + 13 eggs per female) nor sucrose (113 + 10 eggs per female), provided as energy sources to caged moths, influenced total egg production (compared with water - 114 + 9 eggs per female). However, the rate of oviposition per day, from 3 to 5 days after caging, was highest with honey as the carbohydrate source (Figure 3), whereas sucrose did not increase the rate of oviposition. Longevity of moths (2.2-3.0 days) and egg hatch (90-93%) were also non-significantly different between the carbohydrate treatments. Moth density. Moths are highly tolerant of crowding: there was no difference in egg production per female (ranging from 86 _+ 13 to 115 + 21 eggs) or in female longevity (ranging from 2.3 + 0.8 to 3.0 ___0.7 days) in the cylindrical cages (25 X 54 cm). Egg production (Y) increased in a linear fashion, with 114 eggs produced Table 1. Comparison of four' oviposition cage designs on caseworm egg production (,~ ± s.e.), IRRI headhouse" Cage design Eggs laid r.h." (%) (no. per female) insidecage Nylon mesh (fully ventilated Mylar plastic cylinder with 105 ± 13 a c 83 ± 0.7 a top/side mesh Mylar plastic cylinderwith top mesh only Mylar plastic cylinder (non-ventilated) 136 ± 24 a 84 ± 0.5 a 133 ± 21 a 87 ± 0.8 b 67± 8b 91 _ 0 . 8 c " A v e r a g e of five replicates, 15 pairs p e r cage; t'r.h., relative humidity; 'in a c o l u m n , m e a n s followed by a c o m m o n letter do n o t differ significantly (p > 0.05) by T u k e y ' s test (Steel a n d T o r r i e , 1980) 1 I 2 I 3 I 4 I 5 I I I 6 7 Days after caging Figure 3. Rate of oviposition from caseworm moths fed different carbohydrate sources: honey (O), sucrose (V) or water (I). Vertical bars represent s.e.m, for honey ( I ) , sucrose (I) and water ( ! ). Values for the same time point with a common letter do not differ significantly (p > 0.05) by Tukey's test Eggs laid ( n o . / c a g e ) 5000 j 4000 :3000 2000 1000 0 ~ I I I I I 0 5 I0 20 ~0 40 50 Moth density ( t o t a l no. p a i r s / c a g e ) Figure 4. Total egg production from moths placed in oviposition cages in densities from five to 40 pairs per cage: Y = 114 X; r = 0.95**. Vertical bars represent s.e.m. for each additional female (X) per cage (Figure 4). The regression model was Y = 114 X (r = 95) (p < 0.01). Oviposition occurred over a period of 7 nights. The highest rates of egg laying occurred earlier in the more crowded oviposition cages (Figure 5). At 40 females per cage, the greatest oviposition occurred two or three nights after emergence; at 20 females per cage, three C r o p Protection 1994 Volume 13 N u m b e r 7 497 Rice caseworm: J.A. Litsinger et aL Eggs / female (% per day) Table 2. Response of caseworm moths to varying light regimes (X _+ s.e.), IRRI headhouse a 50 I a Photophase (h): scotophase (h) IQ ° 12:12 0:24 24: 0 a b i Z5 ~ 163 ± 13a h 137_+ lOab 75_+ 17b Female longevity (days) 3.2_+_(I.6a 2.6 + 0.5b 4.0± 0.7a Egg hatchability (%) 76 ± I1 a 75 ± 7a 20 ± 12b " A v e r a g e of five replicates, five m a t e d pairs per cage; bin a column, m e a n s followed by a c o m m o n letter do not differ significantly (p > 0.05) by T u k e y ' s test (Steel and Torrie, 1980) I O Eggs laid (no. per (cage) i i i i 2 3 4 5 6 7 Night a f t e r adult emergence Figure 5. Daily oviposition by caged caseworm moths in densities from 5 to 40 pairs per cage: 5 (V), 10 (V), 20 (Q) or 40 cm wide. Intermediate levels occurred with 0.2 and 0.8 cm widths. Before oviposition the moth lands on the floating leaf. Facing in the direction perpendicular to the leaf axis, the female positions her ovipositor along the edge of the leaf blade, moving it under water to deposit one or two rows of eggs (Figure 6). Rarely (39 of 2483 eggs laid) were moths observed to oviposit on larval cases floating on the water. ((3) pairs. Vertical bars as in Figure 3 Larval~pupal rearing. nights after emergence; and at five to ten moths per cage, four nights after emergence. Oviposition rapidly declined, five to seven nights after emergence at all moth densities. Photoperiod and oviposition. The greatest egg production per cage (163 eggs) occurred in the 12 h photophase and 12 h scotophase treatments (Table 2). The least oviposition (75 + 17 eggs) took place in continous light, and only 20 ___ 12% of those eggs were viable. This result indicates that a dark cycle favours mating. Moths laid an intermediate n u m b e r (137) of eggs under continuous dark, but the eggs were as viable (75%) as those laid in the 12 h photophase and the 12 h scotophase treatments (76%). Females died earliest in continuous dark (2.6 days) and lived longest in continuous light (4.0 days). Continuous light apparently inhibited mating as well as oviposition. Females evidently die once their complement of eggs has been laid, but they live longer if oviposition has been delayed or inhibited. Oviposition behaviour. Caseworm moths prefer to oviposit on the undersides of leaves floating on water, rather than on erect leaves. In the free-choice experiment, most (p ~< 0.01, 92 + 7%) of the eggs were laid beneath floating leaf sections and only 2 + 2% were laid on erect leaves (leaf sheath, leaf blade). The remaining 6 +__4% were laid on the cage just below the water line. All eggs laid on the erect plants dried up and failed to hatch. O f those eggs laid in contact with water (n = 761), 92 + 4% hatched. Egg production was similar among moths offered leaves differing in width from 0.2 to 0.8 cm. Production per five females varied from 95 + 23 to 264 + 43 eggs per treatment, but this was not related to leaf width (r e = - 0 . 6 7 ) . Highest egg production was on the leaves 0.6 cm in diameter and the lowest was on the leaves 0.4 498 C r o p Protection 1994 Volume 13 Number 7 The quality of the water was not important to the developing larvae, as there was no difference in survival when reared in chlorinated tap water (95 + 7%), aerated tap water (88 +- 8%), distilled water (84 + 10%), or pesticide-free ricefield water (86 + 6%). Ponding is important, as only 13 _+ 6% of larvae survived when reared on plants under nonflooded conditions on saturated soil. Some larvae were able to fill their cases by capillary action from the limited free water on the soil surface. T h e r e was no difference (p > 0.05) in survival between larvae reared under flooded conditions, whether the ricefield water was changed daily (95 ___ 3%), weekly (93 + 4%), or not at all during the developmental period (94 _+ 3%). Pupae are not aquatic and the pupal case is attached to the rice plant by silk above the floodwater line. The last-instar larva constructs the pupal case from a rice leaf. Both ends of the pupal case are secured with silk as if pinched shut. The pupal cases have an inner lining of silk. A slit is made at the head end of the case to allow the moth to emerge. Figure 6. Caseworm female on a cut leaf turned over to show the rows of eggs laid underwater along the leaf edge Rice caseworm: J.A. Litsinger et al. Larval behaviour Case making. Larvae complete five stadia. In making a new case, larval instars 2 to 5 ascend a plant and, while facing upward, sever the tip of a leaf at right angles. T h e cut leaftip falls into the water. The larva then attaches silk strands to the edges of the cut-topped leaf with silk. As the silk dries, the case slowly forms a tube encircling the larva. Before the case is fully rolled, the larva turns a r o u n d and, while facing downward, cuts the leaf off the plant at right angles. The newly created case then falls into the water with the larva inside. Surface tension retains a reservoir of water around the larva as the case is open at both ends. Neonate larvae, and sometimes the second-instar larvae, make a case from the leaftip without first topping the blade. T h e leaf tip is severed from the plant with the larva inside. After falling into the water, the larva turns around inside the case and feeds. Formation of the case is helped by the fact that a severed rice leaf naturally rolls into a tube from the ventral leaf surface. In contrast to rice leaf-folders, older caseworm larvae do not feed on the case itself. Feeding. Larvae crawl up plants to feed at night. While still in their cases, the larvae secure themselves to a leaf with their legs. Larvae methodically scrape away the green leaf tissue, moving the head from side to side while descending a blade. Larvae chose (n = 100) to feed from the dorsal leaf surface (87.5 + 16.4% of occasions) rather than from the ventral surface. The width of the feeding scar ranged from 1 to 2 mm, averaging 1.7 + 0.2 mm. T h e length of the feeding scars ranged from 7 to 141 mm, averaging 52.6 + 13.4 mm. / ,oa.nout,eoves I ! Drooping leaves / / Ta I0.M io ~b zoo~- 0 II I 2 i\\ I 4 I 6 I 8 10 P l a n t age ( w k a f t e r transplanting ) Figure 7. Results of three experiments comparing rice caseworm egg production on transplanted rice of five ages (2-10 weeks after transplanting) and each offering different ovipositional substrates. The experiments offered floating leaf sections, drooping leaves floating on the water surface, or erect leaves. Within each experiment, the means of various plant ages followed by a common letter do not differ significantly (p > 0.05) by Tukey's test Plant age Oviposition preference. In the three free-choice experiments comparing oviposition among different leaf positions and ages, most eggs per ten females (378 + 42) were laid on floating cut leaves aged 4 W T (Figure 7). A m o n g floating cut leaves, fewest eggs were laid on either younger (2 W T 123 _+ 31 eggs) or much older (10 W T 109 +_ 23 eggs) leaves. Floating leaf sections 6 and 8 W T also received large numbers of eggs (201 + 14 and 278 + 27, respectively). In the second free-choice experiment, where the leaves were drooping onto the water surface, there were relatively fewer eggs laid per ten females (90-189) but no preference in plant age could be detected (Figure 7). Numerically more eggs were laid on 4 W T leaves (189 + 23). In the third free-choice experiment on plants having only erect leaves, low egg production occurred (31-157 eggs per ten females) (Figure 7). Leaves from plants aged 2-4 W T (85 + 9 and 157 _+ 17 eggs per ten females) were preferred (p ~< 0.05). The lowest egg production on erect leaves occurred on leaves aged 6--10 WT. In the fourth experiment, the number of drooping rice leaves (Y) recorded touching or floating on the water surface in a ricefield declined in an exponential fashion from 2 to 10 W T (Figure 8). The model was Y = 741 X -2"9 (r e -----0.96) (p > 0.01) where X is plant age. The n u m b e r of drooping leaves that touch the water D r o o )ing leaves ( n o . / 2 0 8O hills ) 60 40 20 0 --,.~-I 2 4 6 8 10 Age of plants ( w k after transplanting ) Figure 8. Rate of decline of drooping leaves as favourable ovipositional sites with crop maturity; Y = 741X-Zg; r2 = 0.96** Crop Protection 1994 Volume 13 Number 7 499 Rice caseworm: J.A. Litsinger et aL surface (thus making ideal oviposition sites) rapidly decreases during the stem elongation phase when the leaves are lifted out of the water as the plant grows. Less than 10% of drooping leaves are present at 6 WT and virtually none at 8 WT. Larval~pupal development. Nutritionally, leaves 4 and 6 WT resulted in the largest larval weight gain as each 14-day-old larva averaged 309 + 8 to 310 + 7 mg (Figure 9a). Larvae reared on 2 WT leaves weighed 249 + 6 mg, whereas those on 8 WT and 10 WT plants weighed 193 _+ 5 mg and 174 + 4 mg, respectively. The larval developmental period was significantly affected by plant age. The most rapid development occurred on 4 WT plants (15.4 + 3.8 days) followed by 6 WT (17.2 + 4.2 days) and 2 WT (18.7 + 2.4 days) Fecundity. Egg production also was significantly affected by the age of the plants on which caseworms were reared (Figure 9d). More fecund females resulted if reared on plants 2-6 WT (108 ___ 13 to 111 + 22 eggs per female) than on plants 8-10 WT (67 + 16 to 74 + 11 eggs per female). Larval developmental '.2 Larval fresh weight ( mg ) ;SZ~O _ a plants (Figure 9b). The slowest development occurred on plants 8 WT (21.3 + 12.0 days) and 10 WT (20.8 + 1.7 days). Caseworms reared on 2--6 WT plants had the highest survival (63 + 7 to 69 ___ 6%) from neonate larvae to adult emergence, which was significantly (p ~ 0.01) greater than on plants 8-10 WT (36 ___ 8 to 37 + 9%) (Figure 9c). There was a distinct decline in the nutritional value of the rice plant between 6 and 8 WT. period (days) b a~ 310 O ~.0 -- 290 270 ab 18 250 230 16 I_ L 210 190 i 14,- 170 I 150 I I 12 S u r v i v a l larva to adult (%) 80 I l~gogS l a i d ( n o . / ~ ) I ( I 1 10 C 100-- 60 ~ 40 i b 8060 20 I 2 I 4 I 6 I 8 I lO Plant ~ b I I I 2 4 6 age (wk after transplanting ~ b I b 8 10 ) Figure 9. Effect of rice plant age on larval nutrition as measured by weight gain (a), developmental period (b), and survivorship to adulthood (c). Egg production of emerging caseworms reared on five ages of rice (d). Within each experiment, values followed by a common letter do not differ significantly (p > 0.05) by Tukey's test ,500 Crop Protection 1994 Volume 13 Number 7 Rice caseworm: J.A. Litsinger et al. Discussion Mass rearing Rice caseworm is readily mass-reared under tropical conditions. As no rice variety has been found to be resistant to rice caseworm (Heinrichs et al., 1985), the choice of cultivar is not important. Continuous cultures have been kept for several years without any seasonal effects that could occur as a result of dormancy, sensitivity to changes in temperature, or epizootics of pathogens as a result of crowding. Moths readily mate in the small oviposition cages (0.14 m 3) and apparently are highly tolerant of crowding. The oviposition cage should be ventilated and exposed to a photoperiod that includes a dark cycle. Honey as a carbohydrate sustenance for moths hastens oviposition of females up to 5 days after emergence but does not affect total egg production. Honey, therefore, would be useful if oviposition cages were maintained only for 3-5 days rather than for 7 days or until the adults die. The rice caseworm is less sensitive to rearing conditions than the rice leaf-folder, Cnaphalocrocis medinalis (Guen6e), for which a similar mass-rearing method was also developed (Waldbauer and Marciano, 1979). The leaf-folder moth requires high humidity and carbohydrate sustenance for optimal development. Caseworm eggs desiccate if not laid under water; moth therefore prefer to oviposit on floating leaves, either those drooping on the water surface or, more readily, on floating leaf sections. Viraktamath et al. (1974) provided ovipositing moths with cut leaves in order to fit them in the small oviposition cage. They did not report whether caseworm moths oviposited on the undersides of those floating leaves. In this study, when floating leaves were offered, virtually all the eggs were laid under water. Greater egg production occurs on leaves in the vegetative stage. Starting from 40 pairs of moths, >4000 eggs can be harvested in 4 days, a period when >90% of eggs will have been laid; 3% of the eggs are then recycled back to maintain the culture. Larvae require ponding so that water is available to fill their cases. Any normal source of water that is pesticide free can be used. Optimal larval development occurs on rice in the vegetative stage, and pupae should not be submerged. The minimal requirements for optimal rearing conditions were not followed in earlier studies. The low mean fecundity of 52 eggs per female reported by Sison (1938) was probably due to the non-ventilated Petri dishes used as oviposition cages. The fecundity of caseworms from India reported by Viraktamath et al. (1974) (161 eggs per female) and Piilai and Nair (1979) (155 eggs per female) is higher than that in our Philippine study. The difference may be genetic, because the fecundity reported from our study occurred under optimal rearing conditions. Pillai and Nair (1979) provided each moth pair with potted plants of a preferred age (5 WT) but offered only erect plants as oviposition sites. Viraktamath et al. (1974), on the other hand, provided cut leaves of unspecified age in closed Petri dish cages (10 × 5 cm) without water. Because caseworms disperse very little, local populations may exist over its broad range. Perhaps, with better rearing methods, the populations recorded in India may attain even higher fecundity levels. A caseworm moth lays >90% of its eggs in a single egg mass (Viraktamath et al., 1974). This may explain why crowding has little effect on fecundity, as moths, in an oviposition cage, lay eggs daily over a period of 7 days, with ample time for each to oviposit in turn. If space for mass rearing is limited, then >40 pairs of moths per cage may be tried as the egg production curve (Figure 4) may not have reached a peak. The single ovipositional period may also explain why egg production is not increased with a carbohydrate source, as there is usually only one ovipositional flight. Caged moths can live for 7 days without sustenance. Plant age Caseworm biology was examined to understand why this pest is most prevalent at the vegetative crop stage, a fact which diminishes its potential to damage a rice crop. Evidence showed that the caseworm is better adapted to the vegetative stage. Higher rates of oviposition, more rapid larval development, higher survivorship, and larger larvae and more fecund caseworms, resulted when they were bred on vegetative stage rice. The differences in fitness and nutrition cannot fully explain why caseworm is not encountered on reproductive stage rice, inasmuch as it still survives on older rice. The habit of ovipositing under water, however, presents a further argument, as suitable oviposition substrates decline with crop age. Drooping leaves are lifted out of the water during the stem elongation phase and very few are floating on the water surface 6 WT. Moths may not also fly beneath a closed canopy to locate them. Larvae, in topping leaf blades during case construction, create ideal oviposition sites for succeeding generations as the leaf tops fall onto the water surface. However, in an older crop with a closed canopy, many of these leaf blades become lodged in the plant vegetation and may not reach the water surface. Moths, in the absence of floating leaves, could oviposit on rice tillers just below the water surface. Water fluctuates through seepage and irrigation and a drop in the water level would expose eggs. Caseworm will oviposit on erect leaves out of the water if there is no other choice, but those eggs soon dry up. The behaviour of ovipositing under water protects eggs from parasites and many predators. The base of tillers would not be a favourable oviposition site as the eggs could be exposed to natural enemies when the water level falls. Studies on other lepidopterous defoliators that attack vegetative rice show that there is a high rate of survival in the early crop period, because of escape from the more slowly colonizing egg parasites and predators (van den Berg et al., 1988). Egg predators and parasites build up to high levels by the reproductive growth stage, presenting a new mortality factor. Nonsubmerged caseworm eggs have been parasitized in the laboratory by Trichogrammajaponicum (= australicum) (Ash.) (Perez and Cadapan, 1986), which could attack eggs in the field. A number of factors limit caseworm population build-up after the vegetative stage: older rice is a less nutritious food source and oviposition is forced to locations above the water surface, where eggs will Crop Protection 1994 Volume 13 Number 7 501 Rice caseworm: J.A. Litsinger et al. desiccate or be found by increasing numbers of predators and parasites. Reissig, W.H., Heinrichs, E.A., Litsinger, J.A., Moody, K., Fledler, L., Mew, T.W. and Barrion, A.T. (1986) Illustrated Guide to Integrated Pest Management in Rice in Tropical Asia. International Rice Research Institute, Los Bafios, Philippines. 411 pp. Acknowledgements Sen, P. and Chakravorty, S. (1970) Mode of formation of larval shelters in certain Lepidopterous pests of rice. Int. Rice Comm. Newslett. 19, 13-19 We thank the following technical staff for their assistance: Maria Austria, Ben Garcia, Marciano Perez, Danny Amalin and Nonnie Bunyi. Sison, P. (1938) Some observations on the life history, habits, and control of the rice caseworm, Nymphula depunctalis Guen. Philipp. J. Agric. 9, 273-301 References Steel, R.G. and Torrie, J.H. (1980) Principles and Procedures of Statistics, 2nd edn, McGraw-Hill, New York Heinrichs, E.A. and Viajante, V.D. (1987) Yield loss in rice caused by the caseworm Nymphula depunctalis (Guenfe) (Lepidoptera: Pyralidae). J. Pl. Prot. Tropics 4, 15-26 van den Berg, H., Shepard, B.M., Litsinger, J.A. and Pantua, P.C. (1988) Impact of predators and parasitoids on the eggs of Rivula atimeta, Naranga aenescens (Lepidoptera: Noctuidae) and Hydrellia philippina (Diptera: Ephydridae) in rice. J. PI. Prot. Tropics 5, 103--105 Heinrichs, E.A., Medrano, F.G. and Rapusas, H.R. (1985) Genetic Evaluation for Insect Resistance in Rice. International Rice Research Institute, Los Bafios, Philippines Perez, M.L. and Cadapan, E.P. (1986) The efficacy of Trichogramma species as biological control agents against some rice insect pests. Philipp. Entomol. 6, 463-470 Pillai, K.S. and Nair, M.R.G.K. (1979) Biology and habits of the rice case worm Nymphula depunctalis Guen, in Kerala. Entomon 4, 13-16 Ramasubbaiah, K., Sanjeeva Ran, P. and Ahmed, K. (1978) A note on the bionomics and control of rice caseworm. Indian J. Entomol. 40, 91-92 Crop Protection 1994 Volume 13 Number 7 Viraktamath, C.A., Puttarudriah, M. and Channa-Basavanna, G.P. (1974) Studies on the biology of rice caseworm Nymphula depunctalis Guenfe (Pyraustidae: Lepidoptera). Mysore J. Agric. Sci. 8, 234-241 Waldbauer, G.P. and Marciano, A.P. (1979) Rice leaf-folder mass rearing and a proposal for screening for varietal resistance in the greenhouse. IRRI Res. Pap. Ser. No. 27. Received 22 January 1993 Revised October 1993 Accepted 14 November 1993