Proc. Indian Acad. Sci. (Anim. Sci.), Vol. 96, No. I, January 1987, pp. 71-76. © Printed in India. Development and survival of Mythimna sepsret« (Wlk.) on some host plants during winter at Hisar RAM SINGH and J P CHAUDHARY Department of Entomology, Haryana Agricultural University, Hisar 125004, India MS received 18 January 1986; revised 25 October 1986 Abstract. Survival and post embryonic development of the oriental armyworm, Mythimna separata (Wlk.) was studied on 5 host plants, viz sorghum, maize, sugarcane, Johnson grass and napier grass during winter months (15 November to 7 March) under laboratory condi­ tions. Variations in different parameters like larval survival (10'3-60'3%), size of larvae (7'33-29-67 mm), larval weight (25'5-493'1 mg for 26 days old larva), size of pupa (15'0-16·49 rnrn), pupal weight (184'9-249'5 mg for 2-4 days old pupa), pupation (3-3-38-9%), developmental period (74,8-109'8 days) and moth emergence (3·4-50'0%) were significant on different foods. Low temperature and humidity conditions during experimen­ tal period' resulted in complete mortality of female pupae reared on napier grass, Johnson grass and maize whereas in case of males 60-100% moths were crippled winged. However, adults emerged on sorghum and sugarcane f.oods were normal except 28'7% males in sorghum were crippled. Total growth index was also higher on sugarcane (0'45) and sorghum (0'24) indicating their suitability in carrying over of winter pest population. Keywords. Armyworm; Mythimna separata; host plant; growth index. 1. Introduction The oriental armyworm, Mythimna separata (Wlk.) (Lepidoptera: Noctuidae) is a ubiquitous pest of cereal crops and grasses in Asia and Australia. About 16 serious outbreaks of the pest have been recorded in India from 1924-1981 (Sharma and Davies 1983). In northern part of the country it has been reported to cause consi­ derable damage intermittently to many graminaceous crops (Chaudhary and Ramzan 1967; Sarup et al 1969; Verma and Khurana 1971; Bindra and Singh 1973; Butter et al 1979; Chaudhary and Singh 1980; Singh and Manchanda 1981). This armyworm in recent years has started assuming serious proportions quite frequently and little information is available on the extent of survival of winter generation of this pest on various host plants. Keeping this in view, the present investigations were carried out. 2. Material and methods Studies were carried out on 5 host plants, viz Sorghum (Sorghum bicolor (L.) Moench), maize (Zea mays L.), sugarcane (Saccharum oj)icinarum L.), Johnson grass (Sorghum halepense (L.) Pers) and napier grass iPennisetum purpureum Schumach) from 15 November 1984 to first week of March 1985 under fluctuating laboratory conditions at the University. Freshly hatched larvae of armyworm were fed on fresh young leaves of these grasses. First instar larvae were taken from the mass culture maintained in laboratory on leaves of pearl millet and maize. Small pieces of fresh leaves of each grass were put in glass jars (20 x 15 ern). In each jar, 100 freshly 71 72 Ram Singh and J P Chaudhary hatched larvae for each test plant in 3 replications were released. These jars were covered with muslin cloth and placed under laboratory conditions. Temperature and humidity prevailing in the laboratory were recorded daily with the help of maximum-minimum, dry and wet bulb thermometers, respectively. The fresh food was provided daily. Various developmental parameters, viz larval survival, dura­ tions, size and weight; pupation, pupal weight, size and durations; pre-pupal duration, moth emergence, adult longevity etc were recorded for each food separa­ tely. Sexing was done in pupal stage as suggested by Avasthy and Chaudhary (1964). The growth indices on different foods were calculated by the following formulae (Sharma et al 1982). Pupation (%) Larval growth index Mean larval period (days) Moth emergence (%) Total growth index Mean total developmental period (days) 3. Results and discussion From the data summarised in table 1, larval survival when recorded after 5 days of release, ranged from 74·7-97·0% on different foods without any significant difference among them. Thereafter, it decreased gradually except in case of sugarcane where only 10'3% survival was observed on 22nd day of feeding. Sugarcane though a potential host plant of armyworm (Butani 1955; Chaudhary and Ramzan 1967), appeared to be less acceptable by first instar larvae probably due to hard leaf tissues. Kalode et al (1971) also reported 100% mortality of young larvae of Cirphis compta Moore on sugarcane leaves. Maximum (60,3%) larvae survived on napier followed by Johnson grass (54'3%) and sorghum (48'7%) without any significant difference among them. On maize leaves, only 36'7% larvae could survive. Under varying conditions of temperature and humidity larval survival of this pest on maize had been reported to vary from 12·0-60·0% (Bindra and Singh 1973; Dhaliwal and Bains 1978; Alam and Khatri 1980). Table I. Effect of feeding on different host plants on the larval survival, size and weight of M. separata under laboratory conditions. Larval survival (~~) (days) Size of larva Larval weight (mg) (days) (mm) (20 days Test plant 5 10 15 22 old) 14 23 26 Napier grass 97-0 77-7 71·3 60·3 2967 56·3 358·5 493·1 Sorghum 74·7 56·0 49·7 48·7 25·49 54·0 339'5 482·5 Johnson grass 76·3 66·0 59·7 54·3 25-00 59·3 284·0 306-6 Maize 94·0 61·7 47·7 36-7 2233 36·9 254-0 344-2 Sugarcane 90·7 27·7 17·3 10·3 733 5·5 25·4 25·5 Average 86·5 57·8 49·1 42·0 21-96 42·4 252·3 330·4 SEM± 9·8 4·8 5·5 5·9 0-80 4·6 19·4 62·9 CD_ at 5~~ NS 11·1 12-7 13·7 1-86 10·6 44·8 128·1 Mean temperature = 18-6± 3·2e e. Mean relative humidity = 55 ± 5~~. *Based on 300 larvae. Development and survival ofarmyworm in winter 73 Size of the larvae compared after 20 days of release ranged from 7·33 (sugarcane) to 29·67 mm (napier) indicating the slow growth of larvae on sugarcane. Larval weights after 14 days of feeding showed considerable differences on different foods. It ranged from 5·5 mg on sugarcane to 56·3 mg on napier grass. After 26 days of feeding, the range (25-5-493-1 mg) was still wider. Larvae fed on napier gained maximum weight (493'1 mg) followed by sorghum (482-5 mg) without any significant difference between them. On maize and Johnson grass, larva weighed 344-2 and 306-6 mg, respectively and were also significantly heavier than sugarcane (25-5 mg), Pupal size on different foods ranged from 15-0-16-91 mm (table 2)_ Avasthy and Chaudhary (1965) and Alam and Khatri (1980) also reported similar pupal sizes on sorghum, maize and sugarcane. Pupal weight of 2-4 days old pupa varied between 184-9 mg (sugarcane) and 249-5 mg (sorghum) at 18-2± 3-9°C and 63 ± 4-0% relative humidity, However, Tripathi et al (1982) recorded a higher range (319-5-340'7 mg) of pupal weight at 27°C and 85% relative humidity. A considerable decrease in pupal weight was recorded after 20-22 days of pupal formation on different foods. On all foods, male pupa (111·9-153-6 mg) was heavier than female pupa (98-5-127-5 mg). Inspite of highest larval weights on napier grass, pupae of both sexes were lighter in weight in comparison to that of others, However, maximum pupation (38-9%) was recorded on napier followed by sorghum (30-3%), Johnson grass (25-0%) and maize (19-7%)- On sugarcane only 3'3% larvae pupated_ Under favourable conditions more than 80% pupation was recorded on maize and sorghum (Dhaliwal and Bains 1978; Alam and Khatri 1980)_ Ratios of males and females at pupal stage were almost equal on different foods. As is evident from table 3, shortest larval period (28·2 days) was recorded on napier grass followed by sorghum (28·3 days) and Johnson grass (29-3 days) without any significant difference among them whereas, it was longest (49-8 days) on sugarcane at prevailing temperature (18-6± 3-2°q and relative humidity (54 ± 5%)_ During colder months (October hatching) larval stage had been observed to go upto 100 days on maize (Bindra and Singh 1973), but under favourable conditions it was 15'11-15-18 days on Johnson grass and maize (Kushwaha and Bhardwaj 1977), 19'3-20-75 days on sorghum and maize (Alam and Khatri 1980) and 14-22 days on Table 2. Effect of feeding on some grasses on pupal parameters of M. separata under laboratory conditions. Pupal weight (mg) Size of pupa 20-22 days old Sex ratio Larval (mm) (2-4 days 2-4 days Pupation growth Test plant old) old Female Male ("'~) Male: Female index Napier grass 16-49 236·2 98·5 111-9 38·9 1:0·82 1·38 Sorghum 15-66 249·5 1275 153-6 30·3 I: 1·\0 1·07 Johnson grass 15·24 193·7 112·4 127-1 25·0 I: 1·10 0·85 Maize 16·91 229-4 111·3 157·0 19·7 1:0·85 0·61 Sugarcane 15·00 184·9 101·2 131-1 3·3 I: 1-00 0·06 Average 16·07 218·7 110·2 136·1 23·5 1:0·97 SEM± 0·28 13·7 12·1 13-8 3·7 CD.