View of STUDIES ON THE IMPACT OF TENDER, MIDDLE AND MATURE LEAVES ON THE MANDIBLE SIZE AND ECONOMIC TRAITS OF BOMBYX MORI L.

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Received 05 March 2021; Accepted 01 April 2021.

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Studies on the Impact of Tender, Middle and Mature Leaves on the Mandible Size and Economic Traits of Bombyx Mori L.

Dr. M.Thilsath Fatima Quraiza^1,*,Dr.C. Dyana Selin² and Dr. V.K. Stanley Raja³

1 Assistant professor, Department of Zoology, Muslim Arts College, Thiruvithancode, Kanyakumari District, Tamil Nadu, India.

2 Assistant Professor, Department of Zoology, Muslim Arts College, Thiruvithancode, Kanyakumari District, Tamil Nadu, India.

3 Assistant Professor, P. G. and Research Department of Botany, St. Johns college, Palayamkottai, Tamilnadu, India.

Abstract- Silkworm, BombyxmoriL. is an important insect in sericulture industry which belongs to the order Lepidoptera. The mouth parts of Lepidopteran belong to one of the best studied feeding organs of flower- visiting insects, in terms of anatomy, functional morphology, and evolutionary biology. It has been suggested that the body size of herbivorous insects varies in response to the leaf quality of host plants to successfully feed on their specific hosts. Mulberry (Morus alba) is the sole host plant for silkworm and therefore forms the basic material for silkworm growth since it gets its necessary nutrients from the plant. Insect mouthparts are varied, based on their food habits. In this study, size of mandibles (fifth instar) and economic parameters of silkworm reared on different stages (Tender leaves, middle leaves and mature leaves) of mulberry leaves were investigated in the laboratory.In this larval period, the mandible length (1.11±0.09 mm) and width (0.95±0.077 mm) was high in the 17^th and 19^th old day larvae when fed with tender leaves. It also reflects in the economic traits of B.mori. cocoon characteristics such as cocoon weight (1120±55.41 mg), shell weight (200±18.66 mg) and shell ratio (17.85±1.01 %) were increased significantly in the tender leaves treated groups.

Key words: Bombyxmori, Morusindica, Mandibles, Morphometry, Economic traits Introduction

Body size is one of the most important ecological and physiological traits for herbivorous insects to cope with variety of biological and environmental hurdles such as competition and metabolic constraints (Roff, 1992).

The body size of herbivorous insect’s response to the leaf quality of host plants to successfully feed on their specific hosts (Coley et al. 2006). Therefore, the mature leaves in summer have generally been regarded as poor food resources being hard to be utilized by herbivorous insects (Feeny, 1970). However many lepidopteran larvae can overcome to feed on tough leaves even during summer (Forkner et al. 2008).

Some studies have suggested that the leaf toughness affects the body size, and hence population dynamics and community structure of herbivorous insects (Filin and Ovadia, 2007). The mouthparts are exhibited in their most primitive form in Sabatinea, where they are clearly of the mandibulate type. The mandibles are functional denate organs, with evident ginglymus. Several examples showed the morphological difference within an herbivorous insect species to cope with tough leaves. Silkworm, BombyxmoriL. is an important insect in sericulture industry which belongs to the order Lepidoptera. In most Lepidopteran larvae have different exocrine and endocrine glands which take part different body functions and few are still unknown about their function. The mandibular gland is one of the salivary gland present in lepidopteran larvae which is paired tubular glands opening into the lumen of the apodeme of the cranial adductor muscle of the mandible (Vagliante, 2005).

Mulberry (Morus alba) is the sole host plant for silkworm (Zhishen et al. 1999) and therefore forms the basic material for silkworm growth since it gets its necessary nutrients from the plant (Adolkar et al.

2007). The chemical composition of mulberry leaves has a great influence on the larval growth and survival and cocoon qualities (Kherdekar et al. 2000). High leaf quality is also a prerequisite for healthy silkworm growth and production of high quality cocoons.

Insect mouthparts are varied, based on their food habits. In Lepismatidae and Pterygota, the mandibles are articulated with the cranium at two points having a second more anterior articulation with the sub-gena in

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addition to the original posterior one. These mandibles are usually short and strongly sclerotised and often the biting surface is differentiated into a more distal incisor region and a proximal molar region, the mandible of the two sides being symmetrical so as to oppose each other in the midline. In this study, size of mandibles (second to fifth instar) and economic parameters of silkworm reared on different stages (Tender leaves, middle leaves and mature leaves) of mulberry leaves were investigated in the laboratory.

Materials and Methods

Disease free laying (DFLs) of B.mori (PMXCSR2) will be collected from the state Government sericulture centre at Konam, Nagarcoil and incubated in the laboratory the emerging caterpillars will be reared in clean bamboo basket (25 cm diameter and 5cm deep) scaffold with paraffin paper (Krishnaswami,1978) and fed ad libitum. The emerging larvae were selected randomly and grouped into 3 batches for experiment.

Control was also set up. In the experiment group the larvae were fed with tender leaves, middle leaves and mature leaves separately. Each group has 3 replications each with 50 larvae. From 17^thday old larvae to 25^th day were preserved which was the maximum larval period observed from each day development of mandibles.

In this study, the mandibles were isolated from the larval head under a dissection microscope in the younger larvae and without the aid of a microscope for older larvae. The larvae were placed on clean glass slides with the ventral slide upturned. A fine tipped pair of forceps and needle was used to remove the mandibles with the condoyles and muscles. The mandibles were cleaned with alcohol care was taken not to damage the cusp and micrometric measurements of the mandible were made. The mean length and width of mandible were recorded. Photos of the mandible were made by using digital camera.

The cocoons were harvested on the fourth day after spinning and the cocoon characters were recorded in experimental and control groups. Assessment of various cocoon parameters was made by the method ofSonwalker, 1993. The relationship between mandible length and width of B.mori(V^thinstar) was analysis.

Regression co-efficient b, Y intercept a and correlation co-efficient r were calculated and regression lines constructed to establish if there was any relation between the length and width of the various instars. All the economic traits were analyzed statistically by t-test. (Zar, 1984).

Results

Table 1 shows mandible morphometry of fifth instar B.mori larvae fed with different stages of mulberry leaves. During fifth instar larvae had 9 days. Overall this larval period, the mandible length (1.11±0.09 mm) and width (0.95±0.077 mm) was high in the 17^th and 19^th old day larvae when fed with tender leaves. In this stage larval mandible consists of 6 cusps except 24^th and 25^th day old but it had 4 cusps.

Regression analysis of B.mori larval mandible (20^th day old) length and width measurement is presented in Table 1a and Fig.1a. The mandible length (1.005±0.092 mm) and width (0.90±0.085 mm) was high when the larvae fed with tender leaves. Based on the regression analysis a regression co-efficient of 0.84 was recorded and the Y intercept ‘a’ value was 0.06. The correlation co-efficient ‘r’ value was 0.99.

Regression analysis of B.mori larval mandible (25^th day old) length and width measurement is presented in Table 4b and Fig.5. The mandible length (0.92±0.076 mm) and width (0.81±0.057 mm) was high when the larvae fed with tender leaves. Based on the regression analysis a regression co-efficient of 0.67was recorded and the Y intercept ‘a’ value was 0.19. The correlation co-efficient ‘r’ value was 0.86.

The economic parameters of B.morifed with various stages of mulberry leaves are presented in Table 2. The maximum cocoon weight, shell weight and shell ratio was 1120±55.41 mg, 200±18.66 mg and 17.85±1.01 %respectively, when B.mori larvae fed with tender leaves but it was reduced (-0.98, -7.1 and -6.19 per cent respectively) in the mature leaves treated groups.

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Table 1

Mandible morphometry of fifth instar B.mori larvae fed with different stages of mulberry leaves

Age (in days) Type of feed Mandible’s Size Number of

cusps Length (mm) Width (mm)

17

Control 0.94±0.081 0.84±0.071 6

Tender leaves 1.11±0.090 0.86±0.063 6

Middle leaves 0.85±0.066 0.74±0.051 6

Mature leaves 0.75±0.059 0.70±0.049 6

18

Control 0.98±0.080 0.88±0.076 6

Tender leaves 1.06±0.097 0.89±0.059 6

Middle leaves 0.87±0.065 0.75±0.046 6

Mature leaves 0.80±0.073 0.72±0.040 6

19

Control 0.95±0.079 0.88±0.071 6

Tender leaves 1.02±0.080 0.95±0.077 6

Middle leaves 0.85±0.076 0.83±0.063 6

Mature leaves 0.78±0.067 0.72±0.055 6

20

Control 0.93±0.076 0.85±0.059 6

Tender leaves 1.005±0.092 0.90±0.085 6

Middle leaves 0.80±0.065 0.73±0.042 6

Mature leaves 0.75±0.036 0.69±0.061 6

21

Control 0.90±0.068 0.83±0.034 6

Tender leaves 0.99±0.079 0.85±0.059 6

Middle leaves 0.78±0.054 0.70±0.039 6

Mature leaves 0.70±0.063 0.65±0.042 6

22

Control 0.95±0.077 0.80±0.066 6

Tender leaves 0.97±0.075 0.84±0.070 6

Middle leaves 0.75±0.069 0.71±0.053 6

Mature leaves 0.68±0.054 0.64±0.029 6

23

Control 0.93±0.066 0.80±0.074 6

Tender leaves 0.96±0.081 0.84±0.055 6

Middle leaves 0.73±0.049 0.73±0.069 6

Mature leaves 0.67±0.080 0.60±0.072 6

24

Control 0.93±0.077 0.82±0.069 4

Tender leaves 0.94±0.073 0.81±0.057 4

Middle leaves 0.71±0.065 0.68±0.031 4

Mature leaves 0.70±0.050 0.62±0.042 4

25

Control 0.90±0.062 0.79±0.048 4

Tender leaves 0.92±0.076 0.81±0.057 4

Middle leaves 0.69±0.044 0.73±0.060 4

Mature leaves 0.65±0.051 0.57±0.055 4

Table 1a Regression analysis of mandible length and width in 20^th day old (fifth instar) B.morilarvae

Type of feed Length (mm) Width (mm) a b r

Control 0.93±0.076 0.90±0.085

0.06 0.84 0.99

Tender leaves 1.005±0.092 0.85±0.059 Middle leaves 0.80±0.065 0.73±0.042 Mature leaves 0.75±0.036 0.69±0.061

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Fig.1aCorrelation between mandible length and width in 20^th day old (fifth instar) B.morilarvae

Table 1b Regression analysis of mandible length and width in 25^th day old (fifth instar) B.morilarvae

Type of feed Length (mm) Width (mm) a b r

Control 0.90±0.062 0.79±0.048

0.19 0.67 0.86

Tender leaves 0.92±0.076 0.81±0.057 Middle leaves 0.69±0.044 0.73±0.060 Mature leaves 0.65±0.051 0.57±0.055

Fig.1 b Correlation between mandible length and width in 25^th day old (fifth instar) B.morilarvae

Table 2

Economic traits of B.morifed with different stages of mulberry leaves

Treatment

Cocoon weight

(mg) Pupal Weight

(mg)

Shell Weight

(mg) Shell Ratio (%)

Control 1020±63.65 880±60.17 140± 14.81 13.72±1.73

Tender leaves 1120±55.41 (9.8)

920±25.44 (4.4)

200±18.66 (42.60)

17.85±1.01 (30.11) Middle leaves 1090±49.30

(6.86)

910±30 (3.3)

180±12.3 (28.70)

16.51±1.27 (20.34) Tough leaves 1010±43.95

(-0.98)

880±39.07 (0)

130±9.23 (-7.1)

12.87±1.07 (-6.19) a=0.06, b=0.84, r=0.99

0 0.2 0.4 0.6 0.8 1

0 0.5 1 1.5

Width (mm)

Length (mm)

a=0.19, b=0.67, r=0.86

0 0.2 0.4 0.6 0.8 1

Width (mm)

Length (mm)

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Per cent deviation over control values in paretheses N=50

*not significant

All other deviations significant at P≤0.05 (t-test) Discussion

In temperate deciduous forests, leaf qualities show dynamic seasonal changes with leaf ageing, young leaves in early spring are high in nutrient and soft with less fibre and lignin, while mature leaves in summer are low in nutrient, high in defensive compounds and tough (Murakami et al. 2005). For the larvae of spring feeders, young leaves (highly-nitrogen and soft) are available until mid-June, during which they can complete their larval development. On the other hand, the summer feeders lay their eggs in late June to early July and their early instar larvae emerge in mid and late July. Therefore, their larvae should face mature leaves and are forces to free on the low nitrogen and tough leaves. This analysis, the relationships between mandible and leaf toughness showed significant correlations in the fifth instar of B.mori.

Overall this larval period, the mandible length (1.11±0.09 mm) and width (0.95±0.077 mm) was high in the 17^th and 19^th old day larvae when fed with tender leaves. In this stage larval mandible consists of 6 cusps except 24^th and 25^th day old but it had 4 cusps. This work was supported by Eaton, 1988, who reported that the allometric change in head size implies the increase in mandibular adductor muscle. Therefore, the larvae of bigger mandibles are advantageous for processing tougher leaves. Bernays and Hamai (1987) reported that the grass for species of Acridoidea grasshoppers clearly exhibited larger heads and mandibles than forb feeders, attributing to tougher leaves of grasses than forbs. This work was also supported byKause et al. (2001) and they showed that the constraint factors of host leaves for determining body size of herbivorous insects under seasonally varying leaf quality should be divided in adequate nitrogen. If the herbivores are predominantly affected by leaf toughness, the larvae will be forced to increase to their maximum final body sizes for favourably utilizing the deteriorated leaves of host plants. On the other hand, if the inadequate nitrogen acts as major constraint on larval growth, smaller body sizes will be promoted by the shortage of nutrition. Our results suggested that the constraint factor induced by leaf toughness play predominant role for determining the body size in last instar of notodontid larvae.

For cocoon production, all the five larval instars of the silkworm must be fed adequately with mulberry leaves. Usually, people feed the larvae with mulberry leaves haphazardly without considering whether such leaves are young or old. However, the selection of appropriate leaves is very important especially for the development of juvenile or early instars. In this study, maximum cocoon weight, shell weight and shell ratio was 1120±55.41 mg, 200±18.66 mg and 17.85±1.01 %respectively, when B.mori larvae fed with tender leaves. Significant higher cocoon weight obtained when silkworms were fed with tender and middle aged leaves agrees with the findings of Bahar et al. (2011) who observed that the quality and nutrient content of mulberry leaves were influenced by maturity. Tender leaves contain higher protein and water contents which are essential for quality cocoon production (Krishnaswami, 1979), thus resulting in the increase weight of the larvae fed with it. Li and Sano (1984) also reported increase in cocoon weight when silkworm larvae were fed on tender leaves containing high quantities of water and proteins. However, when fed on tough leaves containing high quantities of carbohydrate and low quantity of water and protein, slow larval development and larval and cocoon weight were observed. In the present study, it was found that economic parameters were higher in tender leaves than in older leaves.

Conclusion

The morphological change appears to closely associate with the changes in feeding behaviour of B.mori.In this investigation concluded that the tender leaves of food plants increase the mandible sharpness and in turn to improve economic traits of B.mori but tough leaves such as mature leaves affected the mandible size and it produce poor cocoon.In this work conducted with an objective of examining the day by day development of mandibles (fifth instar) of B.mori fed with tender, middle and tough leaves. The mandibles were subsequently measured using a micrometer to find out the length and width of individual mandibles.

This study was also recorded the economic parameters such as cocoon weigh (mg), pupal weight (mg), shell weight (mg) and shell ratio (%).

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