Morphology and biology of Noorda blitealis (Lepidoptera : Crambidae) immature instar for a biological control perspective

doi:10.21203/rs.3.rs-4182138/v1

Noorda blitealis is a phytophagous insect that causes major losses to Moringa growers worldwide. This study was conducted to determine the bio-ecology of its larvae. A mass rearing was carried out in Central Agricultural Entomology Laboratory located in Kamboinsin, Ouagadougou from July to December 2021. Parameters such as eggs incubation period, number of larval instars and their duration, body length and color, number of Moringa folioles attacked and survival rate were collected. Results showed that Noorda blitealis (Lepidoptera: Crambidae) passed into 5 larval instars before pupating. First instar larva averaged 2.4 ± 0.8 mm in length and were light green. At the 5th instar, these larvae reach 10.8 ± 0.4 mm and their bodies take on a reddish appearance. The average egg incubation was 3.00 ± 0.35 days. Larvae and chrysalis duration phases were respectively, 10.61 ± 2.28 days and 9.78 ± 0.42 days. The larval survival rate reaches 100% for 3rd to 5th instar. A statistically significant difference was found between leaf attacked rates and larval instars (p = 0.001). Larvae in their 6th and 7th days attacked the greatest number of folioles. These data could be used to develop effective biological control methods against this insect pest.

Moringa

insect pest

Noorda blitealis

crop protection

biology

Leaf insect pests are a major constraint of Moringa oleifera (Capparales: Moringaceae) production (Halilou et al. 2022; Kabré et al. 2020; Ke et al. 2016; Ratnadass et al. 2011). According to Ke et al. (2016), more than 40 main pest species were found on Moringa and each species can caused damage to different parts of the plant. Among these insect pests, those belonging to Lepidoptera, Orthoptera, Diptera and Hemiptera orders are the most important on Moringa plant (Ikpeze & Ngenegbo, 2022; Joshi & Baliah, 2019; Kabré et al. 2020). Noorda blitealis (Lepidoptera: Crambidae) is one of these insects. The larvae of this insect pest are considered as the major insect pest of Moringa oleifera (Mridha & Barakah 2017; Outani et al. 2023; Ratnadass et al. 2011) worlwide.

N. blitealis larvae are phyllophagous caterpillar that can become miner. They can also destroy the apical bud and make a gallery in the plant central cylinder, which then dries (Kant et al. 2017). In case of heavy infestations, the larvae can cause defoliation ranging from 70–100% on Moringa plants (Bedane et al. 2013; Satti et al. 2013). In addition, these larvae create silken webs around the leaves, which asphyxiate and eventually died (Dao et al. 2015). The damage is enormous and this is a barrier to Moringa cultivation (Ke et al. 2016).

Moringa growers use methods that range from bioinsecticides (Kabre et al. 2023) to synthetic chemical insecticides (Halilou et al. 2022; Ratnadass et al. 2011) to control the pest, but the results are below their expectations. In Burkina Faso particularly, the extent of leaf damage has forced some growers who earned additional income from this crop to abandon it. However, Moringa is a plant that is being popularized by institution to fight malnutrition among children and women (PNDES-II 2021) and poverty in rural areas by selling different parts of the tree. In view of the socio-economic importance of Moringa for the population (Fadeyi et al. 2023; Tefera 2022), it is important to implement management strategies against Noorda blitealis.

Despite of the impact of the Noorda blitealis larvae on Moringa oleifera plant, work on development parameters remains fairly restrictive. This work is being undertaken to determine the bio-ecology aspects of this insect in particular its larvae. Specifically, the study aims to determine the (i) morphological characteristics of Noorda blitealis immature instars; (ii) duration of immature instars (iii) type of damage caused by each larval instars of N. blitealis on Moringa leaf; (iii) larvae instar that causes the most leaf damage in Moringa oleifera leaf and ; (iv) larvae survival rate.

Experimental site

The experiment was conducted in Central Agricultural Entomology Laboratory (12°27’21,66 N ; 1°32’59,46 W) located in Ouagadougou (Burkina Faso) from July to December 2021. During experimentation, the mean laboratory temperature was 26 ± 1°C, the relative humidity was 62 ± 5% and photoperiod was 12 hours.

Origin of larvae

Larvae used for the rearing were collected from Moringa oleifera planted at the research station of the Environment and Forestry Department (12°22’49,90 N ; 1°30’15,40 W). The trees were less than a year old. The collected larvae were transported to the laboratory in 900 ml laboratory glass containers containing sand and fresh Moringa leaves for rearing.

Rearing of Noorda blitealis larvae and adults

Rearing was realised into 900 ml glass containers. Fresh Moringa leaves were served as food for the larvae. The cut ends of these leaves were wrapped in hydrophilic cotton impregnated with water and covered with pieces of polyethylene. This was done to prevent water loss from the leaf and keep them turgid. Each glass jar was covered by with a piece of muslin cloth fastened with rubber band. One-third part of jar was filled with moist sand, which provides optimal condition for pupation. The grown-up larvae pupated in the jar. The emerged moths (Fig. 1) were released in rearing cages measuring 60cm x 60cm x 60cm. Fresh Moringa plants were used for eggs laying.

Data collection

Morphological characteristics of larval instars

Larvae length and body color were determined. For this purpose, newly emerged larvae were taken and followed through to pupation. These larvae were placed individually in 90 mm diameter Petri dishes and fed daily with fresh Moringa leaves. Food in each petri dish was changed daily by bringing fresh Moringa leaves from the field. The larvae length was recorded on graph paper after immobilization in the cold. Body color was determined visually with the naked eye A total of 15 larvae were used for this experiment.

Immature instars duration

In parallel with the determination of the larvae morphological characteristics, the following development parameters were determined :

Egg incubation: for this determination, leaves containing eggs were removed and placed in 90 mm petri dishes and covered with muslin cloth fastened with rubber band. Observations were made daily. A total of 30 eggs were collected and observed until the larvae emerged.
Each larval instar duration by observing moulting daily using an optika binocular magnifier coupled to a monitor. A larva was considered to have changed instar when the head capsule or exuviae were rejected.
Larvae instar duration (1rst instar to 5th instar) by summing the durations corresponding to the different larval instars
pupal instar by recording thee time elapsed between imaginal moulting until apparition of the imago.

Type of damage

The type of damage caused by each larval instar of Noorda blitealis was assessed using the same larvae as those on which the measurements were made. Direct observation was used to assess the type of damage caused by each larval instar.

Damage and larval age

This parameter was determined according to the protocol described by Dao et al. (2015). The protocol involved depositing a newly emerged larva on a potted Moringa plant in a rearing cage measuring 60 cm x 60 cm x 60 cm. The number of folioles on the plant was counted and marked at the start of the test. Daily observations were made until larvae pupated. During these observations, the attacked folioles were pulled off. A foliole was considered attacked when it showed visible signs of attack. The seedling was replaced when all its leaves had been consumed by the larva. A total of 15 replicates were used for this determination.

Larval survival rate

Newly emerged larvae were placed individually in 90 mm-diameter petri dishes. They were fed daily on fresh Moringa leaves. The larvae were observed daily in the event of death. The Optika binocular magnifier coupled to a monitor was used to observe larvae and count the number of cephalic capsules present in the petri dish. The number of cephalic capsules has been used to determine the corresponding larval instar.

Data analysis

A 5% ANOVA test was used to compare the number of folioles attacked daily by larvae. In the case of statistically significant differences, a Student t test was used to separate the means. Analyses were performed using R sofware.

Morphological characters of the larvae

Based on the number of cephalic capsules rejected, 5 larval instars were determined. Each larval instar differs from the others by the length and the color of its body (Fig. 1). The average length of 1st instar was 2.4 ± 0.8 mm. The 2nd instar larva averaged 4.8 ± 1.2 mm in length, reaching an average of 7.7 ± 0.6 mm in the 3rd instar. At the last instar, larvae averaged 10.8 ± 0.4 mm. Body color changes from light green in 1st instar to red in 5th instar.

Duration of immature instars

The incubation period of the eggs was recorded as 3.00 ± 0.35 days and the incubation period varied from 3 to 4 days. Observations on the different durations are given in the table. 2. N. blitealis larval development duration on Moringa averaged 10.61 ± 2.28 days. The 5th instar was the longest, with an average of 3.12 ± 0.64 days, while the 4th instar was the shortest, with an average duration of 1.62 ± 0.51 days. The average duration of the chrysalis instar was 9.78 ± 0.42 days. The average time from egg laying to adult emergence was 23.14 ± 1.21 days (Table 1)

Table 1

Duration of egg, larvae and pupal instars of *N. blitealis*
Instars	Numbers	Duration ± sd* (day)	Minimum -Maximum. (day)
Egg	15	3.00 ± 0.35	3–4
1st instar	15	2.00 ± 0.35	2–3
2nd instar	15	1.87 ± 0.35	1–2
3rd instar	15	2.00 ± 0.53	1–3
4th instar	15	1.62 ± 0.51	1–2
5th instar	15	3.12 ± 0.64	3–4
Larval to chrysalis (1st to 5th instar)	15	10.61 ± 2.28	8–14
Chrysalis	15	9.78 ± 0.42	9–10
Overall	15	23.39 ± 3,05	19–27

*sd : standard deviation

Type of damage

Observations showed that the type of damage caused to M. oleifera leaves varied with the larval stage. Larvae of 1st and 2nd instar feed on the underside of Moringa leaves. The other instars (3rd, 4th and 5th ) gnaw on leaf blades, causing perforations (Fig. 2).

Moringa leaf damage according to their age

Results showed that the number of folioles attacked varied with larvae age (p = 0.001). In fact, the multiple Student separation test showed that larvae at 6 and 7 days after emergence attacked more folioles than younger or older larvae (Table 2).

Table 2

Mean of folioles attacked by *N. blitealis* larva instars
Days	Mean of folioles consumed*
1rst day	Less than one folioles
2nd day	1.4 ± 0.51 e
3rd day	2.8 ± 0.63 d
4th day	4.8 ± 1.03 bc
5th day	5.9 ± 1.85 ab
6th day	6.3 ± 1.56 a
7th day	6.2 ± 2.25 a
8th day	3.9 ± 1.3 cd
9th day	1.0 ± 0.66 e
10th day	0.5 ± 0.52 e
Total	28,6 ± 10,31
11th day	Pupation
12th day	Pupation
P-value	0,001

*numbers with the same letters are not statistically different

Noorda blitealis larval survival rate

Results on larval survival rates showed that N. blitealis larvae had a survival rate of over 90% in the laboratory. First larvae instar showed a survival rate of 86.67%, while larvae of other stages showed survival rates of 100% (Fig. 3).

The results of this study provide data on Noorda blitealis larvae, an insect that causes major problem to Moringa cultivation. In effect, the results show that these larvae, like all holometabolous insects, develop through successive molt.

Thus, 5 larval instars were observed on the basis of the number of cephalic capsules rejected. This result confirms those of Ratnadass et al. (2011) who based their analysis on the observation of larval behavior, and those of Sharjana & Mikunthan (2019) who based their analysis on the morphometry of the head capsule.

During the experiment, the length of the larvae increased rapidly. These larvae grew from less than 5 mm at eggs eclosion to 8.9 ± 1.3 mm at 5th instar. This rapid development is associated with a variation in larval coloration. The rapid growth could be explained by the presence of Moringa leaves, which they consumed. This variation in the color could be a homocromy mechanism that allows the larvae to be camouflaged from their predators, adapting their coloring to the leaves of the host plant (Vukusic & Chittka, 2012). According to Mustata & Mustata (2012), homochromy and mimicry are evolutionary mechanisms that ensure the survival of species in their battle for existence. Other authors explain this variation in color for thermoregulatory needs (Umbers et al. 2013) and is often controlled by biotic and abiotic factors (Tanaka & Nishide 2012; Verlinden et al. 2009).

The life span of larvae from 1st instar to pupation was 10.61 ± 2.28 days. This result corroborates those of Subramoniam and Chitra (2019) who had obtained durations varying between 9 and 12 days under laboratory conditions almost similar to those of our study. Compared with the larval development duration of other pests of the order Lepidoptera that are important in agricultural terms, this period may seem shorter, but the fact remains that these larvae are major pests of plants in the Moringaceae family. Observations have shown that larvae of all stages consume Moringa leaves by the type of damage varies according to the larval instar considered. Larvae aged between 4 and 7 days are the most voracious on leaves, compared with larvae from other days. Several hypotheses could explain this situation. The first is based on the fact that the buccal pieces of young larvae are not sufficiently developed to cut and crush leaves. The second is that, according to coevolutionary theory, the nutrition of individuals depends on the enzymes they have at their disposal to ensure their proper digestion (Strebler 1980). In this way, the nutrients contained in Moringa leaves are digested by the larval stages, which are able to synthesize the enzymes necessary for their digestion. In addition, this could be explained by the fact that larvae from days 4 to 7 need to store sufficient food as an energy source.

In this study, larval survival rates ranged from 86.67% for 1st instar larvae to 100% for larvae of other instars. Umbers et al. (2013) obtained larval survival rates of 98.33% in Spodoptera frugiperda larvae reared at 25 ± 1ºC, 70 ± 10% relative humidity and a 14 hours photophase. This high survival rate could be explained by the fact that the larvae were reared individually in Petri dishes, thus preventing competition of foods. For their survival and normal development, insects consume specific diets (Behmer 2009) which must contain. Proteins and carbohydrates. These two nutrients provide them with the essential amino acids and energy they need (Wang et al. 2018). In this way, N. blitealis larvae find all the nutrients they need for survival and growth in Moringa oleifera leaves.

The mortality observed in first-day larvae could be explained by the fact that, at this age, larvae are unable to metabolize correctly the toxins or non-nutritive compounds contained in Moringa leaves.

he presents study reports on the biological and morphological aspects of Noorda blitealis larvae on Moringa oleifera trees. The results show that all larval stages feed on Moringa plants, causing significant losses and loss of income for growers. Knowledge of the larvae's biological and morphological characteristics could help develop biological control methods capable of limiting its impact on Moringa plantations.

Acknowledgements

The authors are grateful to the technicians who monitored larval rearing and to anyone who contributed to improving the manuscript.

Conflict of interest

The authors declare no conflict of interest.

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Morphology and biology of Noorda blitealis (Lepidoptera : Crambidae) immature instar for a biological control perspective

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Material and methods

Experimental site

Origin of larvae

Data collection

Morphological characteristics of larval instars

Immature instars duration

Type of damage

Damage and larval age

Larval survival rate

Data analysis

Results

Morphological characters of the larvae

Duration of immature instars

Type of damage

Discussion

Conclusion

Declarations

Acknowledgements

Conflict of interest

References

Status:

Journal Publication

Version 1