Short Communication

Cocoon construction by larvae of Rhynchophorus ferrugineus

(Coleoptera: Curculionidae)

Y.K. Amrutha Kumari and S. Sreekumar

Department of Zoology, University College, Thiruvananthapuram, Kerala, India

Corresponding Author: S. Sreekumar, E-mail: ssreekumar53@gmail.com

Received: 10/08/2024; Revised: 05/10/2024; Accepted: 13/10/2024; Published: 29/01/2025

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Abstract

The older larvae of the red palm weevil Rhynchophorus ferrugineus construct cocoons made from the

discarded chewed fibres of the tender parts of the coconut palm for pupation. The larvae stop feeding prior to

pupation and wander around the medium to find a suitable place for cocoon construction. They start

constructing cocoons by picking up fibres by the mandibles and these are then packed into a cylindrical structure

with both ends open. The packing is done by the random rotatory movement of the body and head. During this

time the larva discharges the gut contents to moistens the fibres. The gut contents thus discharged serve to glue

the fibres and provide a coating on the inner surface of the cocoon to give it a smooth finishing. The openings of

the cylinder are eventually closed by pulling fibres from the rim of the cylinder using mandibles. The

mechanism of closing the open ends of the cylinder is demonstrated here in this study using a pair of needles.

Keywords: Rhynchophorus ferrugineus, coconut pest, cocoon construction, pupation

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Introduction

The red palm weevil Rhynchophorus ferrugineus is one of the most injurious pests of coconut, date and oil

palms. It is a holometabolous insect. The life cycle of Rhynchophorus ferrugineus comprises seven larval

instars when reared in the laboratory on sugar cane.[1] It takes 70-60 days to complete the larval life. When

about to pupate, the larvae construct cocoons made of fibrous chewed materials of the host plant. The fibres are

mostly oriented circularly and are packed tightly towards the interior. The inner surface is smooth and has a

glistening appearance.[2] The fibres covering the top appear somewhat loosely packed which may facilitate the

exit of the adult after eclosion. It is intriguing how the apodous larva, with no appendages except a pair of

mandibles (Figure 1), constructs a cocoon with such perfection. After cocoon construction the larva becomes the

prepupa. The prepupal period ranges from 2 to 11 days and is terminated by the act of pupation. The pupal

period lasts for 11 to 21 days.[3]

The cocoon construction by larvae of Rhynchophorus ferrugineus was observed in this study by introducing

older larvae of the late instar into a glass tube (15 cm x 2.5 cm diameter) containing coconut husk fibres. The

cocoons collected from the field were composed of fibres measuring 2-4 cm in length. Hence, coconut husk

fibres cut into pieces, 2-4 cm long, were provided as the material for cocoon construction. The mouth of the

tube was closed with cloth. The tubes with larvae and coconut husk fibres were kept undisturbed until they

completed the cocoons.

The series of events associated with cocoon construction can be broadly recognized into 4 stages: wandering

stage, packing stage, mechanism of closing, and plastering stage.

Figure 1: Developmental stages of Rhynchophoru sferrugineus. 1.Egg, 2. Larva, 3. Prepupa, 4. Pupa, 5. Adult

Wandering stage

The first stage is known as the wandering stage. The older larvae of the final instar consume little or no food

prior to cocoon construction. The larva now enters the wandering stage during which it exhibits vigorous

crawling movements and occupies the surface of the rearing medium. It lasts for about 3.25 ± 1.19 days. At this

time a gradual decrease in body weight occurs from 5.03 ± 0.47 g to 4.40 ± 0.48 g. The wandering stage enables

the larva to find a suitable site for cocoon construction, probably near the exit that may facilitate easy escape of

the adult after eclosion. Detection of light and airflow may provide this clue to the larva. It is observed in this

study that in almost 95 % of instances, the larvae have constructed cocoons towards the mouth of the tube

The foregut and midgut of the larva contain a clear brown coloured fluid having sticky nature. At the end of the

wandering stage, the larva purges the gut. It then starts packing the fibres in the form of a cylinder.

Packing stage

The larva takes 2.30 ± 1.03 days for packing the fibres. This is achieved by pulling the fibres with mandibles

and pushing them with the head. The fibres are rolled into a cylinder with both ends open (Figure 2). It then

presses the hind end of the body against the cylindrical mass of fibre. The packing is usually carried out at night.

After packing the larva enters a brief period of quiescence lasting for 5 to 10 min. It then starts to orient the

fibres of the inner layer around its body by pulling the fibres with mandibles accompanied by random rotation of

the body. During this time the larva discharges the gut contents to moisten the fibres. The secretion serves to

glue the fibres. The larva then pushes the roll of fibres against the tube to make it more compact. The partially

completed cocoon is now open at both ends. Packing is completed by mid or late night. After packing the larva

becomes inactive and enters a resting period for a few hours or one or two days. Then it closes the two open

ends of the cocoon. In 90% of the cases the larva closes the top end of the cocoon first. But very rarely, it closes

the bottom first. Usually, the bottom end is closed 1 to 2 hr after the closure of the top end. One of the stimuli

for closing cocoon may be the light as it happens mostly at dawn. The larvae can be induced to close

the cocoon by flashing a torch towards the open end. Closing is completed within a few minutes. The adult

emerges out of the cocoon through the top end.

Mechanism of closing

The closing of the two open ends needs some expertise. The larva bites on the rim of the open end and holds 2-4

fibres with the mandibles. It then strongly pulls these fibres towards the centre of the cocoon so that the

circularly oriented fibres of the rim now become vertical in position. This process is repeated until the cocoon is

closed with a dome of fibres. The larva then turns about by a somersault movement and strongly pushes the

interior of the dome with the abdominal tip. By this action the fibres become compactly packed. The method of

closing the open ends of the cocoon is simulated in figure 3, using a pair of needles.

Plastering stage

It includes the finishing works such as pasting of the fibres and plastering the interior of the cocoon. The larva

regurgitates the gut contents which by now have turned highly viscous probably due to the presence of

disintegrated peritrophic membrane, for plastering the inner wall of the cocoon. The peritrophic membrane is

made of chitin fibres set in a protein-carbohydrate matrix. It protects the midgut epithelium from mechanical

damage caused by food particles and also serves as a barrier against the entry of microorganisms.[4] In many

coleopterans, the peritrophic membrane is used to coat the pupal cocoon.[5] It is observed in this study that the

secretions of the mandibular glands are also added to the gut contents for plastering. The mandibular secretion is

found to have antifungal properties. During plastering, the larva knocks on the wall of the cocoon 5-9 times at a

stretch, probably to sense the sturdiness of the cocoon. The larva exhibits a variety of movements which include

somersaults, wriggling and rotation for plastering the cocoon. The larva then becomes quiescent, settles down

and becomes immobile for pupation. The entire process of cocoon construction takes 7.80 ± 2.48 days.

Figure 2. (Left) Initial stage of packing, (Right) Final stage of packing.

Figure 3: Mechanism of closing the open ends of the cocoon using a pair of needles

Figure 4: Cocoons made of substitute materials (A) Used fibres, (B) Jute fibres,

The optimum length of fibres for cocoon construction is found to be 2-4 cm. The larvae fail to construct cocoons

when short fibres of 0.5 to 1 cm long are provided. However, they can utilize very long fibres (10 to 15 cm)

after cutting them into pieces of varying length.

When larvae were placed in a specially designed chamber with a temperature range between 260 and 320 C from

bottom to top, the larvae preferred a temperature of 28- 300C for cocoon construction. The larvae maintained at

total darkness required 7.80 ± 2.30 days for cocoon construction. Larvae exposed to continuous light showed

random movements and they spent more time for wandering, packing and closing. These larvae closed the open

ends incompletely and reopened it several times before making the final attempt. The duration was found to be

12.1 2.66 days.

It is observed in this study that the larvae can make use of a variety of substitute materials such as thick and thin

plastic fibres, jute fibres and used fibres (fibres detached from completed cocoons) for cocoon construction (Fig.

4). No significant variation in duration for cocoon construction is observed with the above-mentioned materials.

In the cocoon made of plastic, the fibres are not properly oriented especially those forming the inner layer of the

cocoon. The fibres appear irregularly packed but strongly glued. During eclosion the adults make a hole near the

upper end of the cocoon with their mouth parts and snout to find a way out. It is known that the behaviour of the

larva during cocoon construction and pupal moult are influenced by hormones. In Lepidopterans, a surge in

ecdysone, called the commitment peak, switches on the behavioural responses from feeding to wandering

activity and potentiates tissues for the pupal moult.[6] The second much larger prepupal peak controls the pupal

moulting.

The pupae of coleopterans are inactive forms and during this period locomotion ceases, feeding is suspended,

respiration slows down and externally they look quiescent, but internally probably as active as any period

subsequent to embryonic development. All available energy is used for establishing totally different adult

morphology and anatomy with modifications in physiology. In other words, in the life cycle of insects, growth

has all been relegated to the larval stage and transformation from larva to adult is greatly abbreviated into a

single and short stage, the pupa.[7] The pupa being the most vulnerable stage in the life cycle of the insect, it

needs to be often protected by an outer casting, which are made of different materials in various groups of

insects. For example, in more advanced group of flies, (Diptera) the skin of the last instar is hardened into a

seed like case called ‘puparium’. The caterpillars of most Lepidoptera, Neuroptera, Trichoptera and some

members of other orders often construct cocoon entirely by secreting silk. For providing strength, extra

materials will be added from the surroundings such as bits of leaf, particles of sand or even faecal pellets. The

majority of coleopterans, however, construct cocoons using extraneous materials such as soil or the food

medium itself (e.g., Oryctes rhinoceros) or the chewed fibrous materials of host plant as in the case of

Rhynchophorus ferrugineus.

Financial support and sponsorship

Nil

Conflict of interest

There are no conflicts of interest.

References

1. Jaya S, Suresh T, Sobhitha Rani, Sreekumar S. Evidence of seven larval instars in the red palm weevil,

Rhynchophorus ferrugineus Oliv., reared in sugar cane. J Ent Res. 2000;24(1):27-31.

2. Nirula KK. Investigations on the pest of coconut palm. Part I. Indian Cocon J. 1955; 8:118-110.

3. Nirula KK. Investigations on the pest of coconut palm. Part IV. Rhynchophorus ferrugineus. Indian Cocon

J. 1955; 9:229-347.

4. Gillet C. Entomology. New York: Plenum Press; 1995. p. 470-80.

5. Kenchington W. Adaptations of insect peritrophic membrane to form cocoon fabrics. In: Hepburn HR,

editor. The Insect Integument. Amsterdam: Elsevier; 1976.

6. Truman JW, Dominic OS. Endocrine mechanisms organizing invertebrate behavior. Bioscience. 1983;

33:546-51

7. Metcalf CL, Flint WP. Destructive and useful insects: their habits and control. New York: McGraw-Hill

Book Company; 1951. p. 146-805.