Weevil

Weevil is the term used to define a diverse group of insects that belongs to the family Curculionidae. Although the term itself is an umbrella term, it is also more commonly known as the snout beetle. The weevils are widely distributed species of insects, some of the common species of this group are rice weevil, red palm weevil, flour weevil and maize weevil. An important point to notice here is that the term is used to define various types of weevils and all of these belongs to different species. There are about 40,000 different species that are classified among the family.

Although various weevils have some structural similarities they vastly differ in the plants they infest, they are known to infect different plants and crops and because of this nature, they are classified into different species. Some of the species of weevils are considered agricultural pests. Examples of such species include rice weevil (Sitophilus oryzae), maize weevil (Sitophilus zeamais) and grain weevil ( Sitophilus granarius). These organisms are responsible for the loss of crops every year, various pest management techniques are used to control the population of these animals.   

The article focuses on the general description of the insect. The article also discusses the taxonomic classification, morphology and the life cycle of ther insect.

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General Description

The weevils are classified under the family called Curculionidae, this family contains about forty thousand species, the weevil and beetles share a common taxonomic order. Since they are classified under the same order they share some morphological resemblance with each other. Due to this, the weevil insects are sometimes referred to as snout beetle.

They are a widespread breed of insects, they live on different landscapes, they are found almost everywhere in the world. The distinctive feature of these insects is the presence of a long antenna. The antenna is anatomically modified in this group because of which they can fold the appendage.  Apart from the presence of long antennas, they are also known for the absence of winged structures. They are considered as pests because they can destroy large quantities of crops and grains by feeding on them. An important fact to be noted is that these insect species are majorly found in the storage houses, they are rarely seen in the wild.

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Taxonomical Classification

The taxonomy is the study of the classification system of the organism based on its similarities and dissimilarities. The study is important because it provides a basic understanding of evolution and evolutionary adaptations. It also lays the framework of the phylogenetic relationships between different species. An important point here is to understand that various species are classified under the same genus. The taxonomic classification discussed here encompasses the general classification system of the weevils. There are species and subspecies classified under the family that have distinctive classification systems. The general classification of the weevil is as follows.

The first level of classification is done at the level of the kingdom, they are classified into the kingdom called Animalia, the basic requirement to be classified as a member of the kingdom includes two aspects, the organism must be multicellular and they must be made of eukaryotic cell type. The kingdom is further classified into phylum.

Weevils belong to the phylum called Arthropoda, this phylum is known to contain almost all the insects on the planet. The characteristic feature of this phylum is the segmented body of its member. Another important characteristic is the presence of a strong exoskeleton which is generally made up of chitin and calcium carbonate. The body of a weevil is segmented and they have a hard exoskeleton covering. A common feature of the phylum is the presence of the joined appendages and compound eyes. Weevils have developed jointed appendages and a compound eye. The only anatomical modification that is found in this insect is the absence of wings in some of the members of the species. The phylum can be further divided into order.

Weevils are classified into the order called Coleoptera, this order belongs to the suborder called Endopterygota. One of the most important members of the order apart from the weevils is beetles. A unique feature that is only seen in this order is the presence of the hardened fore wing cases in some of the species. Although this modification is not found in the weevils for the different species of weevils. They are known for their strong exoskeleton, which is prominent in every species of the weevil including the weevils in flour, which is known as wheat weevil or grain weevil.

The order is further classified into Polyphaga, a suborder of the order Coleoptera. The suborder is the largest diverged suborder. This suborder largely consists of beetles, there are about 144 families in 16 superfamilies that are classified under this taxonomic level. One of the most distinguishing features of this suborder is that members that are classified under this category do not divisible hind coxa. The hind coxa is defined as the base of the leg, they are not differentiated in these animals. They are also known to have anatomical modifications called sternites. It is defined as the first and second ventral plates of these insects that can not divide.

The suborder is divided into an infraorder called Cucujiformia, which consist of plant-eating beetles. Phytophaga is the clade that is classified under the infraorder. The clade is divided into the superfamily, the weevils belong to the superfamily called Curculionoidea.

Curculionoidea is known as the family of true weevils, all the weevils or snout beetles are categorized into this family. This family is the largest family that are categorized into the Animal kingdom, the family consist of more than seven thousand genres under which approximately  83000species are classified.  The characteristic feature of this family is the presence of elongated snouts, they also have geniculate antennae with small clubs. The size of the body of these animals varies according to species.

Some of the common species of the weevil that are categorized under this family are given below, it is important to note that these weevils belong to a different genus. 

• Anthonomus grandis, known as boil weevils

• Austroplatypus incompertus

• Rhynchophorus ferrugineus, known as red palm weevil

• Sitophilus granarius, these are weevils in flour, known as grain weevil

• Sitophilus oryzae, known as rice weevil

• Sitophilus zeamais, known as maize weevil

• Cosmopolites sordidus, known as banana weevil.

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Morphological Features

Although it is evident that weevils are insects with large species diversity as weevil as, genus diversity. There are some features that are present in almost all the weevils that are classified under the superfamily called, Curculionoidea. One of the most important morphological features that are unique to the family is the presence of an elongated antenna, these antennas have a club attached to them. A unique feature is the ability to get folded, these antennas can fold themselves to fit in the grooves of the snout. 

Another unique feature can be observed in the mouth part of these insects, they are anatomically modified as compared to other insects of Arthropoda phylum. They lack proboscis which is a common characteristic of insects instead they have modified mouthparts that are adapted to create a hole in the host. The mouth part is known to chew and create a tunnel.

The average size of weevils is about less than 6mm in length. The size also varies according to the species, the rice weevil body length is approximate 3 and 4.6 mm long. The sexual dimorphism in these groups of insects is not very prominent. Although there are some examples where sexual dimorphism is observed, it is important to note that this is seen in the case of the length and size of the body. Rhopalapion longirostre, which is commonly known as red palm weevil exhibits an extreme case of sexual dimorphism, in this species, the female members of the species are twice as long as the male members. The difference in size is mainly attributed to the presence of long rostrum in the females. 

A very interesting point here is that the sexual dimorphism seen in these animals are not favoured by sexual selection and are rather a result of ecological adaptation. This will be evident in the example of the red palm weevils. The length of the body of females is long because they need to lay eggs in the bud of the common hollyhock flower. This is an ecological adaptation necessary for survival whereas a sexually selected trait decreases the probability of survival by favouring the probability of mating.

Apart from these unique features, a common morphological feature that is present in all the members of Arthropoda including weevil is the presence of an exoskeleton, they have a hard exoskeleton made up of chitin. The main purpose of this structure is to protect from predators. The also jointed appendages, the foreleg and hind legs. The body is segmented, the complete anatomy of the body can be divided into head, thorax and abdomen regions. The head region contains compound eyes.

An interesting feature about their morphology is that most of the weevils do not have wings while there are some species of weevils that are considered to be excellent flyers, the example of such weevils includes rice weevils and red palm weevil.

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Reproduction

They are considered as sexually reproducing insects, the fertilization process is internal. The females lay eggs in the host, since they have the ability to lay eggs they are categorized as oviparous animals. The level of parental care provided by the parental generation is minimal. 

The eggs take up to 1 to 3 days to hatch, they then undergo metamorphosis under the host. The litter size of this insect, that is the number of progeny produced by the female in a single mating is about 2 to 10 eggs. The litter size differs according to the species.

Life Cycle

The average life span of the weevil is about 28 days, the average life span may vary differently depending upon the species. The life span of a weevil fly irrespective of the species can be divided into 3 stages, the stages are as follows, egg stage, larval stage, and adult stage.

The female members of the species undergo fertilization, they then lay eggs. An adult rice weevil can lay up to 250 eggs in the complete life cycle. The granary weevils lay eggs by chewing a tunnel using their mandibular mouth part. The egg is laid by special appendages, the rice weevil cut up the hole in the rice kernel and then seal it with gelatinous secretion, while red palm weevil lays eggs in the bud. Irrespective of species all the species lay eggs in the host body.

The eggs hatch after 3 to 4 days depending upon the species, upon hatching they enter the first larval stage. During the first larval stage, they are comparatively smaller. In this stage, they lack segmentation and legs. The only recognisable structure is the presence of compound eyes. They feed on the host in which they were laid, the weevils in flour feed on wheat grain. Due to this, they are known as grain weevils. 

After 18 days the larval stage is completed. The larvae undergo the second and third larval instar stages. During these stages, the body develops into the size they also develop the physiology in these stages.

After about 18 to 20 days, the larvae enter into the pupal stage. This stage lasts 6 to 8 days depending upon the species. During this stage, the insect gets cover in a smooth wrapping, this is known as a cocoon. Upon completing the development they break open the cocoon.

They exit the host body, whether rice kernel, palm fruit or wheat grain based on the species. They then reproduce to continue their life cycle.

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Behaviour

Weevils are polygamous animals. The life span of these animals are also not very long, these insects undergo metamorphosis. One of the most important features of their behaviour is that they have a specific range of hosts. It is seen that weevils often are classified based on the host as the weevils feed on rice are categorize as rice weevil, other examples include, red palm weevil, weevils in flour, granary weevil. They are not generally found in the wild, they can only sustain their life in places where the host is stored.

Although these animals are not classified as harmful animals, they are considered agricultural pests because of their ability to kill crops and infect the grains. The wevils are not known to shoe insecticide resistance but recent studies indicated that some species of grain weevils have developed resistance to some common drugs used as an insecticide.

Summary: At a Glance

• Weevils are also known as snout beetle.

• They are classified under the family called Curculionidae.

• The most commonly found weevils are rice weevil, red palm weevil and granary weevil.

• The species are classified on the basis of the host, for example, weevils that infect bananas are classified as banana weevil

• They are sexually reproducing insects, the females can lay 250 eggs during the complete life.

• The average life span ranges from 28 days to 4 months based on species.

• They are not generally found in wild.

• They are considered agricultural pests.

• The most intriguing feature is the presence of an elongated antenna that has a club associated with it.