Forest Entomology, Scope, Insect Structure, Function and their Classification

Entomology is a branch of Zoology that deals with the study of insects in general whereas, Forest entomology is that branch of biological science which deals with the study of the effect of insects upon forests and forest products.

Scope of forest entomology: 

Forest Entomology has great scope in the field of agriculture and forestry. So many species of insects damage plants, crops, forest trees, and agricultural products although some species are useful for helping in pollination fruits and crops plants. The knowledge of Entomology helps in the field of agriculture and forestry in the following ways;

1. Harmful and useful insects are identified and this work will help in pest control and management in the forestry and agriculture sectors.
2. Entomological knowledge helps to know the structure and function of different organs of insects (mostly mouthparts) which are used to damage forest trees and crops.
3. Entomology helps to know the life cycle of insects with their damaging stages (adults and larval stages )
4. Entomology helps to identify the vector insects which may transfer the plant diseases from one plant to another
5. Entomology describes insect population, type of species, and their behaviors.

Forest Insect Beetle

Insect’s Structure and function (Morphology)

Insects are those arthropods that are characterized with

• Live in water(aquatic) and on land(Terrestrial)
• The body is divisible into three regions- Head, Thorax, and Abdomen
• One pair of antennae, three pairs of jaws, and one pair of eyes are present in the head; Three pairs of walking legs and one or two pairs of wings are present in the thorax.
• The life cycle consists of four stages- Adult, Egg, larva, and pupa stages.
• Excretion by Malpighian tubules
• Behave with nature as vectors, parasites, and pests although some are useful to human beings and acting as pollinators. For example honey bee and silkworm.
• Sexes are separate and reproduced by sexual methods.

The morphological structure of the insect are described under two major categories; Body surface and covering; and Segmentation.

Body Surface and covering: 

The outer body surface of insects consists of a hardened layer that acts as a skeleton for body support and as a protective covering to the soft internal organs. It is referred to as the exo- skeleton. The outer surface consists of a waxy layer (cuticle ) which is highly impermeable to water. This outer layer is extremely important to insects in minimizing loss of water from the body and its occurrence is one of the reasons insects have been so successful as land animals despite their small size. If the cuticle is damaged in any way, the insect may lose moisture rapidly and finally die.

Segmentation: 

Insects are segmented creatures because of bodies consisting of a series of similar units or segments. The individual segment can be seen most readily in the hind end ( abdomen ) which often presents a horizontally striped appearance. In other parts of the body ( the head in particular ) the segments tend to be more modified and merged together. Internally the segmented pattern is clearly apparent in the nerve cord and the respiratory system. The body segment of insects is grouped into three body regions

Head:

The head consists of at least 6 segments but no direct evidence of their presence can be seen as they are fused into a single structure, which is called a head capsule, and is equivalent to the skull of vertebrates. The upper part of the head capsule consists of two sense organs i.e. eyes and antennae. The lower part of the head is occupied by a group of structures that comprise the mouthparts. Their form varies greatly in different insect groups, in each case being modified according to the nature of food and mode of feeding.

Thorax:

The middle body region of the insect is the thorax and always consists of three segments. These may be more or less distinct but often their margins are difficult to identify as the overall structure of the thorax is rather complex to allow for the attachment and operation of legs and wings. Insects typically possess three pairs of legs one pair to each segment of the thorax. Each leg consists of four main portions with flexible joints between. At the extremity of the leg is the foot(tarsus) which is usually composed of several joints. The tarsus normally terminates in a pair of claws though there may be only a single claw. In addition, pad-like structures to provide extra grip are also often present.

Wings when present arise from the second and third segments of the thorax. There are typically two pairs but in a few groups, such as the true flies(dipterans), they are reduced to a single pair. Many insects are of course wingless. The wings themselves are mostly membranous but in a beetle (Coleoptera) the forewings are modified in the form of hard horny cases which are not involved in flight. In all except the smallest of insects the membranous part of the wing is supported by a series of thickened ribs, called veins. The pattern of arrangement of these veins, their branches, and cross-connections, varies greatly in different insect groups and is an important feature in classification.

Abdomen:

The hind body region of an insect is the abdomen. In its primitive form, it consists of several segments though fewer than this can normally be counted as the hind ones tend to be reduced and retracted one inside the other. A row of spiracles commonly occurs along each side of the abdomen, one pair to each segment which helps in respiration. In the more primitive insect groups, a pair of tail filaments(cerci) arise from the tip of the abdomen, and there may be a central third one. In more highly evolved insects they are reduced in size or absent.

External reproductive organs are present in the abdomen of many insects. In the female a sword-like organ, ovipositor down which eggs pass at the time of egg-laying. In male insects, a pair of claspers of varying shape and size with which the abdomen of the female is grasped during mating. A central copulatory organ(Penis, aedeagus) is located between the claspers.

Forestry Interest Insects (pests) classification

1. Phloem and shoot boring insects
2. Wood-boring insects
3. Root boring insects
4. Defoliating insects
5. Seed and Cone damaging insects

Phloem and shoot boring insects: 

Insects are placed in this feeding group utilize the phloem tissue of plants or the inner part of the host for food and habit during a portion of the life cycle. Insects of orders;

A) Coleoptera

B) Lepidoptera

C) Diptera are very common for phloem and shoot consuming and damaging in nature.

The adult female lays eggs on the trunk or its vicinity and the hatched larvae bore in the bank, often into the sapwood and some penetrate into the heartwood. The borer either meanders around or tunnels up and down, feeding on the wood and degrading it in the process. Some borers have the habit of girdling around the bark from the point of entry. The larval stage can last for several months to over a year and the different larval instars can induce severe damage within the tree, resulting in mortality. If a tree ever recovers, its growth will be impeded and its timber value lowered.

Wood-boring insects:

Wood-boring insects are a range of arthropods that cause damage to the wooden structures or xylem tissues which are also called heart borers.

1. Emetald ash Borer
2. Asian long-horned beetle
3. Common furniture beetle
4. Mountain pine beetle
5. Death watch beetle
6. European spruce bark beetle
7. Xyleborus glabratus
8. Weevil(Rice, wheat, boll vine, maize, etc)

 

Root boring insects

1. Root borer beetle
2. California root borer
3. broad necked root borer
4. Banana root borer
5. Prionus apple root borer etc.

Defoliating insects:

Insects defoliators have long been recognized as important pests in forestry. In west Africa, the

main species of injects defoliators belong to three orders;

• Order-Lepidoptera e.g. Moth and Butterfly
• Order-Orthoptera e.g. Grasshopper
• Order-Coleoptera e.g. leaf beetle

Seed borer insects:

The orders Hymenoptera and Lepidoptera have seed borer insects.

Sawflies, Horntail, Moth borer, Hickory, Shuck worm, Iris borer, Lesser peachtree borer, European corn borer, and Greater peachtree borer is a well-known example of a seed borer insects.

Cone borer Insects

1. Cone beetle(Genus Conophthrous ponderosae)
2. White beetle larvae
3. Scarab beetle 
4. Cone weevil
5. Blister beetles
6. Webbing cone worm
7. Slash pine seed worm
8. Longleaf pine seed worm etc.