Insect pests

Belonging to the large group of Arthropods, which include also the woodlice, mites, millipedes and symphilids (see Table 14.1), insects are horticulturally the most important arthropod group, both as pests, and also as beneficial soil animals.

Structure and biology

The body of the adult insect is made up of segments, and is divided into three main parts: the head, thorax and abdomen (see Figure 14.4). The head bears three pairs of moving mouthparts. The first, the mandibles in insects (such as in caterpillars and beetles) have a biting action (see Figure 14.5). The second and third pairs, the maxillae and labia in these insects help in pushing food into the mouth. In the aphids, the mandibles and maxillae are fused to form a delicate tubular stylet, which sucks up liquids from the plant phloem tissues. Insects remain aware of their environment by means of compound eyes which are sensitive to movement (of predators) and to colour (of flowers). Their antennae may have a touching and smelling function.

The thorax bears three pairs of legs and, in most insects, two pairs of wings. The abdomen bears breathing holes (spiracles) along its length, which lead to a respiratory system of tracheae. The blood is colourless, circulates digested food and has no breathing function. The digestive system, in addition to its food absorbing role, removes waste cell products from the body by means of fine, hair-like growths (malpighian tubules) located near the end of the gut.

Since the animal has an external skeleton made of tough chitin, it must shed and replace its 'skin' (cuticle, see Figure 14.4) periodically by a process called ecdysis, in order to increase in size.

Fore wing

Hind wing

Fore wing

Hind wing

Chitin Cuticle

Mandible

Labrum

Waxy layer

Structural layer Living cells

Cross-section of cuticle (X80)

Figure 14.4 External appearance of an insect. Note the mouthparts, spiracles and cuticle, the three main entry points for insecticides

Labrum

Mandible

Waxy layer

Structural layer Living cells

Cross-section of cuticle (X80)

Figure 14.4 External appearance of an insect. Note the mouthparts, spiracles and cuticle, the three main entry points for insecticides

Aphid Labrum

Aphid

Mandible Insect Mouthpart

Aphid

Figure 14.5 Mouthparts ofthe caterpillarand aphid. Note the different methods of obtaining nutrients. The aphid selectively sucks up dilute sugar solution from the phloem tissue

The two main groups of insect develop from egg to adult in different ways. In the first group (Endopterygeta), typified by the aphids, thrips and earwigs, the egg hatches to form a first stage, or instar, called a nymph, which resembles the adult in all but size, wing development and possession of sexual organs. Successive nymph instars more closely resemble the adult. Two to seven instars (growth stages) occur before the adult emerges (see Figure 14.6). This development method is called incomplete metamorphosis.

In contrast, the second group of insects 'Endopterygota' including the moths, butterflies, flies, beetles and sawflies undergo a complete metamorphosis. The egg hatches to form a first instar, called a larva which usually differs greatly in shape from the adult. For example the larva (caterpillar) of the cabbage white bears little resemblance to the adult butterfly. Some other dam aging larval stages are shown in Figure 14.7 and these can be compared with the often more familiar adult stage. The great change (metamorphosis) necessary to achieve this transformation occurs inside the pupa stage (see Figure 14.6).

The method of overwintering differs between insect groups. The aphids survive mainly as the eggs, while most moths, butterflies and flies survive as the pupa. The speed of increase of insects varies greatly between groups. Aphids may take as little as 20 days to complete a life cycle in summer, often resulting in vast numbers in the period June-September. On the other hand, the wireworm, the larva of the click beetle, usually takes four years to complete its life cycle.

Larva or caterpillar (X1.5)

Larva or caterpillar (X1.5)

Cabbage White Life Cycle

Large cabbage white butterfly

Large cabbage white butterfly

Cabbage Aphids Nymph Photo

Adult (X20)

Peach-potato aphid

Adult (X20)

Peach-potato aphid

Figure 14.6 Life cycle stages of a butterfly and an aphid pest. Note that all four stages of the butterfly's life cycle are very different in appearance. The nymph and adult of the aphid are similar

Figure 14.6 Life cycle stages of a butterfly and an aphid pest. Note that all four stages of the butterfly's life cycle are very different in appearance. The nymph and adult of the aphid are similar

Symphyla Stages

Carrot fly larva (10) white, no legs or mouthparts

Sawfly larva (4)

more than four pairs of false legs

Carrot fly larva (10) white, no legs or mouthparts

Sawfly larva (4)

more than four pairs of false legs

Wireworm larva (3)

long, shiny brown, small legs

Figure 14.7 Insect larvae that damage crops. Identification intothe groups above can be achieved by observing the features of colour, shape, legs and mouthparts

Wireworm larva (3)

long, shiny brown, small legs

Insect groups are classified into their appropriate order (Table 14.1) according to their general appearance and life cycle stages. There follows now a selection of insect pests in which each species' particular features of life cycle are given. Whilst comments on control are mentioned here, the reader should also refer to Chapter 16 for details

Table 14.1 Arthropod groups found in horticulture

Group

Key features of group

Habitat

Damage

Woodlice (Crustacea)

Grey, seven pairs of legs, up to 12 mm in length

Damp organic soils

Eat roots and lower leaves

Millipedes (Diplopoda)

Brown, many pairs of legs, slow moving

Most soils

Occasionally eat underground tubers and seed

Centipedes (Chilopoda)

Brown, many pairs of legs, very active with strong jaws

Most soils

Beneficial

Symphilids (Symphyla)

White, 12 pairs of legs, up to 8 mm in length

Glasshouse soils

Eat fine roots

Mites (Acarina)

Variable colour, usually four pairs of legs (e.g. red spider mites)

Soils and plant tissues

Mottle or distort leaves, buds, flowers and bulbs; soil species are beneficial

Insects (Insecta)

Usually six pairs of legs, two pairs of wings

Springtails (Collembola)

White to brown, 3-10 mm in length

Soils and decaying humus

Eat fine roots; some beneficial

Aphid group (Hemiptera)

Variable colour, sucking mouthparts, produce honeydew (e.g. greenfly)

All habitats

Discolour leaves and stems; prevent flower pollination; transmit viruses

Moths and butterflies (Lepidoptera)

Large wings; larva with three pairs of legs, four pairs of false legs and biting mouthparts (e.g. cabbage-white butterfly)

Mainly leaves and flowers

Defoliate leaves (stems and roots)

Flies (Diptera)

One pair of wings, larvae legless (e.g. leatherjacket)

All habitats

Leaf mining, eat roots

Beetles (Coleoptera)

Horny front pair of wings which meet down centre; well-developed mouthparts in adult and larva (e.g. wireworm)

Mainly in the soil

Eat roots and tubers (and fruit)

Sawflies (Hymenoptera)

Adult like a queen ant; larvae have three pairs of legs, and more than four pairs of false legs (e.g. rose-leaf curling sawfly)

Mainly leaves and flowers

Defoliation

Thrips (Thysanoptera)

Yellow and brown, very small, wriggle their bodies (e.g. onion thrips)

Leaves and flowers

Cause spotting of leaves and petals

Earwigs (Dermaptera)

Brown, with pincers at rear of body

Flowers and soil

Eat flowers

of specific types of control (cultivations, chemicals, etc.) and for explanations of terms used.

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Responses

  • ELISA
    What is unigue about the cabbage white butterfly nymph stage?
    6 years ago
  • RIAN
    What is the function cuticle in insect?
    4 years ago

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