Insect orders of agricultural importance
How insects are classified, the orders that hold crop pests and beneficial insects, their key features, and how to use an identification key.
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About a million insect species have been described, more than half of all known animal species, and many more remain unnamed. Only a small fraction are crop pests, but those few cause enormous losses, while many others pollinate crops, eat pests, recycle dead matter or produce honey and silk. The first step in managing any insect is to identify it, at least to its order, because the order says a great deal about how it feeds, how it develops and how it can be controlled.
Insect orders, their characters and examples are among the most reliably examined topics in agricultural entomology.
What makes an insect
Insects belong to the phylum Arthropoda, animals with jointed legs and an external skeleton, and to the class Insecta. An adult insect has:
- three body regions: head, thorax and abdomen;
- three pairs of legs, on the thorax;
- usually two pairs of wings on the thorax, though some have one pair or none;
- one pair of antennae.
Other arthropods that damage crops are not insects: mites and spiders (Arachnida) have two body regions, eight legs as adults and no antennae; millipedes and centipedes have many legs; and slugs and snails are molluscs. This matters because many insecticides do not control mites.
How orders are distinguished
Insect orders are separated mainly by:
- Wings: their number, texture, venation and how they fold. Many order names end in -ptera, from the Greek for wing: Coleoptera means "sheath wing", Lepidoptera "scale wing", Diptera "two wings".
- Mouthparts: chewing, piercing–sucking, rasping–sucking, siphoning or sponging.
- Metamorphosis: complete (egg, larva, pupa, adult) or incomplete (egg, nymph, adult).
The major orders
Coleoptera (beetles and weevils)
The largest order, with around 400,000 described species. Forewings are hardened elytra that cover the membranous hind wings. Chewing mouthparts in adults and larvae. Pests include weevils of stored grain, the Colorado potato beetle, flea beetles, white grubs and bark beetles; beneficials include ladybird beetles and ground beetles that eat pests.
Lepidoptera (moths and butterflies)
Wings and body covered in scales. Adults usually feed on nectar through a coiled proboscis; larvae (caterpillars) chew and do the damage. Major pests include the fall armyworm, cotton bollworms, the diamondback moth, stem borers, codling moth and pink bollworm. The silkworm is a valuable member.
Hemiptera (true bugs, aphids, whiteflies, hoppers, scales)
A diverse order with piercing–sucking mouthparts forming a beak. Includes Heteroptera (true bugs, with half-hardened forewings) and the groups once called Homoptera (aphids, whiteflies, leafhoppers, planthoppers, psyllids, mealybugs and scale insects). They suck sap, cause honeydew and sooty mould, and are the most important vectors of plant viruses. Examples: cotton aphid, brown planthopper, silverleaf whitefly, Asian citrus psyllid.
Diptera (true flies)
Only one pair of functional wings; the hind wings are reduced to knob-like halteres used for balance. Larvae are legless maggots. Pests include fruit flies (Bactrocera, Ceratitis), leaf miners, the Hessian fly and seed-corn maggots. Hoverflies and tachinid flies are valuable predators and parasitoids, and many flies pollinate crops.
Hymenoptera (bees, wasps, ants, sawflies)
Two pairs of membranous wings, linked by tiny hooks; many have a narrow waist. Sex is determined by haplodiploidy (males develop from unfertilised eggs). Mostly beneficial: honey bees and wild bees pollinate crops, and parasitoid wasps such as Trichogramma and braconids are key biological control agents. Sawfly larvae and leaf-cutting ants are pests in some regions.
Thysanoptera (thrips)
Tiny, slender insects with narrow wings fringed with long hairs, and asymmetrical rasping–sucking mouthparts. They scar leaves and fruit and transmit tospoviruses such as tomato spotted wilt virus. Example: onion thrips.
Orthoptera (grasshoppers, locusts, crickets)
Enlarged jumping hind legs, chewing mouthparts, leathery forewings. Locusts, such as the desert locust, can form swarms of billions that strip crops across whole regions. Many grasshoppers and crickets are minor pests.
Other important orders
- Neuroptera (lacewings): predatory larvae, the "aphid lions", widely used in biological control.
- Blattodea, including termites (formerly order Isoptera): damage roots of crops and trees, and wood.
- Dermaptera (earwigs) and Odonata (dragonflies, predators).
Using a simple identification key
A dichotomous key asks a series of two-way questions. A farmer finds a small insect on chilli leaves:
- Wings present? Yes → go to 2.
- Front wings hard and meeting in a straight line down the back? No → go to 3.
- Only one pair of wings? No → go to 4.
- Wings narrow and fringed with long hairs; insect under 2 mm long? Yes → Thysanoptera (thrips).
Had the answer at step 4 been "no", with a beak tucked under the body and soft, often white-waxy wings, the next couplet would lead to Hemiptera, perhaps whiteflies. Identification to order narrows the control options: thrips hide in flowers and buds, so sprays must reach them there, and many thrips species have evolved resistance to common insecticides.
Why the order matters for management
- Mouthparts decide which insecticides work: systemic insecticides moving in sap control sucking insects better than chewing ones; stomach poisons and Bt control chewing larvae.
- Life cycle decides timing: control caterpillars while young; time fruit fly baits to adult activity.
- Beneficials in the same habitat, especially Hymenoptera, Coleoptera and Neuroptera predators, should be protected by choosing selective products.
Key points
- Insects have three body regions, six legs, usually two pairs of wings and one pair of antennae; mites have eight legs and two body regions.
- Orders are distinguished by wings, mouthparts and metamorphosis.
- Coleoptera (elytra, chewing), Lepidoptera (scales, larvae chew), Hemiptera (piercing–sucking beak; virus vectors), Diptera (one pair of wings, halteres), Hymenoptera (bees, parasitoids), Thysanoptera (fringed wings), Orthoptera (jumping legs, locusts), Neuroptera (lacewings).
- Identification to order guides choice of control and protects beneficials.
What the exams ask
- School-leaving and entrance papers: characteristics of insects, the major orders with examples, and differences between insects and mites.
- Degree and research-fellowship entrances: diagnostic features of orders and important families, mouthparts and metamorphosis by order, and examples of pests and beneficials.
- Professional examinations: field identification, use of keys, and linking taxonomy to management.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Are spiders and mites insects?
No. They are arachnids, with eight legs and two body regions. Spider mites are serious crop pests, but they need acaricides rather than most insecticides, and spiders are useful predators.
Which insect order causes the most crop damage?
Lepidoptera, through their caterpillars, and Hemiptera, through sap feeding and virus transmission, are usually ranked highest, with Coleoptera close behind, especially in stored products.
How can I tell a fly from a wasp?
Flies have one pair of wings and usually large eyes and short antennae; wasps and bees have two pairs of wings, often longer antennae, and many have a narrow waist.
Sources
- Gullan, P. J. and Cranston, P. S. The Insects: An Outline of Entomology, 5th edition. Wiley-Blackwell, 2014.
- Triplehorn, C. A. and Johnson, N. F. Borror and DeLong's Introduction to the Study of Insects, 7th edition. Thomson Brooks/Cole, 2005.
- Dent, D. Insect Pest Management, 2nd edition. CABI, 2000.
- Stork, N. E. "How many species of insects and other terrestrial arthropods are there on Earth?" Annual Review of Entomology 63, 31–45 (2018).
- Hill, D. S. Agricultural Insect Pests of the Tropics and Their Control, 2nd edition. Cambridge University Press, 1983.
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