Sucking pests: aphids, whiteflies, thrips and mites
The pests that feed on plant sap and cell contents: how they damage crops, why they multiply so fast, how they spread viruses, and how to manage them.
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Not all insect pests chew. A large and important group feeds by piercing plant tissue and sucking out sap or cell contents: aphids, whiteflies, leafhoppers, planthoppers, mealybugs, scale insects, psyllids, thrips, and the closely associated spider mites, which are not insects at all. Individually tiny, they build huge populations quickly, and many of them carry the viruses and bacteria behind some of agriculture's most destructive diseases.
Sucking pests are among the most examined groups in agricultural entomology, and they are where pesticide resistance and the value of natural enemies are clearest.
How they feed
Most sucking pests belong to the order Hemiptera (true bugs), whose mouthparts form a slender beak containing needle-like stylets. The stylets pierce the plant and reach, depending on the species, the phloem (rich in sugars), the xylem (water and minerals) or ordinary leaf cells. Saliva injected as they feed can be toxic and can carry plant pathogens.
Thrips (order Thysanoptera) have asymmetrical "punch-and-suck" mouthparts: they rasp or pierce cells and suck out the contents, leaving silvery, scarred surfaces.
Spider mites (class Arachnida, order Acari) have eight legs and pierce individual leaf cells with stylet-like mouthparts, emptying them.
Types of damage
- Direct sap loss: weakens plants, stunts growth and reduces yield, especially of young plants.
- Toxic saliva: leaf curling and distortion (many aphids), hopperburn (yellowing and drying caused by leafhoppers and planthoppers, which in severe brown planthopper outbreaks burns patches of rice), galls (some psyllids and aphids).
- Honeydew and sooty mould: phloem feeders excrete excess sugar as sticky honeydew, on which black sooty moulds grow, blocking light and spoiling fruit, cotton lint and foliage.
- Cell damage: stippling and bronzing (spider mites), silvering and scarring (thrips), which ruin the market value of fruit, flowers and vegetables.
- Transmission of pathogens: often the most serious harm. Aphids carry hundreds of viruses; the whitefly Bemisia tabaci carries begomoviruses such as tomato yellow leaf curl and cassava mosaic; thrips carry tospoviruses; leafhoppers and planthoppers carry viruses of rice and maize and phytoplasmas; psyllids carry the bacteria of citrus greening (huanglongbing).
Why they multiply so fast
Sucking pests combine short life cycles with high reproductive rates.
- Aphids in warm seasons often reproduce by parthenogenesis (females producing young without mating) and give birth to live nymphs. Developing embryos can already contain the next generation ("telescoping of generations"). Crowding and poor host quality trigger winged forms that disperse to new plants.
- Whiteflies and thrips have overlapping generations of a few weeks in warm weather, and thrips pupate in the soil or in flowers where sprays rarely reach.
- Spider mites thrive in hot, dry conditions and can complete a generation in about a week.
How fast an aphid colony grows
Under favourable conditions, many aphid species have an intrinsic rate of increase (rm) of about 0.3 per day.
- Doubling time = ln 2 ⁄ rm = 0.693 ⁄ 0.3 ≈ 2.3 days.
- Starting from 10 aphids on a plant, after 14 days: 10 × e(0.3×14) = 10 × e4.2 ≈ 670 aphids, if nothing checks them.
Real populations are held back by natural enemies, weather and host quality, but this growth rate is why scouting intervals for sucking pests are short, and why a population can go from "a few" to "damaging" within a fortnight.
Monitoring
- Direct counts on a set number of leaves or plants (for example the underside of leaves for whiteflies and mites), walking a systematic pattern across the field.
- Coloured sticky traps: yellow attracts aphids, whiteflies and leafhoppers; blue attracts thrips.
- Beating flowers or shoots onto a white card for thrips.
- Thresholds: economic or action thresholds for the crop and region, and, for virus vectors, much lower or preventive thresholds.
Management
Prevention and cultural control
- Avoid excess nitrogen: soft, lush growth favours aphids, whiteflies and mites.
- Clean planting material and healthy transplants raised under insect-proof netting.
- Remove weeds and crop residues that host pests and viruses; use crop-free periods and avoid overlapping plantings of the same crop.
- Physical barriers and mulches: insect-proof nets in nurseries and protected cropping, reflective mulches that repel incoming aphids and whiteflies, and barrier crops around fields.
- Resistant and tolerant varieties: hairy leaves, resistance genes and virus-tolerant varieties exist for several crops.
Biological control
Sucking pests have many natural enemies, and conserving them is the most important single element of control:
- Predators: lady beetles, lacewings, hoverfly larvae, predatory bugs, spiders.
- Parasitoids: tiny wasps such as Aphidius species in aphids and Encarsia formosa and Eretmocerus species in whiteflies, widely released in greenhouses.
- Predatory mites: for example Phytoseiulus persimilis against spider mites and other phytoseiid mites against thrips and whiteflies.
- Entomopathogenic fungi in humid conditions.
Broad-spectrum insecticides kill these enemies and often cause resurgence of the pest or outbreaks of spider mites, the classic secondary pest.
Chemical control, when needed
- Use selective products where possible, and insecticidal soaps and oils, which work by contact and carry little resistance risk but need good coverage.
- Rotate modes of action (IRAC groups) between pest generations; whiteflies, aphids, thrips and spider mites have developed resistance to many insecticide groups.
- Target the vulnerable stage and the site where the pest lives, often the leaf underside.
- Remember the virus problem: insecticides rarely prevent the spread of non-persistently transmitted viruses, because aphids transmit them in seconds.
- Use only products registered for the crop in your country, as directed on the label.
Key points
- Sucking pests feed on sap or cell contents: Hemiptera (aphids, whiteflies, leafhoppers, planthoppers, mealybugs, scales, psyllids), thrips and spider mites (arachnids).
- Damage: sap loss, toxic saliva (curl, hopperburn, galls), honeydew and sooty mould, cell damage, and above all transmission of viruses and bacteria.
- Fast population growth comes from short generations, parthenogenesis in aphids, and overlapping generations.
- Monitor with leaf counts and coloured sticky traps (yellow for aphids and whiteflies, blue for thrips).
- Manage by prevention, conserving and releasing natural enemies, barriers and resistant varieties; use selective chemicals rotated by mode of action.
- Broad-spectrum sprays often cause resurgence and spider mite outbreaks.
What the exams ask
- School-leaving and entrance papers: examples of sucking pests, their mouthparts and the symptoms of their damage, such as honeydew and sooty mould.
- Degree and research-fellowship entrances: biology of aphids (parthenogenesis, telescoping generations, winged morphs), whiteflies and thrips; virus transmission; resistance; natural enemies and their use.
- Plant protection examinations: monitoring and thresholds, integrated management in field and protected crops, and resistance management.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why do spider mite outbreaks often follow insecticide sprays?
Many insecticides kill predatory mites and insects that keep spider mites in check, while spider mites survive or are resistant. Hot, dry weather then lets them explode.
Are ants a sign of sucking pests?
Often. Ants feed on honeydew and protect aphids, scales and mealybugs from their natural enemies. Controlling ants can make biological control work better.
Why does the same insecticide stop working on whiteflies?
Whiteflies have short generations and are exposed to frequent sprays, so resistant individuals are selected quickly. Rotating modes of action, using non-chemical methods and protecting natural enemies slows this.
Sources
- van Emden, H. F. and Harrington, R. (eds) Aphids as Crop Pests, 2nd edition. CABI, 2017.
- Gerling, D. and Mayer, R. T. (eds) Bemisia: 1995. Taxonomy, Biology, Damage, Control and Management. Intercept, 1996.
- Lewis, T. (ed.) Thrips as Crop Pests. CABI, 1997.
- Pedigo, L. P., Rice, M. E. and Krell, R. K. Entomology and Pest Management, 7th edition. Waveland Press, 2021.
- IRAC. Mode of Action Classification Scheme. Insecticide Resistance Action Committee, updated regularly.
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