Thrips

Identification

Very small (1–2 mm), slender, elongated insects with fringed wings, coloured yellow, brown or black; the larvae are pale yellow and wingless. They suck sap from the cells of leaves, petals and buds. Symptoms: silvery to silvery-grey streaks and blotches on the leaves, tiny black specks of excrement, and distortion of young leaves, buds and flowers. They are especially damaging under glass and on houseplants, and some species transmit plant viruses. The silvery sheen appears once the pest has sucked out the contents of the epidermal cells — the empty, air-filled cells reflect light like metallic streaks; they are accompanied by glistening black specks of excrement, which distinguish thrips feeding from that of most other sap-sucking pests. The mouthparts are piercing-sucking and asymmetric (only a single mandible), so the insect first rasps the tissue and then sucks up the exuding sap. On flowers, feeding produces white or brownish discolouration and distortion of the petals — severe on roses, chrysanthemums, orchids and cyclamens. The life cycle is short and, in warm conditions, the generations overlap: the females lay eggs inserted into the plant tissue, the larvae (two instars) feed on the leaves and then drop to the soil or litter, where they pass through the immobile prepupal and pupal stages. Among the most damaging are the western flower thrips (Frankliniella occidentalis) — a key greenhouse pest and vector of tospoviruses (including tomato spotted wilt, TSWV) — and the onion thrips (Thrips tabaci). Only the larvae acquire the viruses, while the adults transmit them, so reducing the larvae also lowers the risk of viral infection.

Treatment

Blue sticky traps for monitoring and capture (thrips are attracted to the colour blue), predatory mites (Amblyseius) and lacewings, potassium soap or oils, misting the plants (they dislike high humidity), and removing weeds and infested flowers. As a last resort, insecticides applied in rotation because of rapid resistance build-up. Control is complicated by the hidden stages: eggs inserted into the tissue, larvae sheltering in buds and rolled leaves, and pupae in the soil are poorly reached by sprays, so only a strategy that combines several methods and is repeated every few days is effective. Prevention: quarantine and inspection of new plants (ideally with a hand lens), blue or yellow traps for early detection of an incursion, removal of plant debris and weeds that act as a reservoir, and — under glass — humidity control. Raising the humidity deters thrips and favours entomopathogenic fungi, but makes sense mainly under glass and in combination with biological methods; while doing so, avoid wetting the flowers of plants susceptible to grey mould (Botrytis), so as not to provoke fungal diseases. Biological methods work best under glass with early, preventive introduction: predatory mites of the genus Amblyseius/Neoseiulus and the minute pirate bug (Orius) destroy the larvae and some of the adults, lacewing larvae provide support, and the soil stages are targeted by entomopathogenic nematodes (Steinernema) and entomopathogenic fungi (Beauveria). Contact treatments with potassium soap and oils act only where they land and require thorough coverage of both leaf surfaces. Chemically: strict rotation of groups with differing modes of action (e.g. spinosyns, translaminar avermectins) — thrips develop resistance to a single substance exceptionally quickly, and pyrethroids alone can be unreliable. Ecological methods are most effective early and preventively; during a mass incursion they play a supporting role and cannot replace rotational protection.

How to tell it apart from look-alikes

Control — organic and chemical methods

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