Spider Mites on My Strawberry Plants

Strawberry leaves that look bronzed, stippled, or dull rather than fresh green — with fine webbing visible on the undersides in more severe infestations — are being attacked by two-spotted spider mite (Tetranychus urticae). Spider mites are very small (about 0.5 mm) and difficult to see with the naked eye, but the fine yellowish or orange-red speckling on the upper leaf surface caused by their feeding is distinctive. They are at their worst in hot, dry summers and under polytunnels where natural controls are absent.

Identifying spider mite damage

Look at the underside of a leaf from a damaged plant with a hand lens or magnifying glass — you should see tiny moving dots (the mites), their eggs (tiny spherical translucent balls), and shed skins. The upper leaf surface shows pale yellowish flecking where cells have been emptied by mite feeding — the speckling pattern is caused by the mite piercing individual cells. Heavy infestations produce visible webbing on the leaf underside, which is the mite's protective web where they breed. Very severe damage causes entire leaves to look bronzed and dead.

Why hot dry conditions trigger outbreaks

Spider mites breed extremely quickly in warm, dry conditions — a generation can complete in eight to ten days at 30°C. Their natural predators (predatory mites, ladybird larvae) are less active in very dry conditions and may be absent altogether under cover. This combination of rapid breeding and reduced natural control allows populations to explode in July and August during hot summers. Outdoors, natural predators and rainfall that washes mites off leaves usually keep populations below damaging levels in all but drought years.

Control measures

A strong jet of water directed at the leaf underside dislodges mites and eggs and is effective at reducing populations. Apply mite-specific organic sprays (insecticidal soap, rapeseed oil-based formulations) to the leaf underside where mites feed and breed. Under glass or polytunnel, introduce predatory mites (Phytoseiulus persimilis) as biological control — available by mail order from biological control suppliers. Phytoseiulus is highly effective but only works above 12–15°C, so it is primarily a summer/polytunnel solution. Avoid broad-spectrum insecticides which kill predatory mites and make the problem worse.