Tiny tomato rust mites threaten German crops as researchers turn to predatory mites for organic solution

The invisible threat

Tomato rust mites (Aculops lycopersici) measure just 0.16 millimeters as adults, making them impossible to spot without a microscope. Despite their size, they pose an outsized danger: under favorable conditions, 100 mites can multiply into 50,000 within four weeks. The pests attack leaf stalks and stems first, sucking them dry until they turn brown. In severe infestations, leaves take on a bronze to rust color before withering completely, unripe fruits turn brown and corky, and entire plants can die.

They like hot, dry environments.

Once visible symptoms appear, the tomatoes are already unmarketable, making prevention the only viable strategy. The mites also feed on other nightshade plants including potatoes, eggplants, and ornamental species like angel's trumpet.

A growing problem in Germany

Rust mites were once only occasional visitors to Germany. According to the Agricultural Technology Centre (LTZ) Augustenberg in Karlsruhe, they have appeared repeatedly in greenhouse tomato stocks in Baden-Württemberg since 1999, sometimes threatening entire crops. Since 2018, the mites have also been detected in open-field cultivation. Alongside the tomato leaf miner, the rust mite is now considered the most significant animal pest affecting tomatoes.

The organic dilemma

Conventional growers can turn to chemical pesticides, but organic farmers have no effective products available. Sulphur treatments used against powdery mildew do harm mites as a side effect, but they kill beneficial insects as well and require multiple applications. Nikola Lenz, an advisor with the Demeter organic farming association, notes that if only individual plants are affected, they can be removed and sealed in plastic bags on site to prevent spread.

You have to be proactive with rust mites.

Predatory mites enter the picture

From 2022 to 2025, experts working on the "Kretschab" project focused on two predatory mite species that had previously been largely overlooked. These species occur naturally in Germany, tolerate drier air better than other predatory mites, and have already been observed entering greenhouses infested with rust mites. Larger predatory mites often struggle to navigate the fine hairs on tomato plants, but these two species may offer a better fit.

Now we have to see whether this can be applied on a larger scale.

Companies that distribute beneficial insects do not yet include these particular predatory mites in their catalogs, but the research could change that.

Baden-Württemberg's tomato landscape

In 2025, tomatoes were grown under high walk-in protective covers on 67 hectares in Baden-Württemberg, yielding a harvest of just over 11,000 tonnes, according to the State Statistical Office. This compares to 73 hectares and roughly 9,600 tonnes in 2015. The state also imported nearly 152,000 tonnes of tomatoes in 2015; preliminary figures for 2025 show imports exceeding 97,000 tonnes, with Spain, the Netherlands, and Belgium as the main countries of origin.