WHEN pot plants wilt and die and seedlings never appear, biological controls may come to the rescue. So, what is a biological control and how do we use it? It’s a living organism for controlling or eliminating a pest or disease. Nowadays, they’re usually tiny insects, like mites or parasitic wasps, and nematode worms only visible through a microscope. And a bacterium is also engaged to fight fungal disorders.

Long before the invention of microscopes, biological controls were much larger than the ones we use today. Four thousand years ago, the Egyptians recognised that cats were tip-top mousers, so employed these avid predators for the task.

By the third century, ant nests were being sold in Cantonese markets to control insect pests. And 1000 years later, Yemenis started moving ants from surrounding hills to date palms to clear insects from the trees.

Using natural predators progressed apace, especially from the 18th century onwards. Lacewing eggs, described at the time as ‘aphidivorous insect’, were placed in glasshouses to consume the dreaded aphid. And, to the welcome relief of many, the spiked shieldbug, Picromerus bidens, was engaged in the scratching war with bed bugs.

By the Second World War, growers were using 57 predatory species and the publication of Rachel Carson’s Silent Spring focussed many minds. As a result, the concept of integrated pest management emerged, with biological controls providing an environmentally-friendly alternative to damaging synthetic chemicals.

Unfortunately, some of these applications had unforeseen consequences that gave biological control a bad name. Spotted knapweed had been seriously damaging grazing for cattle in parts of the United States. So, in the 1970s, European seed-head flies, Urophora affinis, were released to control them. The flies’ larvae provided a welcome feast for larva-consuming deer mice, Peromyscus maniculatus, and this caused a population explosion. But since deer mice carried the hanta virus which can be fatal for humans, this biological control carried an unacceptable price tag.

And the harlequin ladybird has become a horrifying biological control. This voracious pest was released in mainland Europe and the USA in 1988. And after spreading widely in England, the alien invader has now arrived in this country.

Fortunately, the latest biological controls should have no adverse consequences. They are put through the most rigorous tests, lasting several years, before being marketed. After eliminating specific pests, they usually die.

This applies to one of the most commonly-used biological control, the nematode worm. Nematodes are the most numerous multi-cellular species on earth, playing a critical role everywhere. One species could lay 200,000 eggs in a day. Scientists have selected a few species to combat slugs and vine weevil and crane fly larvae.

Nematodes, like Phasmarhabditis hermaphrodita, that parasitise slugs and snails occur naturally in the soil, so we’re only boosting their numbers when applying Nemaslug. Nematodes enter a slug, exuding bacteria that prevent the mollusc from eating, resulting in its death within a week.

The treatment is effective against most slugs, especially Deroceras reticulatum, the grey field slug. But nematodes need moist soil. So, given the cost of Nemaslug, don’t let the soil dry out during the six-week treatment. These nematodes don’t stray beyond the treatment area and largely die out after killing the molluscs.

Different nematodes, Heterorhabditis megidis and Steinernema kraussei, will deal with a vine weevil attack, and like Nemaslug, are effective just now when the grubs – white with brown heads – are embarking on their evil practices. A second treatment in August is often worthwhile.

Nematodes only work when pests are at the larval stage, so, cranefly or daddy longlegs larvae, leatherjackets should be dealt with in August.

Biological controls have also been developed for greenhouse pests. The eggs of the parasitic wasp, Encarsia, do a fine job by devouring whitefly. And the tiny predatory mite, Phytosieulius readily tucks into red spider mite.

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I have four large bins of grass cuttings collected over the last two years. Normally I have spread them over the vegetable patches in the autumn and the worms have kindly mixed them into the soil (along with last year's collected tree leaves). I was unable to do this last autumn, and spring has arrived. Should I leave the cuttings and spread them at the end of the year? John Berry

Provided the grass has rotted down well, you could dig it in just before planting. If still fairly rough, use it as a mulch round the vegetables: worms will gradually incorporate the mulch into the soil. If the grass contains weed seeds, lay damp newspaper on damp soil, then cover with mulch. This stops seeds rooting and prevents seed germinating beneath the paper. The soil is enriched with carbon from paper and nitrogen from grass.

Polygonatum multiflorum, Solomons Seal.

Arching stems hung with little white bells rise gracefully from the grey green foliage. Needs a damp, shady spot.