Leatherjacket Larvae Cleared from a Ryegrass Sward with Nemasys Grubs and Autumn Watering

By late September, a perennial ryegrass lawn that looked even in July can show irregular yellow areas, commonly 20 to 40 cm across. Lift a suspect patch and the turf may peel away like a loose carpet, with the roots chewed off underneath. The larvae are grey-brown, legless, tough-skinned, and up to 3 cm long. They are leatherjackets, the larvae of the European crane fly, Tipula paludosa. Adult crane flies laid eggs into damp grass through August, and the larvae have fed on grass roots since then.

Drought gets blamed first, since both problems leave yellow, dying patches. The tug test separates them. Drought-damaged grass stays fixed in place, while leatherjacket-damaged turf pulls free with very little resistance. Birds add another clue. Starlings, crows, and magpies working a lawn in tight clusters, leaving small holes, are usually probing for larvae, and their pecking tears up more turf.

Soil temperature sets the treatment window

Nemasys Grubs contains Steinernema feltiae, a parasitic nematode sold as a moist powder for mixing into water. The nematodes enter the leatherjacket through body openings, release bacteria that kill it within a few days, and breed inside the carcass. BASF’s product and similar Steinernema packs from Nematodes Direct give the same working range: soil temperature between 12C and 20C, with no use in colder soil.

That 12C threshold explains why a September treatment often succeeds and an early November one often disappoints. Air temperature can mislead. A mild October afternoon may be above 14C while soil 5 cm down has already fallen to 9C. Push a cheap soil thermometer into the root zone for two minutes and use that reading. In most temperate regions, the main workable period runs from early September to mid-October, with a smaller spring window in April as the soil warms again.

Nematodes are living organisms with a short shelf life, usually four to six weeks under refrigeration. Apply the batch within days of arrival. A pack left in a shed over winter for next season is a poor bet.

Watering makes or breaks the application

Steinernema feltiae travels through the thin water films between soil particles. Once the top few centimetres dry out, the nematodes stall and die before reaching larvae. The watering instructions on the packet are part of the treatment, equal in importance to the dose.

Mow the lawn short before application so the solution can reach the soil. Water the whole area heavily the evening before, giving moisture time to move into the root zone. Mix the nematodes into water and apply with a watering can after removing the rose, or use a hose-end applicator after taking out any fine filter. The nematodes can clog a standard 0.5 mm mesh.

Apply in the evening or under cloud. Ultraviolet light kills nematodes on contact, and darkness gives them time to move down into the soil.

Water again immediately after application to wash the nematodes from the leaves into the soil. Then keep the top 5 cm damp for at least two weeks. During a dry autumn spell, that usually means light watering every second or third day. A single application followed by dry weather gives little reduction in larvae, even when the mix was prepared correctly. September rain is patchy enough that hand watering should be planned from the start.

Larvae usually die over one to three weeks. Dead insects do not appear on the surface, since they break down underground. Bird activity stops because the larvae are gone, and the yellow patches stop spreading.

Open the soil before treating

Compacted soil holds less water and drains unevenly, both of which interfere with the two-week moisture period. Aerating a week before applying nematodes helps water move down to the feeding zone.

A Wolf-Garten hollow tine fork removes cores roughly 8 to 10 cm deep, leaving open channels for water and nematodes to reach the root zone. On a small lawn, crossing the worst patches at 10 cm spacing takes about fifteen minutes. Leave the cores on the surface to weather down, or rake them into the compost heap.

A solid-tine fork or a garden fork rocked back and forth also opens the soil. On heavy clay that has been walked on all summer, hollow tines give cleaner channels because they lift material out of the ground.

Reseed after the larvae are dead

The chewed areas will not simply knit back together, because the root system that supported them has gone. Overseeding is the repair stage, and autumn is well suited to it: soil still carries warmth, moisture is more reliable than in summer, and weed pressure is lower.

Rake each bare patch hard to expose loose soil and pull out dead thatch. Scatter a perennial ryegrass or ryegrass-fescue mix at roughly 25 to 35 grams per square metre. That is heavier than the 20 to 25 grams used for a new lawn, because an existing sward needs fast cover across open patches. Press the seed against the soil by treading or rolling, then keep it damp until germination. Ryegrass normally germinates in seven to fourteen days at autumn soil temperatures.

Ryegrass sown by early October usually establishes before the first hard frost in most temperate zones. Fescue-heavy mixes take longer and suit earlier sowing.

Seedlings are vulnerable if leatherjackets remain in the soil. Any surviving crane fly eggs laid that same August can feed on new roots as soon as they form. Treat first, then reseed.

Feeding recovery without pushing soft growth

New ryegrass seedlings and stressed established turf both need feeding. A high-nitrogen summer feed in October pushes soft top growth that frost damages and disease exploits. Autumn lawn feeds use a low-nitrogen, higher-potassium ratio, often near 3-0-8 or 4-0-8 N-P-K. Potassium helps harden grass against cold and supports roots going into winter.

Comfrey liquid feed is the home-made potassium option. Comfrey leaves, especially the Bocking 14 cultivar, are rich in potassium. Rotting them down produces a dark liquid that is diluted at roughly one part feed to ten parts water. Pack cut comfrey leaves into a bucket with a weight on top, collect the thick liquid that drains over two to three weeks, and dilute it heavily before watering it on. The smell is strong enough to put many people off the method.

As an autumn lawn feed, comfrey liquid supplies potassium without the heavy nitrogen of a spring liquid feed. Its nutrient content varies from batch to batch, so it lacks the consistency of a bagged autumn lawn product.

Reinfestation next August

Killing this year’s larvae does nothing to stop crane flies laying a fresh generation into the same damp lawn next August. The adults are weak fliers and tend to lay where they emerged, so a lawn treated one autumn can show patches again the following year. Keeping the lawn slightly drier in late summer, with deeper and less frequent watering, makes it a less attractive egg site, although the balance between that effect and August weather has not been quantified.

Compost from the same work

Grass clippings can feed the compost heap through the same period. Fresh clippings are wet and high in nitrogen, and on their own they slump into a grey anaerobic mat that stinks. Mixed by volume with roughly two parts brown carbon material, such as autumn leaves, shredded cardboard, or straw, to one part clippings, they heat up and break down into usable compost. The cores lifted by the hollow tine fork can go into the same heap. A heap built through autumn from clippings and leaves at that ratio is turnable by spring.

Clippings from a lawn treated with a persistent selective herbicide can carry residues that damage vegetable seedlings months later. Nematode-treated clippings do not carry that residue problem into a vegetable bed.