Vine weevil in soft fruit: Biological control – nematode application methods

Storage before use

• As soon as received, store nematodes in the fridge at the temperature recommended by the supplier (e.g. 2-6°C, 5°C or 4-10°C depending on supplier and product).
• Do not freeze.
• Use by the expiry date printed on the pack.

Application conditions in substrate or soil

Use of nematodes in substrate-grown crops tends to be more effective than in soil-grown crops due to the smaller root zones of container or bag-grown crops, the comparatively damper conditions due to the more frequent and effective irrigation of substrate crops and to the more open structure of substrates than soils. All these factors help nematodes to move and to find vine weevil larvae.

Apply to moist substrate or soil and if necessary, irrigate before application. Nematodes need moisture to survive and move.

Sufficient moisture

If using nematodes in soil-grown strawberry crops, two irrigation lines per bed should be used rather than one, to help wet up the soil in the root zone before application.

Nematodes move more easily in lighter soils than heavy soils as long as sufficient moisture is available.

The ‘soil conditioner’ Transformer’® is recommended by Bionema for use with their nematode products. This product is claimed to improve the water holding capacity of soil or substrate, thus allowing the nematodes to move and find vine weevil larvae and other insect hosts more effectively. ‘Transformer’ is also available from other distributors as an individual product so could be used with other nematode products if required.

Apply when substrate or soil temperatures are within the optimum range for the species being used for at least several hours a day and for two to four weeks (depending on product recommendations) after application. See Table 1 on the Vine weevil in soft fruit: Biological control – nematodes page for species information.

Nematode application rates

• Use the rate of nematodes recommended by the supplier. Most suppliers recommend a number of nematodes per plant for strawberry crops and one supplier (e-nema) recommends higher numbers per plant for blueberries.
• For other crops, the rate is recommended as numbers per m² for use in soil or substrate-grown crops or some suppliers recommend numbers per litre of substrate for container-grown crops.
• The recommended rate for use in soil is usually higher than that recommended for use in substrate in pots or containers.
• Some growers of strawberry and blueberry crops are using reduced rates of nematodes in a ‘little and often’ approach rather than using recommended rates at the traditional timings.
• Use the whole pack(s). Do not divide them as nematodes may be unevenly distributed within the pack.

Water application rates

Once the nematodes have been mixed with water at the recommended rate, apply in the recommended volume of water per m². This water volume is specified by some suppliers and not others.

Some suppliers recommend a specific water volume per plant rather than per m² for use on strawberry crops. Other suppliers give recommended volumes of water per pot depending on pot volume and offer details of how many pots of a certain volume can be treated per pack of nematodes.

Water volume

As a rule of thumb for application to containers, the total volume of water applied (including that used for nematode application and that used for irrigation afterwards) should be approximately 10% of the pot volume, but this will depend on substrate moisture content prior to application. For example, for a 7-litre pot, a total of 700 ml of water should be applied.

Some growers prefer to use higher water volumes (e.g. one third of the pot volume), to ensure that the nematodes reach the vine weevil larvae in the pots. Care should be taken when using higher water volumes to ensure that the drench does not run out of the bottom of the pot.

Using acid, fertilisers and plant protection products

• If adding acid to lower water pH, do not add acid directly to the nematode mixing tank.
• The pH should not be below 4.5 to ensure that nematodes are not harmed by acidified water.
• Similarly, application of fertilisers together with nematodes should be avoided, especially if this would lower the pH below 4.5.
• A few plant protection products can have harmful effects on nematodes, so check with your supplier for details.
• Do not tank mix with plant protection products or concentrated fertilisers, unless specifically recommended by the supplier (check the supplier’s website or contact them for details).

Drip irrigation systems

Growers of most soft fruit crops apply nematodes through the drip or trickle irrigation system as this is much more convenient and less labour-intensive than drenching. Check with your supplier that your drip irrigation system is suitable for nematode application.

Before nematode application, remove filters smaller than 50 mesh (equivalent to 0.3mm) downstream of the injection point to avoid nematode blockages.

Using food grade dye and monitoring blockages

Before nematode application, use a food grade dye to test how long it takes the water to reach the intended area of the field. The dye can also be used to check that all drippers are working and are not blocked.

It will take longer for the nematodes to reach fields further away from the injection point than nearer fields. Similarly, it will take longer for the nematodes to reach irrigation blocks further from the on/off valves than those nearer the valves. If more than one irrigation block is to be treated in the same field, treating individual blocks at a time will give a more uniform application than treating several blocks at a time (e.g. five 3-minute irrigation cycles for five successive blocks, rather than one 15-minute cycle for all five blocks). Start with the valve nearest to the nematode injection point and the block nearest the valve.

Concentrated nematode suspension

Considering the injection rate, the area to treat and the recommended dose rate, calculate the number of nematode packs and volume of water required in the concentrated nematode mixing tank.

Make up the concentrated nematode suspension by initially mixing with 1-3 litres of water in the nematode mixing tank to break up the formulation and to rehydrate the nematodes and mix thoroughly to ensure they are well dispersed. Then add the remaining volume of water required. If the suspension is too viscous add an additional known volume of water and adjust the injection rate accordingly. Set the injection rate on the dosing unit to the appropriate dilution (normally 1% is suitable for nematode application).

Irrigation system management

Inject the nematodes using a suitable dosing unit during the first half of the irrigation cycle. Then run the irrigation system for the second half of the cycle with no nematodes injected, in order to flush the nematodes out of the lines and into the crop.

It is important to run the irrigation for long enough (for at least as long as it took for the dyed water to reach the field) to allow the nematodes to flow down each irrigation line and out of the drippers to apply a uniform and accurate dose.

• Consider applying nematodes in the first irrigation of the day so that any subsequent irrigation will help to wash the nematodes out of the lines as quickly as possible, as nematodes drown if left for too long in stagnant water.
• Ensure that the nematode suspension is agitated prior to and during injection into the irrigation lines, either by hand stirring, using a mechanical propeller at low revs or by bubbling using an air pump.
• Delivery of nematodes through replicate drippers can be checked by collecting the suspension into small containers. The help of a supplier or consultant may be needed in order to do this.
• After application, rinse the nematode mixing tank and inject through the system.
• Consider spot treatments with a spray applicator at the ends of the drip lines as fewer nematodes are applied here due to the water ‘bouncing’ back at the end of the lines.

Drenching using spray application equipment

As an alternative to using drip irrigation, nematodes can be applied to soft fruit crops as a drench using conventional spray application equipment. For example, drenching is often used on blueberry for effective application to the entire root system in the large containers used, including the central root zone where vine weevil larvae do the most damage and the edges where larvae can also occur. In addition, drenching is used in some strawberry crops (e.g. to tray plants if infested with larvae), or to outdoor planted crops where drip irrigation is not available.

Conventional spray equipment

Unlike drip irrigation, which is best applied in the first irrigation of the day, when using conventional spray equipment, apply in the late afternoon or evening to avoid harmful effects of high temperatures and UV light.

There is no need to mix the nematodes in the dark but ideally this should be done away from bright sunlight.

All equipment, regardless of application method, should be cleaned/flushed prior to nematode application, particularly if it has previously been used for a plant protection product that is harmful to nematodes.

Application to the root zone

The nematodes must be applied to the root zone where needed as they do not move far in the soil or substrate after application.

1. Remove all fine filters in the spray lines and nozzles (i.e. 0.3mm or smaller), to prevent nematode blockages. If you do not know the size of filters they are best removed. Use nozzles with apertures of at least 0.5mm or 0.8mm diameter (depending on supplier recommendations) and apply as a medium / coarse spray.
2. Do not use high pump pressures (i.e. over 5, 12 or 20 bar - depending on supplier recommendations).
3. Empty the whole pack(s) of nematodes into a bucket, rinse the packs with a small volume of water and then add 5-10 litres of water (depending on product recommendations) and stir well.
4. Partially fill the sprayer tank with water (5-15°C, 5-25°C or 15-20°C depending on product and supplier). Start the agitator and add the nematode suspension to the tank through the sieve. Rinse the bucket well and add the rinsings to the tank.
5. Keep the agitator running and add the remaining required volume of water to the tank.
6. Apply immediately for best results and do not leave the nematode suspension longer than four hours before use.
7. Keep the nematode suspension agitated throughout the application procedure to prevent the nematodes settling out.

When applying to strawberry crops, drench each plant individually around the base of the plant and avoid run-off over any polythene around the planting hole.

Irrigate immediately

If irrigation is available, irrigate immediately after application to wash nematodes off any foliage treated and to help to disperse the nematodes into the substrate or soil. If applying to containers, when irrigating take care not to cause run-off from the tops of the containers. If irrigation is not available, try to apply during rain for uncovered crops. Keep the soil or substrate moist for at least two weeks after application.

Original author/s

Jude Bennison, Janet Allen & John Atwood, ADAS

Tom Pope, Harper Adams University

Please note this information was last updated in 2018. Biocidal and plant protection products must only be used in accordance with the authorised conditions of use. Regular changes occur in the authorisation status of biocides and plant protection products. For the most up to date information, please check with your professional supplier, BASIS registered adviser or the Chemical Regulation Division (CRD) of HSE (https://www.hse.gov.uk/crd/) before use.

While the Agriculture and Horticulture Development Board seeks to ensure that the information outlined on this page is accurate at the time of publishing, no warranty is given in respect thereof and, to the maximum extent permitted by law the Agriculture and Horticulture Development Board accepts no liability for loss, damage or injury howsoever caused (including that caused by negligence) or suffered directly or indirectly in relation to information and opinions contained in or omitted from this information.