When to Apply Nematodes in Controlling Grubs and Larvae

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When to Apply Nematodes in Controlling Grubs and Larvae

What is the best time to apply nematodes to control larvae and grubs? In this article, we will look at the mechanism of the beneficial nematodes and explain the best months and time of day to apply nematodes to control pests in your grass. You will see how the selection of the optimal application time for the application of the nematodes is more of a scientific process.  

The debate between chemical pest control measures and the more sustainable biological control continues unabated. In the meantime, farmers, homeowners, and gardeners still have to stay on top of resilient and destructive underground pests whose impact can be catastrophic. If effective pest control measures are not taken immediately then you may find yourself in some deep water.

When it comes to environmentally-friendly pest control, one of the ‘cleanest’ and most effective ways of controlling pests, particularly the white grubs and other soil-based baddies, is the use of beneficial nematodes. They destroy the underground larval stage of some of the most destructive insects just in time before they “hibernate” and later metamorphose into their most destructive stage – adulthood. You can target a variety of insect pests with the nematodes including Japanese beetles, sciarid fly, onion flies, caterpillars, sawflies, and codling moths, and leather jackets among others. However, when to apply nematodes to control each of these pests varies depending on their individual life cycles.

For years, beneficial nematodes have traditionally been used by ‘fringe’ organic gardening purists. Theses individuals would want to avoid the use of chemicals and other environmentally-unsustainable methods at all costs. However, the war on pests continues and some pesticide formulations prove ineffective. The use of nematodes is increasingly being adopted by mainstream gardeners and even some commercial farmers to conquer pests that spoil the gardens and ornamentals.

What are Nematodes?

Nematodes are naturally-occurring multicellular and microscopic worms that prey on the grubs and larvae at various stages of development. Nematodes are soil-dwelling predators. They hunt for and prey on the different kinds of insects. This includes the Japanese beetle that is likely to devastate your grass and foliage. They do this by getting into their larvae or pupae and then releasing lethal bacteria inside their bodies. They kill host insects by releasing toxins inside them and by feeding on the host tissue. The toxins released in the process will generally kill off most of these host insects in a day or two. The nematodes complete their lifecycle by giving birth to their offspring inside the insect host. These will subsequently multiply and begin searching for a new host.

This is how nematodes can be very effective in eliminating your grub problem. Once introduced into the garden or grass, their life cycle sets off a chain reaction that will take out a good proportion of the grubs in your grass, depending on how extensively and effectively the nematodes have been used.

At the moment, there are more than 20,000 classified nematodes and some of these can be harnessed and used effectively to biologically control pests in your grass or garden. For the control of insects that are likely to emerge in summer and feed off your foliage, you will need to use nematodes that specifically prey on grubs. Using nematodes is akin to launching biological warfare on the pests afflicting your plants.

The Procedure for Using Nematodes

One of the beautiful aspects of the use of beneficial nematodes is that it is possible to target specific pests. They are not a clumsy ‘train-wreck’ that will destroy everything on their path, including other beneficial insects.

When shopping for nematodes, ensure that you purchase the right one that is specific to the pest you are targeting. For example, if you are targeting the Japanese beetles, you will need the following nematodes:-

  • Heterorhabditis bacteriophora
  • Steinernema carpocapsae

The reason why you should shop for specific nematodes is that every nematode is specially adapted to the prey that it is targeting. You will therefore get the optimal results if you match the nematode to its specific prey.

The second factor that you should keep in mind is timing. You have to target the grubs or the larvae at the stage when they are most vulnerable. The grubs are most vulnerable when they are most active. This is usually the time when they are moving deep into the overwintering sites in the soil.

In the case of Japanese beetles, the best time to target them is usually when they still young and not in the late instar (phase). Japanese grubs in the third instar are constantly on the move in search of overwintering places in the soil and this is when they are at their most vulnerable.

Thirdly, the nematode use must be matched to the right conditions. The soil temperature should at least be above 10 degrees Celsius.

Because the nematodes are light-sensitive, it is advisable to use them either early in the morning or at dusk when there is low lighting.

Summary of Points

  • When the larvae or grubs are young
  • When the larvae or grubs are active
  • Soil temperatures of above 10 degrees Celsius and which stay the same for a while
  • In moist soil
  • In low lighting conditions such as the early morning or at dusk

Some pests are attacked by more than one species of nematodes and you will maximize their impact if you apply the species together. The explanation for this is very simple. Every nematode finds the insect hosts using its unique specific method. When you use nematodes to target the same insect type the different prey-hunting strategies have a combined effect. This will have a devastating impact on the targeted pests.

In the Japanese beetle, for example, the infective juveniles of the S. carpocapsae nematodes lie in wait and ambush highly mobile Japanese beetle larvae while the infective juveniles of the H. bacteriophora have a ‘cruise foraging’ prey-hunting strategy whereby they actively move and search for the immobile insect stages that they can infect such as the overwintering pupae or grubs of the Japanese beetles that are buried in the soil.

How they are Packaged

Nematode are packaged in a sachet where they are suspended in a paste. Many of the stockists sell them via mail order. They must be applied at an optimum time taking into account the soil temperatures. To avoid guesswork, you can use a soil thermometer to measure the soil temperatures. It becomes fairly easy once you get a hang of it, particularly during later applications.

Once the sachet arrives, it is recommended that you use them straight away. If you are not planning to use them immediately, store them in a fridge for a few weeks. However, do not store them in the freezer!

The nematode paste should be mixed with a small amount of water. This will make them slurry. You can then add more water and stir the solution before application.  The nematodes immediately spring int action once you add the water. You can apply the nematode solution using a watering can or a bucket. Apply when soil is moist early in the morning or in the evening.

It is best to apply in the evening as the soil is unlikely to dry at night.

If it’s a dry season, you will need to keep the soil moist and warm for a few weeks to enable the beneficial nematode to remain active for long and ensure their pest control action takes maximum effect. You can also make repeat applications of the nematodes to maximize the pest destruction.

Results of Nematode Application

If correctly applied, you should begin seeing positive results within 3 to 7 days. The maximum effect will happen after 2 to 4 weeks. You will certainly not see ‘corpses’ of larvae, pupae or insects as the nematodes disintegrate their prey from the inside out.

Frequency of Nematode Application

Use beneficial nematodes whenever it is necessary. If you suspect that grubs and larvae are present in your garden or grass, then you should apply the nematodes. This mostly happens during spring and autumn.

It’s hard to spot the grubs and larvae since they destroy the plant roots underground. You can detect signs of infestation by checking if there are any adult insects around such as the Japanese beetles. Adult insects generally lay their eggs just before the onset of winter or in winter and the eggs begin hatching in spring or summer. You may also have to make repeated applications so as to maximize the effect of the beneficial nematodes.

When to Apply Nematodes

When applying nematodes depends on the lifecycle of the target insect. You need to take time to understand the insect lifecycle and know when they lay eggs, the months when the larvae are still young when the larvae are on the move and so on. Knowing the insect’s exact lifecycle enables you to target them with a high degree of precision.

When to Apply Nematodes to Control Japanese Beetles

Let’s take the example of the Japanese beetles and see the best time to apply nematodes for maximum effect.

The Japanese beetle is one of the most pervasive insects in the eastern US and also one of the best understood.

The complete egg-to-egg life cycle of the Japanese beetle runs for one year as follows:

Late June to late July: The adult Japanese beetles begin emerging from late pupae and start feeding on the foliage including leaves, flowers, and fruits. The beetles begin mating and female beetles lay eggs that are buried 1-2 inches beneath the soil.

Early August: The eggs begin hatching. They hatch within two weeks after they have been laid. The eggs will hatch into first instar (phase) grubs. These will immediately begin devouring the grassroots in the soil.

August to October: As they feed on the grassroots, the Japanese beetle will develop into the second and third instars in the month of August through to October.

Late September to October: From late September, temperatures will start cooling down through to October. The third instar grubs will begin moving deeper into the soil in order to find the overwintering sites.

November through March: During these months, the third instar grubs are in the overwintering sites deep in the soil and become immobile. They will not feed until temperatures start to warm up again in early spring. The grubs will sense the warming temperatures and start moving up again towards the grass root zone. There, they will resume their feeding on the grassroots until they reach maturity.

Early June: In early June, the mature grubs start to pupate in the soil.

Late June: Towards the end of June, they start emerging as adult Japanese beetles. They mate and the females lay the eggs thereby continuing the life cycle.

The Best Months to Apply Nematodes

By looking at the above life cycle, it’s clear that the best time to apply beneficial nematodes to control the Japanese beetles is in early October when the third instar grubs start moving deeper into the soil to the overwintering sites.

However, this ranges depending on the life cycle of the insect. Therefore, before you apply the beneficial nematodes to control particular insects or underground larvae, it is critical to understand the life cycle of the insect. This will help you know the precise time period when you should intervene and apply the nematodes. For some insect species such as leatherjackets and chafer grubs, the best time to apply nematodes is usually in April and October respectively as these months correspond to the most vulnerable parts of their life cycle.

Best Time of Day to Apply Nematodes

As already stated, nematodes are highly susceptible to UV radiation. Therefore apply them at the time of the day when there is very low radiation, preferably in the evenings.

Film of water or moisture in the soil that will facilitate the easy movement of the nematodes after application. Once you have applied them, it is generally recommended that you irrigate the grass to maximize their efficacy as the irrigation water also washes the nematodes from the grass leaves into the soil.

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